Electron Transfer Reactions Involving Cblorophylls a and b and Carotenoids BY JOSEPH LAFFERTY AND T. GEORGE TRUSCOTT" Chemistry Department, Paisley College, Paisley PA1 2BE EDWARD J. LAND AND Paterson Laboratories, Christie Hospital and Holt Radium Institute, Manchester M20 9BX Received 3rd March, 1978 Rates of positive and negative charge transfer from the radical cations and anions respectively of several carotenoids of varying double bond chain lengths to chlorophyll a and b are reported. The role of carotenoids in photosynthesis has been the subject of discussion for many years. It is probable that carotenoids have at least two functions, namely that of accessory pigments in which light energy is absorbed and transferred to chlorophyll and that of a protective species in which the potentially damaging effects of singlet oxygen, produced from the chlorophyll triplet, are quenched.It is thought that the primary process of photosynthesis involves electron ejection from chlorophyll a and various oxidation/reduction reactions. Thus the properties of the cations of carotenoids and chlorophylls (chi*+) are of importance. that all-trans-p-carotene and lycopene undergo charge transfer reactions of the type Our recent observations C*++chla -+ C+chla-+ (1) C*-+chla + C+chla-- (2) where C-+ and C-- are the radical cation and anion, respectively, of P-carotene and lycopene. imply that other roles of P-carotene in photosynthesis may be related to such charge transfer reactions. We now report further observations of electron transfer reactions involving both chlorophyll a.and chlorophyll b together with six polyenes varying from 7,7'-dihydro- /?-carotene (8 conjugated double bonds) to decapreno-P-carotene (1 5 conjugated double bonds). EXPERIMENTAL Chlorophyll a and b were extracted from spinach by a procedure similar to that described by Zscheile and Comar ;2 the purity was satisfactory as checked by the ratio of the blue/red absorption maxima. All polyenes were donated by Hoffmann-La Roche and were used without further purification. The solvent, hexane, was B.D.H. (Special for Spectroscopy) and was used as supplied. The pulse radiolysis equipment has also been described previ~usly.~ RESULTS In our earlier publication,' we discussed the rates of positive charge transfer from p-carotene and lycopene radical cations to chlorophyll a, and negative charge transfer 2760J .LAFFERTY, T. G . TRUSCOTT AND E . J . LAND 276 1 from the radial anions of the same pdyenes ta chlorophyll a. We now report the rates of + ve and - ve charge transfer from the radical cations and anions, respectively of several other polyenes of varying double bond chain lengths to chlorophyll a and chlorophyll b. The rate constants obtained, assuming monmeric chlorophylls, are listed in table 1 (chlorophyll a) and table 2 (chlorophyll b). As before the carotenoid radical ions were generated by pulse radiolysis of - mol dm-3 solutions of the corresponding carotenoids in hexane solution.4* The transfer rate constants were then obtained by monitoring the increased carotenoid radical ion decay rates resulting from addition of various concentrations el chlorophyll a or b.TABLE SE SECOND ORDER RATE CONSTANTS (k) FOR THE REACTION BETWEEN CAROTENOID RADICAL IONS AND CHLOROPHYU a number of monitoring conjugated wavelength/nm k/dm3 mol-1 s-1 carotenoid double bonds cation amon reaction (1) reaction (2) 7,7’-dihydro$-caro t ene 8 830 785 5.4 8.0 sep t apreno-/I-car o tene 9 915 785 10.7 7.0 1 5,l Y-ci.+fl-carotene 11 1040 880 11.8 8.7 all-trans-j?-caro tene 11 1040 880 6.0 8.5 all-tram-1 ycopene 11 1070 950 1.7 7.0 decapreno-#I-carotene 15 1250 1130 4.7 5.4 TABLE 2.-sECOND ORDER RATE CONSTANTS (k) FOR THE REACTION BETWEEN CAROTENOID RADICAL IONS AND CHLOROPHYLL b carotenoid number of monitoring conjugated wavelength/nm 10-10 k/dm3 mol-1 s-1 double bonds cation amon reaction (1) reaction (2) 7 ,? ’-di hy dro-b-car0 t ene 8 830 785 1 .o 2.5 septapreno-#I-carotene 9 915 785 0.6 8.0 1 5,I 5‘-cis-fl-carotene 11 1040 880 CO.01 1.45 all-trans-p-caro tene 11 1040 880 <0.01 1.75 decapren 0-p-car o t ene 15 1250 1130 ca.01 1 .o Solutions of chIorophy11 a and b alone in hexane and various other solvents including benzene, ether, acetonitrile and dimethyl sulphoxide were also pulsed in order to examine the absorptions of the chlorophyll a and b radical cations and anions.The transient absorptions thus obtained from chla solutions, although generally consistent with the spectra of c h h - and chlas+ found by Seki et oZ.,~ were too weak, in comparison with the intense absorptions of the earotenoid ions, to allow studies of the growth of chlaq- or chlae+ on pulse radiolysis of carotenoid+chlorophyll a mixtures.The transient absorptions from chlorophyll b solutions, were likewise very weak. For a single pair of carotenoids @carotene and septapreno-P-carotene) positive charge transfer from one carotenoid to another was investigated. Addition of lob5 mol dm-3 P-carotene to 2.5 x mol dm-3 septapreno-P-carotene resulted in an increase in the decay of the septapreno-/?-carotene cation absorption at 915 nm and a matching growth of 8-carotene cation absorption at 1050 nm.4 The rate of + ve charge transfer from the septapreno-P-carotene cation to p-carotene was thus found to be 5.2 x lo9 dm3 mol-l s-l. DISCUSSION Our earlier work involving the reaction of chlorophyll a with the p-carotene and lycopene radical cation [reaction (l)] and radical anion [reaction (2)] showed2762 ELECTRON TRANSFER I N CHLOROPHYLLS AND CAROTENOIDS that both processes occur at rates close to the diffusion limit for both carotenoids (k = 2-8 x lo9 dm3 mol-' s-l).Our new results show the same behaviour for all the carotenoids studied in that all such radical ions were quenched by chlorophyll a at similar rates (k = 2-12 x lo9 dm3 mol-' s-'). However, a marked difference was observed with chlorophyll b ; in this case the radical anions of all carotenoids were quenched while only the radical cation of the shorter polyenes (septapreno-P-carotene and 7,7'-dihydro-P-carotene) reacted with chlorophyll b. The important aspect is that we could detect no reaction between P-carotene.+ and chlorophyll b.Since septapreno-P-carotene*+ was quenched by chlorophyll b while /?-carotene*+ was not, this implied that septapreno-p-carotene*+ can gain an electron more readily than P-carotene-+. In an attempt to confirm this we studied the reaction between these two carotenoids and, in agreement with their reaction rates with chlorophyll b, we found that the process septapreno-P-carotene*+ + p-carotene -+ septapreno-/?-carotene + p-carotene-+ occurred at close to the diffusion limited rate. Recently, Beddard et aL7 have reported the quenching of chlorophyll a fluores- cence by P-carotene. Since the corresponding singlet energy levels preclude singlet- singlet energy transfer and P-carotene has a low ionisation potential, they interpret their observations in terms of an electron transfer process : chlorophyll a (S,) + P-carotene (So) -+ chlorophyll a*- +,&carotene-+. Thus, following our observation of the reaction p-carotenea+ + chlorophyll a. -+ /?-carotene + chlorophyll am+ Beddard et nl.speculate that the overall result of such electron transfer processes is to produce a charge separated pair, chla*+. . . chla.0-, which may be involved in the reaction centre of photosystem I1 (PSII). It is well known that the reaction centres in both PSI and PSII involve chlorophyll a and not chlorophyll b. Thus it is interesting that our results on the failure of p- carotene-+ to react with chlorophyll b preclude the formation of a chlb-+ . . . chlb*- charge separated pair and thus may offer an explanation for chlorophyll b not being involved in the primary process of PSII.Note added in proof.- One could also speculate that, after the eletron transfer processes described above have occurred, a " sandwich " type system chla*-. . . p-carotene . . chla*+ is produced, the carotene molecule preventing immediate charge recombination. Similar " sandwich " type systems can be envisaged involving an associated chlorophyll a radical cation.8 We thank the Cancer Research Campaign, the Medical Research Council and the S.R.C. for support. J. L. acknowledges a S.R.C. research studentship. J. LafTerty, E. J. Land and T. G. Truscott, J.C.S. Chem. Comm., 1976, 70. * F. P. Zscheile and C. L. Comar, Botan. Gas., 1941, 102,463. J. P. Keene, J. Sci. Instr., 1964, 41, 493. E. A. Dawe and E. J. Land, J.C.S. Faraday I, 1975, 71,2162. J . Lafferty, A. C. Roach, R. S. Sinclair, T. G. Truscott and E. J. Land, J.C.S. Faradayl, 1977, 73, 41 6. H. Seki, S. Arai, T. Shida and M. Imamura, J. Amer. Chem. Soc., 1973, 95, 3404. G. S. Beddard, R. S. Davidson and K. R. Trethewey, Nature, 1977, 267, 373. D. Holten and M. W. Windsor, Ann. Rev. Biophys. Bioeng., 1978,7,189. (PAPER 8/398)