excited states of biomolecules—v

YEARLY REVIEW

EXCITED STATES OF BIOMOLECULES-V

The series on excited states of some biological molecules continues much as previous reviews [130, 351, except that this review is of necessity more abbre- viated than in the past. See Brown [l5] for an extensive review on spectroscopy of chlorophylls in uiuo and in oitro. The accompanying Yearly Review by Suzuki reviews quantum chemical aspects of biomolecules.

Porphyrins and chlorophylls

The resonance energy of tetraphenylporphyrin has been calculated from thermodynamic (combustion) data as -1021 kJ.mol-’ 1841. This value is compared with values determined by other methods.

Single site spectra of Zn-porphin in triphenylene at 4.2 K [70] has been shown to exhibit three principal site species at 570.1, 568.7 and 567.8 nm, which can be selectively excited with a dye laser. Vibrational assignments are reported. T~ for each species is also reported as < 10 ns. Fluorescence of Chl a and h by site selection spectroscopy has also been reported [37] in ethanol and methyltetrahydrofuran glasses at - 4 K. The possibility of analytically utilizing the different vibrational frequencies as a “fingerprint” for identification of the two species in uiuo is promising.

The effects of the central metal and substituents on the quasiline absorption spectra of Zn, Cu, Ni, Pd and Pt-porphyrins at 15 and 35 K in n-octane [4] show linewidths of - 12 cm for Pt-porphyrin and - 4 cm- for Pd-porphyrin and Zn-porphyrin. Ni- and Cu-porphyrins show no quasiline structure, due to an efficient electronic radiationless decay. Zn- Tetraphenynlporphin shows no quasiline structure due to inhomogeneous broadening. The & was estimated as 2 7 x

The luminescence of Y, Lu and Th meso tetra- phenylporphin was examined and s, -+ S o fluorescence (-425 nm) was reported [91] with & 1 for all species. No heavy atom effect was observed, even though Z increased from 39-90, presumably due to efficient internal conversion from S2-+S1. The S1 + So emission clearly shows a heavy atom effect, as expected. Phosphorescence was also reported, with T~ = 29 ms, 4p = 0.01 for Y ; T~ = 2.8 ms. q5p _z 0.02 for Lu and T~ I 0.3 ms, bp = 0.03 for Th.

Kampas and Gouterman [67] report that amor- phous octaethylporphin films lack an exciton band at 460nm found in crystal films, and that the Soret band is considerably sharpened. Gouterman and co- workers have done much work on the absorption and emission of porphyrins in film. Lack of space prevents

for Pt-porphyrin.

803 P A P 2 7 6 1

detailed discussion here. The interested reader is referred to references in [67] and other more recent work.

The molecular origin of the anomalous polarization in resonance Raman spectra of metalloporphyrins has been theoretically evaluated [145]. This is due to a doubly degenerate electronic level and a rotational- type normal mode which leads to equal rotation of the two orthogonal transition moments (i.e. any molecule with degenerate in-plane electronic transitions and an A,, vibration).

Low temperature (77 K ) MCD has been reported [69] for Mg, Zn. Cu. Pt and Pd porphins in EPA. The authors state that the Jahn-Teller coupling in S , of Pd and Pt porphins is weak, while in Zn porphin the coupling is sizable, with the active vibrational mode being 170-180cm-‘. The Jahn-Teller distortion parameter LY is <0.3 for Pd and Pt porphin and 1.2 0.3 for Zn porphin. High resolution Zee- man spectra of the singlet, triplet and quartet states of metalloporphyrins [22] show a Jahn-Teller stabilization energy of 3 0 4 0 c m - ‘ using a distortion parameter LY = 1.2-1.4

Shipman [ 1251 has evaluated the oscillator and dipole strengths for the Q, (0,O) band of Chl a, Bchl a, pyrochl a. phe a and pyroBchl a. A medium correc- tion for the solvent was also explicitly included in the calculation. Previous calculations are critically reviewed and a detailed methodology of the calculation is discussed. Chi a f values in many different solvents are enhanced at 77 K.

Thi: spin-orbit effects in metallophthalocyanines have been evaluated. as their structural similarities to Chls, hemes, vitamin BIZ. etc. make them valuable as models [60]. The widely varying luminescence properties of metalloporphyrins are reconciled, based on the assumption that spin-orbit coupling and crystal field stabilization are the primary perturbers gov- erning the zero-field triplet splitting and intersystem crossing. Three distinct categories of luminescence emerge, largely in agreement with experimental data.

The intersystem crossing rate constant of Chl a has been shown [13] to exhibit a strong solvent dependence in pyridine, increasing from 630 s f 1 to 1 11 1 s - l

(75% increase) from dry pyridine to wet pyridine and a weak deuterium isotope effect -20% (504 s- l ) , due to the molecular geometry. Burgner and Goncalves [17] have shown that the deuterium isotope effect on radiationless decay of the triplet state of porphyrin free bases critically depends on the position of iso-

804 Yearly review

topic substitution, the N-H(D) positions being an order of magnitude more efficient.

Polarization of the phosphorescence of octaethylporphin and octaethylchlorin has been described [ 1281 by a model where the T, -+ So oscillator is par- tially anisotropic; M,, M, # 0, M , = 0. For octaethylporphin, M J M , = 0.7, and for octaethylchlorin M , / M z = 1.7. implying that the space part of the T, wavefunction is not the same as the S, wavefunction. For octaethylporphin (DZh) T, symmetry is Bzu, while for the chlorin (C,,), it is A, at 77 K. The phosphorescence polarization has also been reported [42] for rhodoporphyrin XV. For different metal derivatives, the polarization Poo is reported as - 3 1 % for Pd, 44% for Cu and 0% for Zn, indicating significant differences in the mechanism of intercombination transitions. The authors interpret the Pd result as indicating S 2 and S 3 significantly mixed with TI, while for Cu, the high value is attributed to electrostatic interaction. In the Zn rhodoporphyrin, the TI oscillator is proposed to be perpendicular to S, -+ So. with the n,T* of the carbonyl admixed with TI.

Phosphorescence for Mg, Zn, Cu and Pd complexes of octaethylchlorin and Zn and Pd complexes of iso- bacteriooctaethylchlorin have also been reported [41] at 77 K. The singlet-triplet interval (AEsT) for octaethylchlorin is -4100cm-’ and 4300cm-l for iso- bacteriooctaethylchlorin, but is less in the free base. Fluorescence and phosphorescence (0,O) maxima, AEST and T~ are also available. Additionally, the phosphorescence maximum for Chl a is reported as 895 nm. Lebedev rt a!. [85] report the phosphorescence maximum (950nm) of Chl a in cellulose triacetate polymer films both at room temperature and 77K, with T~ = 2.2 ms. The most recent report in this Jour- nal [95] reports the phosphorescence of Chls a and h and pheophytins a and b in ETOH and EPA at 77 K, with &. = 5 x For Chl a, A,, = 960 rim. T~ = 1.7 ms; Chl h A,,,= 901 nm, T~ = 2.1 ms; phe a i,,,= 932nm, T~ = 1.5 ms and for phe h, A,,, = 900 nm, T~ = 1.6 ms, in substantial agreement with the above-mentioned values.

Katz and co-workers [52] have observed stimulated emission and lasing near the fluorescence maxima from Chls a, h, Bchl, pheophytin a and metal pheophorbide a with N, laser pumping. For Chls in pyridine, lasing is observed at 681 nm from the S , + So (0,O) level, while for the metal-free species, lasing is observed from the S , -t So (0,O) and (0,l) vibronic levels. They attribute this to differences in aggregation behavior. The authors also submit that stimulated emission must be considered in the inter- pretation of fluorescence lifetime ( T ~ ) data for both in uitro and in vivo Chl systems when high photon fluxes are used.

Shipman and Katz [ 1261 have further supported the special-pair model by performing exciton calculations for the low energy manifold of Chl a . H 2 0 and Bchl a . H 2 0 stacks, and comparing with the electronic spectra. For Chl a, the low energy transition

for (Chl a.H,O), ranges from 693 to 721 nm for n = 1 to cu. while for (Bchl a . H 2 0 ) , stacks, the range was 848-908 nm. The authors conclude that Chl a stacks are probably not major components in green plants. but that Bchl a - H z O stacks may be identified with the more red-shifted components of antenna Bchl a in the purple photosynthetic bacteria.

Koester and Fong [76] have furthered Fong’s Chl model by examining the exciton interaction in sym- metric dimeric aggregates of Chl a monohydrate. The theoretical analysis was based on the point dipole approximation in first order perturbation theory of the red absorption band of in oitro 700nm (C, symmetry) dimeric aggregates of Chl a. H20.

Concentration quenching of Chl fluorescence in bilayer vesicles and liposomes and its relationship to the light harvesting mechanism has been investigated [lo]. Quenching values (50% quenching) are reported for various lipids. At 0.1 M Chl, the lifetime becomes non-exponential with components T, = 761 and T~ = 132ps in egg lecithin liposomes. The authors view the mechanism as Forster-type resonance energy migration, followed by trapping by a pair of molecules (referred to as a “statistical pair trap” [9] at less than the critical distance). Aggregate formation as a possible mechanism is eliminated by the simi- larity of zF and & vs concentration curves.

The EPR spectra of the lowest triplet state of Pd porphin in n-octane at 1.3 K has been discussed [79] in terms of the 3E, orbital degeneracy being lifted by the crystal field, and in n-octane the energy difference between Ix> and ( y > is 58 & 2 cm-’. Zero field parameters have been determined as D = -24.38 & 0.03GHz and [El = 320 & 60MH.z. with g, , 1.677 f 0.001 and g1 = 1.989 & 0.002. These results are discussed with respect to the spin-orbit coupling matrix element between (x> and 1y>.

Zero field splitting parameters for the triplet state of Mg porphin . ETOH in n-octane have been deduced [64] by microwave-induced changes in fluorescence intensity, and they find D = 0.035cm-I and IEJ = O.OlOcm-’. The decay rate of the upper two spin components is - 20 s-’, while the bottom (in-plane) level is 2s-’. Chls a and b decay one to two orders of magnitude faster than this, which cannot be accounted for by either the Mg or the So - To split. According to the authors, this must be due to the phytol side chain and carbonyl groups on Chls.

Phe h, Ca Chl b Cd Chl b have been examined by optically detected zero field magnetic resonance at 2 K [26]; the central metal ion exhibits the con- trolling effect on the one center spin-orbit coupling mechanism for determining kIsc.

Kleibeuker et al. [74] have measured the optically induced electron spin polarization of the triplet states of Chls a and b, tetraphenylporphyrin and chlorin, free bases in organic glasses at 15 K. Both Chls have the greatest triplet steady state population in the top zero field level, differing from reports in earlier Yearly Reviews (see [35, 1301 for references).

Yearly review 805

ASMO-SCF-CI calculations have been performed on bacteriochlorophyll and bacteriopheophytin for neutral, +1 and - 1 charged species [104]. Compar- ing the theoretical results with experimental evidence. the authors conclude that the x-cation radical of Bchl is formed at the initial stage of excitation.

Song et al. [ 1321 have applied the method of configuration analysis to Chl and Bchl. They show that the excited state wavefunctions ('Q, and 'Q,) are largely determined by the macrocycle. Spin density distributions for 'Chl a and Chi a+ are also compared with experimental results.

Chl a-H,O complexes have been examined by both fluorescence and fluorescence detected magnetic resonance at 4.2 K [80]. The zero field splitting parameters were calculated for Chl a . 2 H z 0 as D = 0.0291 f O.OOO5 cm-'. E = 0.0038 f O.OOO5 cm-I, for Chl a.H,O: D = 0.0309 0.0004cm-', E = 0.0042 f 0.0004cm-' and (Chl a.H,O),: D = 0.0286 f O.OOO5 cm-' and E = O.OOO31 f O.OOO5 cm-'. It can be seen that ZFS (dimer) < ZFS (monomer), favoring Katz's model of the Chl dimer with H-bonded H 2 0 .

Katz's group [ 1051 has spectrally characterized phe a'+ by adding trifluoroacetic acid to Chl a to yield phe a'+. On titration, they report no phe a+ formed. The zero field parameters for phe a'+ measured by EPR at 5 K show D = 0.0288 f 0.0009cm-I and E = 0.0046 O.OOO1 cm-', more similar to ZFS parameters for Chl a than phe a.

Flash photolysis studies of the photooxidation of Chl a by p-benzoquinone [3] in dioxane and toluene show that in nonpolar solvent the product is a triplet exciplex, which is formed in an essentially diffusion controlled reaction. The triplet-triplet absorption spectrum of the exciplex is also shown. The rate constant for"intercombination degradation" is 3 x lo3 s- ' and k , for 'Chl is 2.4 x lo9 M- ' .s - ' .

Electron transfer from singlet and triplet bacteriopheophytin to pbenzoquinone [55] quenches Bphe with k , = 1.7 x 10" M-' .s- ' . & was determined as 0.6 and klsc = 3 x 108s-'. The 'Bphe yields Bphe ?.

A laser-induced photochemical isomerization of free base porphyrin (H,P) in n-octane at 4.2K has recently been discussed [140]. The authors observed that in slowly grown crystals, the solute was coplanar. with the N-H . . . H-N axes of the pair perpendicular to each other, and separated by - 65 cm- ' in their absorption and fluorescence spectra. Irradiation caused a reversible proton shift to yield only one species. The reaction was not significantly enhanced at 77K.

Phytochrome and hiliproteins

Phytochrome has been theoretically studied by the configuration analysis method [ 1321 and the results correlated with the photomorphogenic transforma- tion of P,-+P,,. In the 'Q, excited state, ring A becomes "activated" due to a significant local exci-

tation. Consistent with a nucleophilic mechanism is the local excitation and charge transfer from ring A to the viofin moiety, lowering the pK, of the pyrrolic nitrogen. The pKa* increase in ring B can also be accounted for by the charge transfer.

Land [82] has characterized the triplet excited state ( - 150 kJ . mol- ') of bilirubin by pulse radiolysis, and reports a T -+ T A,,, = 500 nm. The extinction coefficient is c = 8800 M- ' .cm- ' . The triplet lifetime is 7T = 9 ps, and the upper limit for +,sc = 0.1. Oxygen quenching occurs with k , = 8.2 x I O 8 M - ' . s - ' in benzene. The relevance of these data are used to dis- cuss the photodestruction mechanism of bilirubin.

An anomalous solvent shift effect and Tl--+So transition in bilirubin IX-r has been examined [6]. The ET = 230kJ.mol-I, confirming that bilirubin is capable of '0, ('Ag) formation. The transient absorption spectrum in deoxygenated methanolic ammonia is also reported with a &,ax= 485 nm. This was enhanced by ethyliodide.

Dimerization of 2.3-dihydrooctaethyl - 1,19(21. 24Hhbilindione (Amax= 594 nm) in the presence of K,[Fe(CN),] (as an oxidant) yields a dimeric violin which is shortened by one ( A 4 ) double bond, as is PI, [120]. This thermally reverts to the original compound. The authors propose that this may serve as a useful model for the P, -+ P,, phototransformation.

The '0, reactivity of biliverdin has been studied [93] by direct excitation of 0, in trichloro- and tri- fluoro-ethane and in D,O, as a function of pD. In the organic solvent, biliverdin quenches '0, with

105 M - 1 . - 1 to yield a red product. In D,O, k , ranges from 1.5 to 6 x 1O1OM-'.s-' and k R X , 3 to 5 x 108M-1.s-I , depending on pD, to yield a color- less product.

An autocatalytic photooxidation of xanthobilirubic acid has been shown [46] to proceed with + R X = 0.2 to yield two products: methylethylimaleimide and 5-formyl-2,4 dimethyl- 1 H-pyrrole-3-propanoic acid.

Lightner [87] has written a review on the photo- reactivity of bilirubin and related pyrroles. A very up- to-date discussion of the spectroscopic properties and photochemistry of these molecules is included.

Although not directly related to this review. Ken- drick and Spruit [68] and Pratt [in press, January, 1978, this Journal] have written excellent, comprehen- sive reviews on the phototransformation of phytochrome. The molecular nature of the phototransformation. techniques for study of the intermediates and a very informative look at the present viewpoint of phytochrome phototransformation are discussed.

Polyenes and carotenoids

Two-photon excitation studies by Holtom and McClain have located a low-lying excited 'A, state in 1,6-diphenylhexatriene (DPH) and 1.8-diphenyloctatetraene (DPO) by observing the very strong A, symmetry allowed two-photon absorption bands which are accompanied by shoulders in the low

kQ = 8 X 1O8M-'.s- ' and reacts with kRx = 6 X

806 Yearly review

energy region at 25,900cm-' in DPH and 22,600 cm-' in DPO [54]. Christensen and Kohler have studied the mixed crystal spectra of 2,lO-dimethylun- decahexaene (DMH) and 2.1 2-dimethyltridecatetraene (DMO) with high resolution fluorescence and show additional information to support the 'A, state assignment for the lowest excited singlet state in these and other linear polyenes. The energy gap between the 'A, and the higher lying 'B, state increases from 3,000cm-' in the pentaene to 4,000cm-' in the hex- aene [25]. The two-photon absorption spectrum has also been reported on 1,3,5-hexatriene using thermal blooming techniques [ 1381. However, the authors failed to observe the states lying at about 4.5 eV which are expected to appear in a two-photon spectrum on the basis of symmetry arguments and predicted by the single plus double CI-PPP calculations. Small values of two-photon cross sections indicate that the two-photon transition to this low-lying 'A; state would be weak in spite of the fact that the transition is symmetry allowed.

1,3,5-Hexatriene (cis- and trans- mixture) has been subjected to an electron-impact investigation with a low energy (incident energy of 20, 40, 50 and 70eV), variable angle (&8O0) technique [34]. The angular- and impact-energy dependence of the scattering intensity has confirmed the existence of two S--+ Ttransi- tions with maximum intensities at 2.61 eV and 4.1 I eV, which correspond to the Z'A, --+ 1 3B, and f A, -+ 3A, transitions, respectively. In contrast to a previous electron impact study [ 1161, the authors were unable to detect any singlet --+ singlet transitions in the energy-loss range of 4.24.6eV; no evidence has been found for .f1A,+2'A, excitation in the vicinity of 4.4 eV.

The multiphoton ionization (MPI) technique has been applied to trans-1,3,5-hexatriene (THT) [lo91 and trans-1,3-butadiene (TBD) [65]. The MPI spectrum of THT from 6.0eV to 6.6eV reveals a two- photon allowed transition beginning at 6.23 eV (50,280 cm-I). The excited state in this transition is assigned to either the 'A, state as predicted by the PPP theory with singly excited configurations only C38, 1221, or more likely a g-Rydberg state [6].

Johnson [65] has studied the MPI spectrum of rrans-1,3-butadiene over the four-photon ionization region to energies lower than 405nm, and many three-photon resonances within the Rydberg state are observed and a new Rydberg series with a quantum defect of 0.04 is identified. Above 405nm in energy, vibronic structure reveals that the state allowed two- photon resonance is 'B, symmetry. The author con- cludes that the existence of a 'A, state below 48,765 cm-' is very unlikely. All the structure in the MPI spectrum and vacuum UV spectrum [97, 981 of trans-1,3-butadiene can be accounted for in terms of several Rydberg series and 'B, valence transition [65]. An ah initio theoretical description of the low- lying R,R* excited states of frans-l,3,5-hexatriene has been reported using Slater-type orbitals [90]. Exci-

tation energies and out-of-plane spatial character (Z') values of the lowest n,n* singlet and triplet states are l'A, (0.00eV, 33.2a:), 13B, (4.11 eV. 34.1 a:). 13A, (5.80eV. 34.443, 2'A, (6.83eV, 35.7~:) and l'B, (7.40 eV, 54.8 a:). The authors suggest that hexatriene may be the shortest polyene to provide evidence for the K,R* singlet state ordering.

Controlled electron impact exatation has been applied to trans- and cis-1.3.5-hexatriene radical cation in the gas phase [2]. The emission bands observed are assigned to the X2B,--+g2Au and A 2 A 2 -+ g2B, transitions of the cations of the frans- and cis-isomers, respectively. The lifetimes of the zer- oth vibrational levels of the excited A states of the trans- and cis-cations are estimated to be 17 3 ns and 5 6 ns, respectively. The triplet-triplet absorption spectra of 1,6-diphenylhexatriene (DPH) and 1,8-diphenyloctatetraene (DPO) have been recorded using flash photolysis and pulse radiolysis [ 113. Small values for the quantum efficiency of intersystem crossing are reported; q5,sc (DPH: 0.019 in benzene, 0.020 in ethanol, DPO: 0.005 in benzene, 0.006 in ethanol) and no significant solvent effect on &c are observed; the results show that as polyene chainlength increases, &c and the triplet energy level decrease and the triplet extinction coefficient increases.

Donor energy effects on the triplet-sensitized isomerization of 1 1-cis-retinal have been studied as a function of the excess energy of the sensitizers [119]. A systematic decrease in the photoisomerization yield at higher triplet energies is interpreted in terms of an efficient isomerization path involving higher vibronic triplet levels.

Flash photolysis of hindered cis-isomers of retinal and all-trans retinal has been investigated both by direct and sensitized excitation [48]. The spectra of all the isomers are indistinguishable, as are the transient decay rates. The ps transients detected in all these cases are probably an equilibrated mixture of the planar isomeric retinal triplets. The photoisomerization quantum yields for isomeric retinals by direct excitation in polar and nonpolar solvents and biacetyl triplet-sensitization in polar solvent have been reported [142]. The four cis-retinals (9,11,13-cis and 9,13-dicts) have almost identical quantum yields of 20% in nonpolar solvents and 54% in polar solvents. while triplet-sensitized quantum yields of the cis isomers and all-trans-retinal are ca. 20% and 0.3%. respectively. Irradiation of the trans isomer in methanol or hexane at 350nm yielded only 13-cis-retinal which is detected by high pressure liquid chroma- tography, but careful analysis of the photoproducts using a more sensitive detector system reveals that 11-cis-retinal is found to be a primary major photoproduct along with the 134s. 9-cis and 7-cis isomers [143], as reported previously [16, 611. The photoisomerization of cis-retinals occurs via the singlet as well as the triplet state [142]. Liu and co-workers have reported the results of photoisomerization of retinal [28], 3-dehydroretinal and 3-dehydro C, ketone

Yearly review 807

[88] in acetonitrile. Retinal gives all the mono-cis isomers including the hindered 11-cis and 7-cis isomers. For 3-dehydroretinal irradiated first with light of >380nm, 9-cis isomer is the major product, but following irradiation at >480nm, the 7-cis isomer is recovered as the major peak in the HPLC chromatogram, while 3-dehydro C1 ketone gives 7-cis isomer as major product along with 9-isomer, under irradiation at 360 nm. Isomerization kinetics of protonated 1 I-cis-retinylidene Schiff bases have been studied by ps and ns spectroscopy [63]. The formation and bleaching of absorption species (55M60 nm) within lops and corresponding decay and recovery within 11 ns indicate that the isomerization mechanism of the protonated 1 I-cis-retinal Schiff bases is not identical to rhodopsin which isomerizes within 6 ps of irradiation [ 181.

A single 8ps pulse at 353nm has been applied to excite all trans-retinal [53]. The two observed transients are studied over the time range 0-2000 ps. One transient corresponds to T , + Tabsorption, and triplet (TI) build up shows an exponential growth with t = 34 ps ( & 5), the other red transient of I,,, 640 nm appears within 6 ps and decays in about 20 ps. Appar- ently, the molecules excited by the laser pulse relax into two or more different excited states, which then decay independently to the ground state.

Electric-field spectroscopy has been applied to determine the ground and excited state dipole moments of all-trans-retinal, its unprotonated and protonated Schiff bases and 1 1-cis-retinal [94]. All four species observed become highly dipolar at the vertically excited singlet (IApl = 15.6, 9.9, 12 and 12.7D, respectively). The authors suggest that a significant mixing of the polyene ‘B, and ‘A, state occurred, and that a negative charge towards the car- bony1 of the Schiff base terminus and positive charge at the ionone end of these molecules would result in the polar excited state observed.

Becker and co-workers have not observed fluorescence in very carefully dried sample of all-trans- retinal in 3-methylpentane, dichloromethane and acetonitrile, while fluorescence could be observed if water or phenol is added to the dry retinal in EPA (77 K) or in ethanol, methanol (243 K) or trifluoroethanol (295 K). The authors interpret that fluorescence occurs only from the H-bonded species in retinal [135].

Both temperature and solvent effects on the natural radiative lifetime to and the other spectral properties of polyenes [62], retinal isomers and Schiff bases of retinal have been discussed [8]. The authors proposed a simplified coupling model which comprises the two states, ‘A, and ‘B,, being coupled by solvent-solute perturbation. The dependence of T~ on the solvent and temperature for retinal and diphenylpolyenes may be associated with the state mixing by the solvent perturbations, as the strength of solvent induced state mixing increases, ‘50 decreases, and as temperature decreases, T~ decreases owing to the density in-

crease [62]. The authors have also interpreted the temperature and solvent effects on the spectral properties in terms of solvent polarity, H-bonded complex formation and correlation with change of state order of ‘A, and ‘B, [135, 81.

Solvent effects on the spectral shift of retinylin- dene-1-amino-2-propanol Schiff base and its protonated form have been discussed [75]. High concen- trations of phenol, indole and /?-mercaptoethanol are described as effective solvents in red shifting the absorption spectra.

ASMO-SCF-CI calculations have been applied to charge transfer models for rhodopsin [78]. The calculation shows that both models proposed by Akhtar et al. [ l ] and Hooper and Buckser [57] cannot explain the characteristic red-shift of absorption maximum for retinal.

Various rhodopsin models have been compared and discussed in terms of the spectral properties of protonated Schiff bases and chromophore-protein interaction [56]. The effects of charge separation and chromophore conformation change on spectral shift are computed by the PPP-SCF MO method, in which additional charges are incorporated into the diagonal elements of the Fock matrix.

All-trans-1 1,12-dehydroretinal has been synthesized and incubated with bovine opsin suspension for 3 h at 37°C [39]. The resulting pigments gives a new absorption maximum at 345nm and is non-bleach- able.

Stone and Dratz have recently written a review on rhodopsin photochemistry [ 1331 and the subject will not be further reviewed here. A review article on resonance Raman studies of the visual pigments has also recently been published by Callender and Honig

New bands are observed when gas absorption occurs on solid polyene films [58. 591. The new band for trans-/?-carotene is 536 nm, for 15,15’-cis-/?-carotene 537nm and for retinyl compounds, 375nm. Overlap between absorption and emission is greatest in /?-carotene and least in retinal. However, the emission is undoubtedly due to impurities.

The effect of molecular polarization on the elec- trochromism of oriented all-trans-/?-apo-carotenoic acid was studied [118]. The asymmetric insertion of carboxylic group in the poiyene chain causes a strong linear electrochromic effect. The permanent dipole moment differences between the ground state and the excited state of the carotenoic acid molecule is Ap = 3.6 x C.m (10.7 Debyes) (+_20”/,) the polarizability difference parallel to the long axis of the molecule is Aall = 1.17 x 10-37C.m2.V-1 ( - 1050 A3).

The linear dichroic spectra of the crystalline samples of three cross-conjugated carotenoids, renier- apurpurin-20-a1 (I), 20-(2,3,4-trimethylbenzal)reniera- purpurin (11) and 8,8’-diapocarotene-8,20,8’-trial (111) have been reported [23]. The polarization angle between the transition moments of the main (‘B +-A)

c201.

808 Yearly review

and "cis" ( 'C + A ) bands of these carotenals is estimated to be in the range of 32-38' which is in good agreement with the predicted values based on the SCF-MO-CI PPP calculation. The authors deduced the configuration of I and 11 to be 1 3 4 s from the linear dichroic spectra and 'H NMR data.

Carotenoids are usually nonfluorescent. The ex- tremely short singlet excited state of carotenoids is responsible for failure to transfer the excitation energy to donors such as chlorophylls in solution. However, peridinin excitons in an unique topographical array in the photoreceptor proteins of dinoflagellates are able to transfer the excitation energy to Chl a with 10004 efficiency [77. 1311.

Indoles and tryptophan

Theoretical analyses of the photophysical properties of indole, indole radical and radical cation have been described on the basis of the CNDO/S and SCF-PPP-CI MO treatment by Evleth and co-workers [33]. The authors suggest that the observed T - + T transition (430450 nm) in indole may be attributed to transitions to very high-lying computed triplet states ( T6 +- Tl), and 500-600 nm transitions in the radical may correspond to transitions to the second excited doublet states.

MO calculation based on Murrell's localized orbital model has also been applied to indole 1813. The model predicts a pure Rydberg 3 s +- 'A transition at 217 nm, polarized perpendicular to the molecular plane with the 3dn orbital conferring a long range character to the 'L, excited state.

Valeur and Weber have described a method to resolve the excitation spectrum of indole into 'L, and 'Lb excitation bands from the combined measure- ments of the excitation and excitation polarization spectra in propylene glycol at - 185°C [139].

A comparison of the temperature effect on the fluorescence quenching of indole, N-methyl-, 5-methoxyindole and benzimidazole have been reported [73]. The authors conclude that temperature activated quenching could be attributed to a monomeric hydrogen bonded water. The fraction of monomeric species has been estimated by a Stern-Volmer treatment to be less than 17; of the molecules of the liquid at tem- peratures up to 80°C.

Hershberger and Lumry have determined the photophysical behavior and solvent effect on the fluorescence of 5-methoxyindole [5 13. The fluorescence quantum yield q5F in H,O is 0.29 with rF of 4.0ns. 4F in cyclohexane is 0.63 with TF of 4.0ns at 25T , and the activation energy for fluorescence quenching in water is 15.9 & 0.5 kJ/mol. In contrast to indole and methyl substituted indoles. 5-methoxyindole does not form an exciplex with polar solvents.

Solvents effects on the fluorescent state of indole and its derivatives including indole carboxylic acids (2-, 3- and 5- ), and 5-cyano and 5-bromo-indoles have been determined [134]. The dipole moments of the excited singlet states are estimated from the solvent-

dependent Stokes' shifts. All indoles show a substantial increase in dipole moment upon excitation to the emitting state. Large Stokes' shifts of indoles are interpreted in terms of solvent-lattice relaxation during the lifetime of their excited state.

The fluorescence spectrum of serotonin (S-hydroxy- tryptamine) in acidic medium has been measured and compared with the value obtained by MO calculation [72]. The red region emission (,Imax - 562nm) is assigned to the protonated form of serotonin at C4 in the excited state, and the band at 335.6nm is the emission from the neutral molecule.

Excited state interaction of 7-azaindole with ethanol gives rise to the tautomer through double proton transfer between one molecule of 7-azaindole and one molecule of alcohol [S].

Six indolizine derivatives in n-hexane and methanol exhibit a slower nonradiative decay than do their indole counterparts or a number of aromatic hydro- carbons [86]. The SCF-PPP-CI MO calculation indi- cates that the T2 state in the indolizine could lie above S1 so that intersystem crossing from Sl to TI could occur without the benefit of intermediate triplet states.

Pulse radiolysis of trp has been reported to yield a radical cation trp+ with a 570nm (E = 2 5 0 0 M - ' cm-I). free radical trp. with 520nm band (E = 1870 M - ' cm-') and a pK, (trp+) of 4.3 [115].

Laser (265nm) flash photolysis of the dipeptide tryptophyl tyr generates radical spectra with a 580 nm band of trp+ and strong 410 nm band, characteristic of tyr phenoxyl radical [IlO]. The authors suggest that singlet state(s) are involved in the photoejection process and energy transfer processes play a role in the photoionization. The closeness of the two rings in trp-tyr allows direct electron transfer from one ring to the other.

Baugher and Grossweiner have reported photolysis mechanism of aqueous trp as an extension of their previous work [44]. Flash photolysis of trp in H20 with 265 nm laser excitation yields transient trp radical cation trp' (A,,, 580nm, E = 2700 f 200 M-' cm-'. & 0.12 f 0.01) and hydrated electron eaJ& 0.10 f 0.01) from pH 4-8, and the neutral radical trp+ (,Imax 510nm, E = 1800 50 M - ' cm-') is formed by deprotonation with rate constant 1.5 x lo6 s- '. The disappearance of ea; follows a complex decay, inconsistent with homogeneous kinetics [7, 45, 661. The authors conclude that monophotonic photoionization is the principal initial step of aqueous trp photolysis at near neutral pH and has a possible biphotonic contribution in alkaline solution, and an electron-radical recombination kinetic model for ea; is proposed. Evans and co-workers have studied the transients of N-acetyl-L-tryptophanamide (NATA) by flash photolysis and report the transient at 570nm assigned for the radical cation is in rapid equilibrium with its deprotonated neutral radical (A,,, 510nm) with a pK of 4.3, indicating that substitution at the 3-position of the indole ring does not greatly alter

Yearly review 809

its pK [30] (cf. radical cation of trp [llS, 44, 1171) and Br- (>0.1 M) promote neutral radical yields but a corresponding enhancement of the ea; yield is not observed [32, 1411. The authors suggest that neutral and positive radicals originate from the same precursors and one of these precursors is an oxygen and acrylamide sensitive, long-lived state which does not involve e& formation [31, 32, 1411.

The possibility of complex formation in the photosensitized degradation of DNA induced by N'-formyl- kynurenine (NFK) has been suggested by demonstrat- ing photosensitizing properties of the NFK derivative (decarboxy NFK, or 3-anthraniolylethylamine) towards free DNA constituent and thymine dimer

Unusual solvent effects on the spectral properties of acetophenone-2'-formylamino derivatives having the same chromophore as NFK have been observed [ I l l , 1121. An abnormal red-shift of fluorescence observed at both low and room temperature is interpreted as a consequence of the excited state proton transfer from the NH of the formamido group to the o-carbonyl of the side chain.

The UV action spectra for photolytic destruction of trp in aerated aqueous solution are observed to be identical to the corresponding action spectra for chromophore production [12]. The products detected may not be secondary reaction products but direct photodegradation products.

A new fluorescent emission (A,,, - 380nm) for aqueous solutions of L-trp excited in the long wavelength region at room temperature has been reported and assumed to be a photoproduct, possibly a 3- hydroxylated derivative of indole [ 137-J.

Flavins

The photophysics of alloxazines and isoalloxazines have received considerable attention recently. Moore et al. [I023 in a continuing series of papers on the photophysics of riboflavin have recently re-evaluated 4F and t # ~ ~ at 298 and 77 K. Additionally, they determined the lifetime of the triplet state and 41sc by EPR. The riboflavin c $ ~ in water and alcohol is reported as 0.25 and 0.32, respectively, and they see no temperature dependence for fluorescence. The disc at 77K was determined by EPR comparing with Michler's ketone, and found to be 0.7, consistent with the dF, value while 41sc in water is - 0.5-0.6. By EPR, T~ for riboflavin is -0.14s. The quenching of phosphorescence of FMN by trp and indole-acetic acid is also discussed.

Transient spectroscopy of alloxazine, lumichrome, lumiflavin and riboflavin has been elucidated [43] in acidic, aqueous and ethanol solutions. A frequency- doubled ruby laser (347 nm, 25 ns pulse) was used as the excitation source. T-T absorption spectra from 260-750nm are reported. blSc for riboflavin and lumichrome (0.7) in neutral, aqueous solution agrees very well with the above-mentioned values. The 41sc for alloxazine is reported as ~ 0 . 4 5 . 4F, T~ and TF

c1441.

are also reported, and are substantially lower than the previously published values, which the authors attribute to H + quenching. The triplet decay kinetics show for alloxazine k , = 1.1 x lo5 s-' (at 560nm), lumichrome 8.3 x IO4s-' (640nm), lumiflavin 5 x lo4 s-' (670 nm) and riboflavin kT = 5.3 x lo4 s - ' (at 660nm). Triplet quenching by O2 has k - 1.7 f 0.2 x to9 M - ' s - I for alloxazine.

The effects of la-substitution on the flavin triplet state and semiquinone properties have been investigated [29] by flash photoiysis. In comparison to riboflavin, N(3) or N( 1 ) isomers of 82-histidylriboflavin have an increased triplet decay rate ( - 1 x 10'- -4.6 x lo5 s-l) and a relative triplet yield of only about 10%. Acetylation of the a-amino and ribityl OH groups increases both the decay rate and yield by a factor of two. The semiquinone yield of 8a-substituted riboflavin was about half that of riboflavin, and the pK for neutral semiquinone ionization was about 1-1.5 pH units lower as compared to unsubsti- tuted flavins. The protonation and deprotonation equilibria of lumichrome have been investigated [83] to examine the possibility of a ground state tautomer- ization between alloxazine and isoalloxazine. The pK, for protonation was found to be -2.4 for the ground state. The ground state deprotonation occurs in two steps, with pK's of 8.4 and 212 for the monoanion and dianion, respectively. The deprotonation is described as giving rise to the N, anion and an N, anion, which rearranges to give an NIo (isoalloxazine) anion. This is substantiated by comparisons with 1- and 3-methyl lumichrome absorption spectra and fluorescence lifetime data. They conclude that the electron distribution of lumichrome is highly dependent on the charge of N, and Nlo, with protonation or deprotonation, respectively, leading to the isoalloxazine-type charge distribution. Furthermore, they demonstrate that the above-mentioned equilibria are not displaced in the excited state. The Stern-Volmer quenching constant for quenching of lumichrome by H + is K Q = 60 & 5 M-', leading to a rate constant for H+ quenching of 2.5 f 0.5 x 10" M - l s - ' .

The phototautomerism of the alloxazine chromophore [129] catalyzed by acetic acid or pyridine to give isoalloxazine has been further confirmed [36] by measuring the fluorescence lifetimes of the emitting species in the above-mentioned solvent systems and deuterated acetic acid. If the anion were the emissive species, one would not expect a deuterated solvent to affect the lifetime.

The photodegradation of isoalloxazines in various hydroxylic solvents has been further examined [lo01 under anaerobic conditions. In various deuterated solvents, no significant solvent-isotope effect was found. The products were lumichrome and formyl- methylflavin, both irreversible photodegradation products. The authors summarize by stating that the hydrogen abstraction process occurs from available hydrogens on the NIo side chain, even in the presence of good hydrogen donors. The solvent does have an

810 Yearly review

effect on the quantum yield, however, possibly reflec- ting a change in the conformation of the side chain. This is reasonable in light of C D data [47] that indicate the C D of the lowest K - - * K * transition to be highly solvent dependent.

In a very interesting report [146], the lumiflavin sensitized photooxygenation of trp, his, met and guanine has been shown to be enhanced - 2 fold in the presence of adenine. The reaction yields the same products with or without adenine. N o mechanism for the enhancement was mentioned. It would be interesting to see if the same effect could be observed between FMN and FAD.

The photoreduction of cytochrome c by flavin has been studied [I211 as a possible model system for the blue-light photoreceptor system. Aerobically, O2 is formed from the reduced flavin, reducing cytochrome c. Anaerobically, FMNHl reduces cytochrome c directly. Azide inhibits all of these photoresponses, presumably due to inhibition of flavin reduction by azide. These in uitro studies argue strongly for a flavin as the photoreceptor pigment (but not necessarily the same pigment for all responses) in the various in uivo photoresponses.

McCormick has written a rather extensive review [96] dealing with spectral and photochemical aspects of the interactions of flavins with amino acids, both inter- and intra-molecular cases being considered.

Nucleic acid haws

The vacuum UV absorption spectra of thin films of DNA, RNA. nucleosides and nucleotides are reported [71] for the wavelength region from 28e160nm. The region below 160nm is shown by comparison to corresponding phosphoric acid and carbohydrate to consist mainly of these components.

The microwave spectrum from 8 to 40GHz of 2-methyl pyrimidine [2 13 has allowed the calculation of the dipole moment, p = 1.676D from the m = 0 line. The internal rotation barrier was also calculated to be 4.454cal.mol-' (from Iml = 3, IKI = 1 line).

The valence electron structure of methyl uracils has been examined by UV photoelectron spectroscopy [ 1141. Uracil seems to be most sensitive toward substitution at positions 1 and 5. This suggests that in nucleosides and nucleotides. where a sugar is at position 1, uracil and thymine are significantly better electron donors than the base. Adenine has also been studied by U V photoelectron techniques [ 11 31. Highly resolved bands arise from the six highest occu- pied M O s . The authors report that in 6-methyl- aminopurine and 9-methyladenine, the first, third and fifth bands arise from K orbitals, while alternate orbitals arise from nitrogen lone-pair orbitals, which agrees with theoretical calculations. Methyl derivatives also have a lower ionization potential than adenine. The importance of these results with respect to base stacking is also discussed.

The effects of solvent and substituent effects on the phosphorescence properties of purines have been

studied by phosphorescence microwave double resonance [W]. The ZFS parameters and phosphorescence decay rate constant for benzimidazole, purine, guanine, adenine, xanthine, hypoxanthine, cytosine, thymine, C 4 , A-T, U-A, poly A, poly G, poly C, poly U, p o l y ( C S ) and poly(U-A) in ETOH at 1.4 K are tabulated.

The phosphorescence of 4- and 5-amino pyrimidines (pyr) and their sterically hindered methyl derivatives has been examined [127], and as the steric hindrance increases, rP decreases and the polarization value increases. For example, for 4-amino pyrimidine T* = 0.298 s and the phosphorescence to fluorescence polarization ratio (P/F) = 5.0, while for 4-amino 5-methyl pyr. T~ = 0.237 s and P/F = 2.16,4-dimethyl- amino pyr T~ = 0.166 s and P/F = 0.12 and for 4-di- methylamino 5-methyl pyr T~ = 0.095 s and P/F = 0.056. The model applied allows calculation of the spin-orbit coupling matrix element, which is 0.2-0.8 cm-'. These data show that spin-orbit coupling of higher '(n,n*) states with the emitting triplet is approximately ten times stronger than with the lowest '(La:) state.

A phosphorescence microscope has been used to examine the phosphorescence of nucleosides at room temperature and - 175°C [ 1471. Polycrystalline powders of A. G, T, U, C, dA, d G and dC were examined. The phosphorescence lifetime is 0.15 s for adenine and cytosine. T~ is approximately one order of magnitude less for nucleic acid bases. For guanine, the emission maximum was 470nm; for dG, the emission maximum is red-shifted to 498 nm, while for adenine A,,, = 490 nm and for dA ,I,,, = 475 nm. The phosphorescence lifetimes at low temperature are approximately 0.2 s for G and -0.3 s for C, -0.35 s for dC, -0.4 s for A and dA and -0.45 s for dG.

The isothermal- and thermoluminescence of poly- cyrstalline powders of the nucleic acid bases on y-irradiation at 77 K has been shown [I361 to result from the same radiative transition. Thermoluminescence corresponds to the observed phosphorescence. Pyri- midines show an additional component, corresponding to their fluorescence. A model based on the loca- tion of electron traps in relation to the excited states is proposed by the authors to explain the observed thermoluminescence.

Absorption and emission properties of dipyrimidine adducts at both 300 and 7 7 K have been classified into two main types by Hauswirth and Wang [49]. C5-6 saturated double bonds in the pyrimidine-2,4 dione (pyr) and a pyrimidine-2-one (Pyo) ring behaves as a substituted Pyo, while Pyr-Pyo (Pyr unsatur- ated) behaves as a bichromophoric molecule. The spectral properties are dependent on the torsional angle between the rings, analogous with the more classical ortho-substituted biphenyl system.

The photohydration of 1,3-dimethyluracil (DMU) has often been used as a secondary actinometer in aqueous nucleic acid photochemistry (see references in [ 191). Now Burr et al. [ 191 report that the quan-

Yearly review 81 1

tum yield for the reaction varies by a factor of approximately three with concentration. In the concentration range 0.1-1 mM, q5RX = 3.8 x while when [DMU] > 1 mM, q5RX = 11 x

Electron donating indoles split cis-syn uracil dimers [24] with the following reactivity: indole > 3-methyl- indone > indoleacetic acid > 5-hydroxytryptophan > tryptophan. Electron withdrawing indoles, such as indole-3-carboxylic acid and oxindole are inactive in splitting the uracil dimers.

The quantum yield for photolytic tritium exchange at C5 of [S-T] cytidine has been calculated [SO] to be approximately equal to that for photohydration (0.005) after correcting for inner filter effects and spontaneous 3H exchange, both due to the cytidine photohydrate produced during irradiation.

Carvone, camphor, 3-methylcyclohexanone (ET = 326 kJ.mol-') benzoin (E7 = 285 kJ.mol-') and 3- methylindanone (ET - 318 kJ.mol- ')*, all ketone- triplet sensitizers, were examined [123] for their abi- lity to sensitize splitting of dimethylthymine dimers. Splitting was not achieved when 1 > 300nm was used. The authors conclude that ketone-triplet sensitized splitting of cis-syn, cis-anti and trans-anti dimers of dimethylthymine does not occur in solution. In the cases where photosensitized splitting is observed, the mechanism seems to involve electron transfer from the sensitizer singlet.

The UV photolysis of 02, 2'-anhydrouridine in aqueous ethanol gives rise to three spots on TLC [106]. The origin seems to be a mixture of as yet unidentified spots. anhydro-5,6-dihydrouridine and another derivative. Dimethyluracil in methanol yields cyclobutane-type dimers, 1,3-dimethyl-6 hydroxy-5- hydrouracil, and the 6-methoxy derivative. possibly via a free radical mechanism 11241; however, the excited state(s) involved or mechanism IS not known.

A rather extensive review by Lober and Kittler recently appeared [89] on the general area of photochemistry of nucleic acids. Specific topics discussed included photochemistry of aza analogues and minor bases, photoaddition of pyrimidines, photoalkylation of purines, furocoumarin addition, excited states of ligand-nucleic acid complexes and their role in the fluorescence staining patterns in chromosomes, energy transfer processes, photoreactivation and photodynamic action.

Furocoumarins and other carcinogens

Moore et al. [ 1011 have examined the triplet states of 8-methoxypsoralen (8MOP), psoralen, 8-hydroxy- psoralen (8HP) (and its anion) and 4,4'-dimethyliso- psoralen by ODMR and determined zero field splitting parameters, which they reported as the axis-inde- pendent parameter, D* [(D*)2 = (D2 + 3E2)]. For SMOP, D* = 0.142cm-' and for psoralen and 8HP

* It should be noted that the triplet energy levels cited here are only estimates derived from comparison with analogous compounds, with the exception of benzoin.

D* = 0.141 cm-', suggesting a highly localized triplet. The inactivity of 8HP as a skin sensitizer is explained as being due to 8HP anion, with D* = 0.120cm-', similar to coumarin, with D* - 0.124 cm- '. Thus, the skin-sensitizing activity of psoralens is nicely correlated with the localization of the triplet state in these molecules.

Dimethoxycoumarin (DMC) has been reported to intercalate with DNA [107]. Unlike psoralens, DMC has a c $ ~ of 0.65, and very weak phosphorescence, due to the large gap between S1 (n,n*) and Sz(n,n*). This results in an inefficient klsc. This is further supported by the fine structure in the absorption spectrum, and a more negative phosphorescence polarization than is observed for any other coumaryl derivative.

An exhaustive compilation (49 compounds) of the photophysical properties (singlet and triplet energy levels, AEsT and T ~ ) of polycyclic aromatic hydro- carbons and correlation of their carcinogenic activity are available [103]. When Es < 312 kJ.rnol-', the compound is five times more likely to be carcinogenic and when the singlet energy level is in the interval 297 I Es I 310, the compound is 22.8 times more likely carcinogenic. N o correlation was found between carcinogenic activity and ET or T ~ .

Miscellaneous

Excited state interactions of laser generated. '0, and a-tocopherol (vitamin E) in freon-113 have been reported [14]. The k , is calculated as 3.1 f 1.2 x lo7 M . s - ' by the competitive method with bilirubin [92]. The rate constant for chemical reaction is 1.9 f 0.5 x 106M-'.s-'.

Phosphorescence has been reported for various hydroxy-substituted flavones [40]. Phosphorescence is reported to arise from a 3(n,n*) state of these nonfluorescent molecules. For flavone, A:,, - 469 nm and T~ - 0.6 s. As -OH groups are added, I.'' and rP increase. For example, luteolin (5,7,3',4'-tetrahy- droxy flavone) has a A:,, = 495 nm and T~ = 2.02 s.

The fluorescence of vitamin B6 in vivo has been seen in Ehrlich ascites cells, L-fibroblasts and Staphly- lococcus albus cells [ 1081 with a A,, 395-400 nm. The spectral properties are reported to be identical to pyridoxine. HCl in vitro.

A very weak transition (E - 1 M - * . c m - ' ) at 435- 445 nm has been detected for 1,4-dihydronicotinamide [27]. This transition is not enhanced by ethyl iodide, and shows a weak Cotton effect, f3 = 36' cm2.dmol-', which the authors attribute to a T + S o transition.

Department of Chemistry, Seoul National University, Seoul, Korea

TAE YOUNG LEE

Department of Chemistry, Texas Tech University, Lubbock, TX 79409, U.S.A.

ROBERT D. FUGATE

812 Yearly review

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excited states of biomolecules—v

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