LIGHT-INDUCED CHARGE SEPARATION IN RHODOPSEUDOMONAS-VIRIDIS REACTION CENTERS MONITORED BY FOURIER-TRANSFORM INFRARED DIFFERENCE SPECTROSCOPY - THE QUINONE VIBRATIONS

Static FTIR light-induced difference spectra have been recorded for reaction centers from Rhodopseudomonas viridis in the following charge-separated states: P+Q(A)--PQ(A), P+Q(B)--PQ(B), I--I, I-Q(A)--IQ(A), and I-Q(A)2--IQ(A). A comparison of the I--I difference spectra with the I-Q(A)--IQ(A) difference spectra reveals new bands which can be assigned to Q(A)- vibrations; these vibrations are also observed in the P+Q(A)--PQ(A) and P+Q(B)--PQ(B) difference spectra. Through an analysis of all of the static difference spectra, the electron-transfer pathway can be monitored in the infrared from the primary donor, P, to the secondary acceptor, Q(B), via the intermediate acceptor, 1, and the primary acceptor, Q(A). The difference spectra are dominated by absorbance changes of prosthetic groups, with very few identifiable contributions from amino acids and little overall structural change in the protein backbone, involving only one or two residues for the various charge-separated states. Oxidation of the primary donor in the reaction center shows the characteristic absorbance changes of the 9-keto and 10-ester carbonyl groups observed upon oxidation of bacteriochlorophyll b in a non-hydrogen-bonded environment [Ballschmiter, K. H., & Katz, J. J. (1969) J. Am. Chem. Soc. 91, 2661-2677]. Reduction of the quinones in the reaction center yields absorbance changes of the carbonyls observed during reduction of quinones in a hydrogen-bonded environment [Bauscher, M., Nabedryk, E., Bagley, K., Breton, J., & Mantele, W. (1990) FEBS Lett. 261, 191-195]. It is thus concluded that the protein acts as an optimized solvent to facilitate electron transfer from the primary donor to the secondary quinone acceptor.