Temperature-dependent behavior of bacteriochlorophyll and bacteriopheophytin in the photosynthetic reaction center from Rhodobacter sphaeroides

Abstract

We have reexamined the temperature dependence of resonance Raman (RR) spectra of the bacteriochlorin cofactors bound to reaction centers from Rhodobacter sphaeroides. Three types of resonant excitations were performed, namely, Soret band, bacteriopheophytin Qx-band, and near-infrared, Qy-band (pre)resonances. Sample temperature was varied from 300 to 10 K. In both Soret-resonant and Qy-preresonant Raman spectra, the ca. 1610-cm(-1) band corresponding to a bacteriochlorophyll CaCm methine bridge stretching mode is observed to increase in frequency by 4-6 cm(-1) as temperature is decreased from 300 to 15 K. This upshift is interpreted as arising from a change in conformation of the bacteriochlorophyll macrocycles. It may be nonspecific to the protein-bound cofactors, since a similar 4-cm(-1) upshift was observed in the same temperature range for BChl a in solution. Qx-resonant Raman spectra of either of the two bacteriopheophytin (BPhe) cofactors were obtained selectively using excitations at 537 and 546 nm. No significant frequency shift was observed for the CaCm stretching mode of BPheL between 200 and 15 K. We conclude, at variance with a previous report, that the macrocycle of the BPheL primary electron acceptor does not undergo any significant conformational change in the 200-15 K temperature range. Qy-preresonant excitation of RCs at 1064 nm provided selective Raman information on the primary electron donor (P primary). The stretching frequencies of the two conjugated keto and acetyl carbonyl groups of the M-branch primary donor BChl cofactor (P(M)) did not significantly change between 300 and 10 K. In contrast the keto carbonyl stretching frequency of cofactor P(L) was observed to upshift by 5 cm(-1), while its acetyl carbonyl frequency downshifted by 2 cm(-1). The latter shift indicated that the strong H-bond between the acetyl group of P(L) and His L168 may have slightly strengthened at 10 K. Excitation at 1064 nm of chemically oxidized RCs selectively provided RR spectra of the primary donor in its radical P.+ state. These spectra can be interpreted as a decrease of the localization of the positive charge on P(L) from 78% to 63% when the temperature decreased from 300 to 10 K resulting in a more electronically symmetric dimer. Possible origins of the temperature dependence of the positive charge delocalization in P.+ are discussed.