Subpicosecond electron transfer in reaction centers of photosynthetic bacteria

Abstract

Sub-picosecond dynamics of the primary electron transfer (ET) in reaction centers (RCs) of Rhodopseudomonas viridis were studied in frequency domain by burning of spectral holes within the absorption band of the primary electron donor, P960, at 2K. Reversible and irreversible absorbance changes induced by continuous light at 1014 nm (in comparison with white light) were measured in RCs poised at various red-ox states. Spectral holes with a full width at half maximum less than 10 cm -1 observed in RCs with pre-reduced quinone QA or bacteriopheophytin HL correspond to rates of ET from P960° revealed by femtosecond spectroscopy. Wider holes are found in RCs with an open ET chain, and they presumably correspond to earlier relaxation processes (faster than 200 fs) e.g., quasi-resonant transitions to charge-transfer states such as P+P_ and P+B-. A variety of excited states with weak and strong electron-phonon and vibronic coupling may exist either in each RC or in fractions of RCs with different conformations.