0031 -8655184 $03.00+0.011 Copyright 0 1984 Pergamon Press Ltd

Piiorochermsfry and Photobiology Vol. 40, No. 4. pp. 549-550, 1984 Printed in Great Britain. All rights reserved

RESEARCH NOTE

A WAVELENGTH EFFECT ON UROCANIC ACID E/Z PHOTOISOMERIZATION"

HARRY MORRISONt, CHRISTIAN BERNASCONI and GANESH PANDEY Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA

(Received 12 March 1984; accepted 15 May 1984)

Abstract-E-Urocanic acid exhibits a single, featureless, long-wavelength absorption band with A,,, - 268 nm in water. However, the quantum efficiency for E+Z photoisomerization is wavelength dependent in this region, with the maximum value at the low energy edge of the band (e.g. 313 nm) and appreciably lower efficiencies measured at 6 300 nm.

INTRODUCTION

The photochemistry of urocanic acid (UA)$, a metabolite of histidine found in the skin, has been a subject of increasing interest. Recent papers have dealt with E -+ Z photoisomerization (Morrison et a l . , 1980), photocycloaddition of UA t o N,N-dimethylthymine (Morrison et al. , 1983), and UA photosensitized inactivation of +X 174 phage (Tessman ef al., 1983). The E -+ Z isomerization reaction has become particularly significant because of its potential involvement in ultraviolet light immunosuppression (De Fabo and Noonan, 1983). In this regard, we have reported (Morrison et al., 1980) that photoisomerization of the naturally occurring E isomer (Eq. 1 ) is more efficient at 313 nm than at 254 nm, wavelengths which bracket a single unstructured long wavelength band for E-UA (A,,, - 268 nm). In this Note, we extend this observation by reporting quantum efficiencies at a number of intermediate wavelengths.

r l -c02H hu H N d N ' HN-N

E-UA Z-UA

(1) MATERIALS AND METHODS

Materials and instruments. E-UA (Aldrich) was recrystallized from water; uranyl sulfate (Alfa) and oxalic acid (Baker) were used as received. Analysis for 2-UA was by HPLC using equipment and methodology previously described (Morrison et al., 1980a). Photolyses employed a 3.5 KW high pressure Hg/Xe lamp coupled to a Schoeffel 0.25 m monochromator. The exit beam was passed through

*Organic Photochemistry 60. Part 59, Morrison and DeCardenas, (1984) Tetrahedron Lett. 25, 2527-2530; Part 58, Morrison et al. J . Chem. Ed., In press.

tTo whom correspondence should be addressed. $Abbrev ia t ions : H P L C , high pressure l iquid

chromatography; UA, urocanic acid.

a 1.89 mm slit to provide light having a dispersion of k 3 nm. Photolysis and actinometer solutions ( 5 m e ) were contained in matched, 1 cm, square quartz cells.

Quantum efficiency measurements. Aqueous, 5 me solutions of E-UA (6 x M) were degassed by bubbling with Ar for 10 min and irradiated at ambient temperature with the exit beam from the monochromator. Light intensities ranged from 1.7 x 10" photonsis to 2.0 x 10I6 photonsis, depending on the wavelength being utilized. Irradiation times were between 5 and 46 min and were varied so as to achieve E + 2 conversions ranging from < 1 to -18%. For conversions exceeding 4%, correction was made for the photolytic back reaction of Z + E, by using Eq. 2 (Lamola, 1965)

a p' = a(2.303) log [-]

a- P

In Eq. 2, p' is the corrected percent conversion to Z, p is the measured conversion and a is the percentage of Z at the photostationary state. We used a = 66% 2, the value previously observed using 313 nm excitation (Morrison et al., 1980). Though the pss is wavelength dependent (a = 42% Z with 254 nm excitation; Morrison et ul., 1980). conversions were generally sufficiently low so that the corrections were small and essentially insensitive to the choice of a. In one instance, where A = 302 nm, an increase in +E--rz of 10% would result from the use of the high frequency value for a, but the 313 nm value is undoubtedly more realistic. Uranyl oxalate actinometry (Murov, 1973) was employed throughout, using actinometer quantum efficiencies adjusted for the known wavelength dependence of uranyl oxalate decomposition.

RESULTS AND DISCUSSION

Quantum efficiencies for E-UA -+ Z-UA photoisomerization are given in Table 1. The previously reported large reduction in + E + ~ upon changing from 313 to 254 nm excitation is confirmed, and one now observes that most of this change occurs near -300 nm.

W a v e l e n g t h e f f ec t s in so lu t ion phase photochemistry are not the norm, but by no means exceptional (Turro et al . , 1978). Most commonly, such effects are seen when there are two or more transitions distinguishable in the absorption spectrum and decay within the excited state manifold is slow compared to the chemical reaction rate. Less

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5.50 HARRY MORRISON et al.

Table I . Quantum efficiencies for E + Z isomerization of UA as a function of wavelength of excitation (Aexc)

313 0.49 k 0.00,; 0.52 & 0.01$ 302 0.31 k 0.080 289 0.079 k 0.017 276 0.072 k 0.020 264 0.044 f 0.013 254 0.043s L 0.010; 0.058 k O.OOl$

tValues are the average and average deviation of dupli- cate runs. The average deviations for the monochromator derived data exceed those from our previous (turntable) results because of low light intensities and the difficulty in achieving reproducible placement of the sample and actino- meter vessels in the exit beam.

$Values reported previously using a filtered Hanovia medium pressure Hg lamp or a Hanovia low pressure Hg lamp and rotating turntable (Morrison et al . , 1980).

frequent is a situation such as exists here, where the wavelength effect occurs within a single absorption envelope. In this event, one sometimes finds evidence for multiple electronic transitions buried in the single absorption band, with internal conversion between these states again unusually slow or non-competitive with alternative decay or reaction pathways. It is also common for two conformers or tautomers to co-exist in solution, with their absorption bands merged but not completely overlapping, so that one or the other is preferentially excited on the edge of the envelope.

A useful tool for uncovering overlapping transitions has been magnetic circular dichroism (MCD); an MCD study of urocanic acid has given no evidence for a second transition within the UA long wavelength absorption band (J. Michl, private conimunication), Though this appears to rule out close-lying r r , rr* singlet states in UA, a pair of nearly degenerate n , n* and r r , rr* singlet states would probably have not been detected because of the weak oscillator strength characteristic of n+rr* transitions ( J . Michl, private communication). In fact,

overlapping n , rr* and r r , IT* transitions have been suggested to account for an observed wavelength effect on the photochemistry of sodium cinnamate ( U l l r n a n e t a l . , 1 9 6 9 ) , a s t r u c t u r e ( A r - C H = C H - C 0 2 - ) ra ther similar t o the zwitterionic form of UA (ImHf-CH=CH-C02-). Sodium cinnamate has an absorption maximum at ca. 265 nm and different photochemical reactions are observed depending on whether one excites on the long or short wavelength edge of this band. Since we see no new photoproduct when using 254 nm excitation on UA, we have attempted to uncover a rapid, reversible photochemical reaction which might make E/Z isomerization uncompetitive when using shorter wavelength light. However, neither photolysis in DzO (to observe reversible hydration of the double bond) nor photolysis at low temperature (to observe transients by absorption spectroscopy) have proven fruitful. Possibilities now under study are reversible proton transfer to the solvent and photoionization.

Acknowledgements-We are grateful to Professor B. Freiser for the use of his monochromator and lamp, Professor J. Michl for the MCD study, and the Public Health Service, DHHS, Grant Number CA 18267 awarded by the National Cancer Institute, for support of this research.

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Pandey (1983) Photochem. Photobiol. 38, 23-27. Murov, S . L. (1973) Handbook of Photochemistry,

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Turro, N . J. , V. Ramamurthy, W . Cherry and W. Farneth (1978) Cbem. Rev. 78, 125-14.5.

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