Indian Journal of ChemistryVol. 31A. January 1992, pp. 28-33

Mixed ligan? compl~xe~ ~fplatinum (II) with histidine as primary ligand andpunnes, pyrimidines and nuc1eosides as secondary ligands

Badar Taqui Khan", S Vijaya Kumari & K Murali MohanDepartment of Chemistry, University College of Science, Osmania University, Hyderabad 500 007. India

and

G Narsa GoudDepartment of Chemistry, Nizam College, Osmania University, Hyderabad 500 001, India

Received II June 1990; revised and accepted 15 July 1991

Mixed ligand complexes of Pt(II) with histidine as the primary ligand and nucleic acidconstituents-adenine, guanine, cytosine, uracil, adenosine, guanosine, inosine, cytidine and uridine assec.ondaryy~a~ds have been synthesized and characterized. The complex-cis-[Pt(HisH)-CI21.H20 (1)(Hisl-l = histidine) has been prepared and reacted in basic solution with purines, pyrimidinesor nucleosidesIna I: I m~lar ratio to form mixedligandcomplexes.In all thesecomplexeshistidineacts as a bidentate ligandcoordinating to Pt(I1)through the imidazole ring nitrogen (N3) and the amino group. The secondary ligandadenine, adenosine or inosine coordinates to the Pt(II) ion through ring nitrogen (N7). In pyrimidines andcorresponding nucleoside complexes coordination to platinum (II) is through ring nitrogen (N3)' In theguanine and guanosine mixed ligand complexes of Pt(H), these ligandsbind to the metal ion in a bidentatemanner through N7 and C60-. forming a five-memberedchelate ring. In all these complexes carboxylicgroup of bidentate histidine is free.

The interaction of cis-Pt(NH3hCh (cis-platin) withDNA and its property of inhibition of DNAmultiplication I created much interest in the study ofthe interaction of cis-platin and its analogues withnucleic acids and their constituents- -15. Earlier, wehad reported the interaction of Pt(lI) amino acidcomplexes of glycine/alanine/methionine/ethioninewith purines, pyrimidines and nucleosides'". In thispaper, we report the preparation andcharacterization of cis-[Pt(HisH)Cl2l.H20 (whereHisH = histidine) and its interaction with adenine,guanine, cytosine, uracil and the nucleosides-adenosine. guanosine, inosine, cytidine and uridine.

Materials and MethodsChromatographically pure ligands, DL-histidine,

adenine, guanine, cytosine, uracil, adenosine,guanosine, inosine, cytidine and uridine werepurchased from Sigma Chemical Company (USA).Potassium hexachloroplatinate (AR, 98% pure) wasobtained from Alfa Ventron (USA) and wasconverted to potassium tetrachloroplatinate by thereported procedure t7. Micro-analyses of thecomplexes were performed at the Australian MineralDevelopment Laboratories, Australia. Thepercentage of chlorine in the complexes wasestimated by a published procedure 18. Infrared (in

KBr) and electronic spectra of the complexes wererecorded on Perkin-Elmer (337) and Beckman(Model DB) spectrophotometers, respectively. TheIH NMR spectra of the complexes were recorded onVarian 100 and 270 MHz spectrometers. Theconductivities of the complexes in water weredetermined using a Digisun digital conductivitymeter No. Dl 909.

cis-(Dichloro) (histidine )platinum (If)monohydrate (I)

Potassium tetrachloroplatinate (II) (0.423 g,1 mmol) was dissolved in 15 ml water and to this asolution of histidine (0.195 g, 1.25 mmol) in 10 ml ofwater was added. The resulting solution was heatedon a water bath for half an hour when the crimson redcolour of the solution changed to yellow and the pH ofthe solution decreased from 6 to 4. The pH of thereaction was maintained around 5 by the dropwiseaddition of 0.05 N KOH. The grey precipitate thatseparated was washed with hot water, ethanol,acetone and dried.

(Chloro) (histidinato) (adenine)platinum (If)dihydrate (2), (Chloro) (histidinato) (cytosine)-platinum (If) monohydrate (3), (Chloro)(histidinato) (uracil)platinum (If) monohydrate (4)

TAQUI KHAN 1'1 al.: MIXED LIGAND COMPLEXES OF PLATINUM(II) 29

The following general method was followed for thepreparation of the above complexes. cis-(Dichloro)(histidine)platinum (II) monohydrate (0.21 g,0.5 mmol) was taken in 10ml water and dissolved byadding an equivalent amount of 0.05 N KOH. To thisa solution of adenine (0.067 g, 0.5 mmol dissolved in10 ml of hot water) or cytosine (0.056 g, 0.5 mmoldissolved in 5 ml of hot water) or uracil (0.057 g,0.5 mmol in 5 ml of water) was added and theresulting solution heated when the colour of thesolution changed from dark yellow to brown or darkbrown. The reaction mixture was cooled in arefrigerator and filtered. The brown or dark browncomplexes were precipitated out by the addition of4: 1acetone-water mixture. The complex was filteredwashed with 90% cold ethanol until free fromchloride and dried.

(Chloro) (histidinato) (adenosine)platinum (II)dihydrate (5), (Chloro) (histidinato)(inosine)platinum (II) monohydrate (6), (Chloro)(histidinato) (cytidine)platinum (II) monohydrate(7), (Chloro) (histidinato) (uridine)platinum (II)trihydrate (8)

The following general method was followed for thepreparation of these complexes. cis-(Dichloro)(histidine)platinum (II) monohydrate (0.211 g,0.5 mmol) in 1.5 ml water was dissolved by addingequivalent 0.05 N KOH. To this, a solution ofadenosine (0.134 g, 0.5 mmol) in 5 ml water orinosine (0.135 g, 0.5 mmol) in 10 ml or cytidine(0.122 g, 0.5 mmol) in 5 ml water or uridine (0.122 g,0.5 mmol in 5 ml of water was added. The resultingreaction mixture was heated on a water bath for 6-10hr. During the course of the reaction the colour of thesolution changed from dark yellow to brown. Thesolution was cooled in the refrigerator and thecomplex precipitated from the solution by adding 4: 1acetone-water mixture and filtered. The complex waswashed with 90% ethanol until free from chloride anddried.

(Guaninato) (histidinato)platinum (II)monohydrate (9)

cis-(Dichloro) (histidine )platinum (II)monohydrate (0.211 g, 0.5 mmol) in 15 ml water wasdissolved by adding 0.05 N KOH. To this a 10 mlsolution of guanine hydrochloride (0.84 g,0.05 mmol) dissolved in 1.5 mmol KOH (0.05 N)solution was added. The reaction mixture was heatedon a water bath for 4 hr. The colour of the solutionchanged from dark yellow to brown while heating

and a dark brown complex precipitated out. Thecomplex was filtered, washed with 90% ethanol anddried.

(Guanosinato) thistidinato )platinum (I I) dihydrate(10)

cis-(Dichloro) (histidine )platinum (II)monohydrate (0.211 g, 0.5 mmol) was dissolved in15 ml water by adding equivalent amount of 0.05NKOH. A solution of guanosine (0.142 g, 0.5 mmol) in10 ml of water dissolved by adding equivalentamount of KOH, was added to the above solution.The resulting reaction mixture was heated on a waterbath for 4 hr. On heating, the colour of the solutionchanged from dark yellow to brown and a dark browncoloured complex precipitated out. The complex wascooled, filtered and washed with ethanol, acetone anddried. The complex was found to be sparingly solublein water, but on addition ofOAN HCl the complexreprecipitated from the solution.

Results and DiscussionThe analytical and conductivity data of the

complexes are in agreement with the stoichiometriesproposed for the complexes (Table I). The molarconductivities (15-50 ohm - 'ern-mol" J) show thatall the complexes are non-electrolytes. Slightlyhigher than expected values of conductivities areattributed to the presence ofionised carboxyl groupof coordinated histidine in these complexes.

The IR spectrum of DL-histidine shows a strongand broad peak at 3425em -I due to the \oH mode ofthe H - bonded carboxylic group and a strong bandin the region 3100-3000 cm -I due to the NH2/NH3+asymmetric stretching frequency 19. Two peaksobserved in the range 1645-1580em - 1 are assigned tothe Vas COO and VS COO modes of COO- group. Twopeaks of medium intensity observed at 1510and 1420em - 1 are due to the vC = C and vC = N modes of theimidazole ring.

In the IR spectrum of complex (1) the \0 - H modeoccurs as a broad band at 350U-3400em - I. The NHzstretching frequency is lowered by 90 cm - 1 whichindicates that NH2 is involved in coordination toPt(ll) metal ion. A strong band observed at 1700em -I is assigned to vC = 0 mode of free carboxylicgroup which shows that it is not involved incoordination>". The vC=C and vC= N frequenciesof histidine are observed at 1450 and 1400 em - 1

respectively and are lowered by 40-20 cm - 1 ascompared to their positions in free histidine,indicating the involvement of nitrogen atoms of theimidazole ring in coordination. A band observed at

30 INDIAN J CHEM, SEe. A, JANUARY 1992

Table I-Analytical data of the complexesComplex Complex Found (Calc.), %

No. (colour)C H CI

cis-[Pt(HisH)C12). H2O 16.60 2.57 16.40(Grey) (16.38) (2.52) (16.14)

2 [Pt(His)(Ade)CI).2H2O 23.52 2.91 6.51(Dark brown) (23.74) (3.08) (6.37)

3 [Pt(His)(Cyt)CI).H2O 23.15 2.84 6.95(Dark brown) (23.35) (2.92) (6.90)

4 [Pt(His)(Ura)CI).HzO 22.47 2.89 6.72(Dark brown) (22.51) (3.00) (6.65)

5 [Pt(His)(Ado)CIJ.2H2O 29.90 3.63 5.15(Dark brown) (29.61) (3.51) (5.00)

6 [Pt(His)(Ino)CI).H2O 28.38 3.35 5.15(Dark brown) (28.61) (3.28) (5.28)

7 [Pt(His)(Cyd)CI).H2O 27.85 3.56 5.48(Dark brown) (28.00) (3.40) (5.38)

8 [Pt(His)(Urd)CI).3HzO 26.22 3.93 5.35(Dark brown) (26.35) (3.81 ) (5.19)

9 [Pt(His)(Gua - ).H2O 25.03 2.54(Dark brown) (25.50) (2.32)

10 [Pt(His)(Guo - »).2HzO 28.58 3.65(Dark brown) (28.71) (3.58)

Abbreviations: His = histidinato anion, HisH = Histidine,Ade = adenine, Gua = guaninato anion, Cyt = cytosine,Ura = uracil, Ado = adenosine, Guo - = guanosinato anionIno = inosine, Cytd = cytidine, Urd = urdine.

535 em -I in complex (1) is assigned to vPt-N. Twomore peaks appearing at 340 and 310 em - 1 areassigned to two equitorial (cis)M - CI stretchingfrequencies. As the COO- stretching frequency doesnot shift compared to its position in the ligand andpeaks due to vM - 0 are absent in the region 400-500cm -I, the possibility of coordination of carboxylicacid group to the metal ion is ruled out.

Histidine exists as a zwitter ion and coordinates tothe metal ion through imidazole and the NH2nitrogens. The metal ion competes with the proton onNH2 and displaces it. As a result during thepreparation of the complex, the pH decreasesconsiderably, and is kept around 5-5.5 by adding 0.05N KOH dropwise throughout the reaction.

IR spectra of the mixed ligand complexes (2)-(10)show broad bands in the region 3500-3000 em - 1 dueto the NH2 and OH stretching vibrations withextensive hydrogen bonding. In all these complexesvC = C and vC = N frequencies are loweredindicating that both imidazole ring of histidine andpurine/pyrimidine ring nitrogens are involved incoordination. A strong broad peak in the region

550-600 cm - I is assigned to vM - N vibrations whichindicates the coordination of histidine and purines,pyrimidines or nucleosides. Presence of a mediumpeak in the region 300-360 em - 1 indicates that exceptone, all the chlorides in platinum(II) complex aresubstituted with histidine and purine/pyrimidine/nucleoside. UV spectral data of complexes 2, 4, 5, 6, 7and 10 show peaks in the region 220-375 nm, whichare assigned to the 1t~1t* transitions of histidine andpurine/pyrimidine/nucleoside.

The IH NMR spectrum of histidine atpH 7.4 shows(Table 2) a signal at {)7.92 ppm assigned to C2Hproton of the imidazole ring. The signal at 8 7.73 ppmis attributed to the CsH proton. The proton once-carbon and methylene protons of histidine give atriplet and doublet at 84.15 and 8 3.3 ppmrespectively.

In the IH NMR spectrum of the complex (1), thesignal due to the imidazole C2H proton is shiftedupfield and appears at 8 7.6 ppm and that of CsHproton appears at {)6.95 ppm. Since the signal due tothe C2H proton shifts upfield as compared to that inthe ligand, it is proposed that imidazole nitrogen (N3)is involved in coordination to the metal ion?". Theshift in the signal due to the proton on o-carbon from8 4.5 to {)3.52 ppm shows that NH2 group is alsoinvolved in coordination.

In the 1H NMR spectrum of complex (2), twobroad signals observed at 8 8.64 and 8 8.08 ppm areassigned to CsH and C2H protons of adenine. Theseare shifted downfield (0.32 ppm) and upfield (0.12ppm) respectively as compared to position of C2proton; N7 is suggested as the site of coordination incomplex (2). All signals due to histidine protons arepresented in Table 2. Signals due to C2H and CsHprotons of the imidazole moiety of histidine areobserved at 07.06 and 86.05 ppm; these are shifteddownfield indicating the involvement of imidazolering N3 in coordination. The slight shift in the signalsdue to the CH (8 4.24 ppm) and CH2 (8 3.3 ppm)protons of the alanyl side chain indicates that NH2 isalso involved in coordination.

In the IH NMR spectrum of complex (3) (Table 2),CsH and C6H protons of cytosine moiety show signalsat 05.52 and 8 7.55 ppm. Since the signal due to theCsH proton is shifted more (0.36 ppm) as compared toC6H proton (0.14 ppm), N3 of cytosine) is proposed asthe binding site to the metal ion. The shift of thesignals of the imidazole ring protons (C2H 8 7.80 ppmand CsH 86.02 ppm) and alanyl side chain protons(CH s 4.45 ppm and CH2 8 3.40 ppm) of thecoordinated histidine suggests that it acts as abidentate ligand coordinating through N3 ofimidazole ring and amino groups.

TAQUI KHAN et al.: MIXED LIGAND CGMPLEXES OF PLATINUM(II) 31

Table 2-1 H NMR spectral data (0, ppm) of Pt(I1)-histidine-purine/pyrimidine/nucleoside mixed ligand complexes

Complex Complex/Ligand Purine base/nucleoside Pyrimidine base/nucleoside HistidineNo.

CsH C2H C)-H' CsH C6H C)-H' Imidazole Alanyl

C2H CsH Cl-CH -CH2

Histidine 7.92 7.33 4.15 3.3His- 7.80 7.03 3.65 3.0[Pt(HisH)C12]. H20* 7.60 6.95 3.52 3.28Adenine 8.32 8.20

2 [Pt(HisXAde)CI].2H2O 8.64 8.08 7.06 6.05 4.24 3.30Cytosine 5.88 7.69

3 [Pt(His)(Cyt)CI].H2O 5.52 7.80 7.59 6.02 4.49 3.40Inosine 8.22 8.11 5.93

6 [Pt(His)(lno)C1].H20 8.68 8.20 6.05 7.09 6.11 4.25 3.68Cytidine 6.04 4.64 5.91

7 [Pt(HisXCytd)CI].H2O 5.56 4.82 5.80 7.32 6.82 4.12 3.62Guanosine 7.80 5.0

10 [Pt(HisXGuo - )].2H2O 8.86 5.90 7.22 6.60 3.60 3.39

Solvent: D20: *NaOD.

•,

In complex (2), adenine binds to metal ion throughN7, whereas in complex (3) and (4) cytosine anduracil, respectively, coordinate to metal ion throughN3·

In its IH NMR spectrum, complex (6) (Table 2)exhibits signals at 8 8.68 and 8 8.20 ppm, assigned tothe CsH and C2H protons of inosine, respectively.Since CgH protons are shifted to greater extent (from88.22 to 8 8.68 ppm), N7 of the ligand is proposed asthe coordination site.

In the IH NMR spectrum of complex (7) (Table 2)signals observed at 85.56 ppm and 84.82 ppm areassigned to the CsH and C6H protons, respectively,since the signal due to the CsH proton is shifted more(from 8 6.04 to 85.56 ppm) compared to C6H proton(from 8 4.64 to 84.82 ppm), it is proposed that N3 ofcytidine is coordinated to the metal ion. Signal due tothe C, - H of the ribose ring of cytidine might havemerged with the CsH proton signal.

In complex (9) also, histidine acts as a bidentateligand, coor-dinating through imidazole ringnitrogen (N3) and amino group. In this complexguaninato anion acts as a bidentate. ligandcoordinating to the metal ion through N7 and C60-.It is proposed that carboxylate group of histidine isfree in this complex and involves in hydrogenbonding with the proton on the N, of guanine,adenine, guanosine, inosine (IR data of thecomplexes also support this). This is supported by thebroadness of the peak in the region 1620-1575em-I.

"

\.

~r\

IH NMR spectrum of complex (10) shows thesignal due to the CsH proton at 8 8.86 ppm and of C, Hof the ribose at 8 5.68 ppm. As the signal due to theC8 H proton is shifted down field compared to that offree guanosine (87.80 ppm), N7 of guanosine isproposed as the site of coordination. In the) H NMRspectra of these complexes signals due to the CH2 andCH protons of alanyl side chain of histidine areshifted downfield as compared to signals of histidinein complex (1). So it is inferred that electron density onthe carboxylate group deshields these protons. Basedon the analytical and spectral data it is concluded thatin complex (10), guanosine is coordinated to Pt(II)ion through N7 and C60 -,forming a five-memberedchelate ring.

Based on the analytical, conductivity and spectraldata structure (I) is proposed for complexes (2), (5)and (6) whereas structure (II) is proposed forcomplexes (3), (4), (7) and (8). Structure (III) isproposed for complexes (9) and (10).

The IH NMR spectra of the mixed ligandcomplexes (2), (3), (6) and (7) show that Ci. - CH and-'-CH2 protons of alanyl side chain of histidine areeither shifted downfield or show no significant shiftcompared to the complex (1). From this we canconclude that upon coordination to the Pt(II) ion, thesecondary ligand (purine/pyrimidine/nucleoside)labilizes the bond trans to itself, reducing thedonation of electron density from the amino group ofhistidine to the metal ion.

32 INDIAN J CHEM. SEe. A, JANUARY 1992

(JJ)

(Q)[pt (His)(C yl) CI]. ~O;R,= OH,R2=NH2

(b)/!,I(HiS)(UralC~ H20; R, = R2=OH

(C>[Pt(HisHCyd =l Hp;R=Ribos~.R,= OH. R2=NH2

(d>[PI(HiS)(UrdlCI].3H2

0; R =Ribas~.~= ~= OH

In the 1H NMR spectrum of complex (to), in whichsecondary ligand coordinates to the metal ion in abidentate manner, alanyl CJ. - CH and - CH2 protonsof the histidine are not shifted significantly comparedto positions in complex (1). The imidazole C2H andC5H protons of the complex (10) are shifted downfieldto a lesser extent compared to their positions incomplexes (2) and (6) where the secondary ligandbinds to the metal ion in a monodentate manner. Itcan be concluded that bidentate coordination ofsecondary ligand (guanosine) to the metal ionlabilizes bonds trans to its coordination site, reducingthe extent of electron donation by both imidazole andamino nitrogens of the histidine.

Execpt for complexes (9) and (10), only onechloride is displaced by secondary ligand in all thecomplexes. In the case of complexes (9) and (10), thesecondary Iigands, guaninato and guanosinatoanions, respectively, displace two cis chlorides of theparent complex and coordinate through N7 and

/

/~- ••. -....::a

III

(a>[pt (His l (Gua- lJ.H20; R = H

(blft(HiS)(GUO - lJ 2H2

0;R = Ribose

C60 -. In all these complexes the histidine iscoordinated to the metal ion in a bidentate mannerand its carboxylate group remains free as the squareplanar geometry of Pt(lI) does not allow itscoordination due to the steric strain involved.Compared to K[Pt(Gly)C1:!] and K[Pt(Ala)C12], thereaction of K[Pt(His)CI2] (because[Pt(HisH)CI2].H20 exists in this form when dissolvedin basic medium) with purines or pyrimidines ornucleosides is slow'".

AcknowledgementTwo of us (G N G and K M M) are thankful to the

CSIR, New Delhi, for financial support to carryoutthis work.

ReferencesI Rosenberg B. Vancamp L. Troska' J E & Mansour V H.

Nature, 222 (1969) 385.2 Rosenberg B & Vancamp L. Cancer Res, 30 (\970) 1977.3 Cleare M J, Coord Chem Rev, 12 (\974) 349.4 Hodgson D L, Progress in inorganic chemistry, Vol 23,edited

by S J Lippard (Wiley-Interscience. NY) 1977,211.5 Cleare M J & Hydes PC. Metal ions in biological SYSTems,Vol

II, Chap I, edited by H Sigel (Marcel Dekker, NY). 1980(4)

6 Barton J K & Lippard S J, Metal-nucleic acid interactions. VolI, edited by T Spiro (John Wiley. NY), 1980,32.

7 BarnardCFJ.CleareMJ&HydesPCChemyBriT,12(1968)1001.

8 Sherman S E & Lippard S J. Chem Rev, 87 (1987) 1153.9 Reedijk J. Pure Appl Chem, 59 (1987) 181.

JO Pneumatikakis G. Polvhedron, 3 (1984) 9 & 15.II Mogilcvkina M F. Korcheraya L M & Sidorik 0 A. Koord

Khim (Russ), II (1985) 1381.12 Ulrinch K J & Bruce M R. Inorg chim ACTO,80 (1983) I.J3 (a) Lalit Kumar & Srivastava T S.lnorg chim ACTa,80(1983)

47.(b) Srivastava T S, Inorg Biochem, 26 (1986) 45.

14 Stetrenko A I, Adamov 0 M, Dimitrieva E S, Muratova M V,Koord Khim (Russ). 13 (1987) 377.

15 (a) Sandra, Miller K & Marzilli L, Inorg chem, 24 (1985)2421.

\,

TAQUI KHAN et al.: MIXED LIGAND COMPLEXES OF PLATINUM(II)

t•~,1\

(b) Kasselouri S, Geroutis A & Hadjiliadis N, Inorg chim Acta,L 23-25 (1987) 135.

16 (a) Khan B T, Vijaya Kumari S & Narsa Goud G, J coordChern, 2 (1982) 19.

(b) Khan B T, Narsa Goud G & Vijaya Kumari, Inorg chimActa, 80 (1983) 145.

(c) Narsa Goud G, Ph.D. Thesis, Mixed ligand complexes ofplatinum(JI) with amino, acids and nucleic acid constituents,Osmania University, Hyderabad, India, (1983).

(d) Khan BT, Murali Mohan K& Narsa Goud G, Trans MetChern, 15 (1990) 407.

(e) Khan B T, Najmuddin K, Shamsuddin S & Zakeeruddin SM, Inorg chim Acta, 170 (1990) 129.

.I

/

33

(I) Khan B T, Murali Mohan K & Narsa Goud G,lndian J Chern,(IC-6799/91 in Press).

17 Gurbe H L, Handbook of preparative inorganic chemistry, Vol2, edited by G Brawer (Academic Press, NY), 1965, 1571.

18 Steyermark A, CaIcancetle R A & Eontreras E M, J AssocAnalyt Chern, 55 (1972) 680.

19 Bellamy L J, The infrared spectra of complex molecules (JohnWiley, NY) 1970,234.

20 Balice V & Theophanides T, J inorg nucl Chern, 32 (1970)1237.

21 Erickson L E, McDonald J W, Howie J K & Clow R P,J Amchern Soc, 90 (1968) 6371.

-