Picosecond Spectral Relaxation in a Photosynthetic Reaction Center

Abstract

In the reaction center protein of the photosynthetic bacterium Rhodobacter sphaeroides, the initial electron transfer process takes place in 3.5 ps at room temperature and speeds up to 2 ps at 77 K 1. When the tyrosine residue at site M210 near the acceptor pigments is replaced by an isoleucine, by site-directed mutagenesis, the electron transfer rate slows down 5-fold at room temperature and is further slowed down to about 50 ps at temperatures below 175 K. This slowing can be explained by an increase in the height of the barrier for the electron transfer reaction. A further interesting observation is that at low temperatures the decay of the excited electron donor (or ’special pair’ P) varies from being distinctly nonexponential on the red side of the stimulated emission band of P* to being exponential on the blue side of the same band. This behavior has its origin in the shifting of the stimulated emission spectrum to shorter wavelengths at longer times. At room temperature the stimuated emission band remains unshifted. These are the first clear indications that (temperature-dependent) nuclear relaxation takes place on the time scale of electron transfer. Possible explanations for this phenomenon include conformational interconversion in the excited state or competing nonradiative relaxation to a vibrationally hot ground electronic state followed by slower nuclear relaxation.