NEAR-INFRARED RESONANCE RAMAN-SPECTROSCOPY OF THE SPECIAL PAIR AND THE ACCESSORY BACTERIOCHLOROPHYLLS IN PHOTOSYNTHETIC REACTION CENTERS

Rapid-flow resonance Raman spectra of the primary electron donor (a ba cteriochlorophyll dimer known as P) and of the monomeric accessory bac teriochlorophylls (B) in the bacterial photosynthetic reaction center of Rb. sphaeroides have been obtained at 5 degrees C. The spectra were obtained using a shifted excitation Raman difference technique with e xcitation at 850 nm for the P spectrum and 800 nm for the B spectrum. Raman bands at 187, 204, 332, 564, 684, 730, 899, and 1163 cm(-1) are found in common in the P and B spectra, while unique modes appear in t he low-frequency region of the special pair at 34, 71, 95, 128, and 48 4 cm(-1). The remaining strongly Raman-active monomer modes at 353, 38 5, 621, 761, 1010, 1114, and 1132 cm(-1) were not detected in the dime r spectrum. No substantial resonance Raman activity is observed above 1200 cm(-1) for either chromophore, indicating that high-frequency mod es are not strongly coupled to the optical excitation in the Q(y) abso rptions f of B or P. The Raman spectrum shows that the electronic exci tation of P is coupled to at least 14 vibrational degrees of freedom, including low-frequency modes at 34, 71, 95, and 128 cm(-1). The Raman scattering cross sections for the modes of B are approximately an ord er of magnitude larger than those for analogous modes of P. This diffe rence suggests that the excited electronic state of P is damped by rap id vibronic relaxation processes that are not present in B. The comple te analysis of these resonance Raman results will lead to the developm ent of specific multimode models for the excited-state structural dyna mics and relaxation of the chromophores in reaction centers.