A Ultrafast Infrared Spectroscopic Study of Photosynthetic Reaction Centers

Protein and cofactor vibrational dynamics associated with photoexcitation and electron transfer events in the photosynthetic reaction center (RC) have been investigated with time resolved infrared spectroscopy. Existing questions regarding the nature and extent of the perturbation of nuclear coordinates and potentials associated with electron transfer, effect of changing such perturbations (by changing the hydration) on electron transfer rates, time scale of vibrational dephasing of various vibrational modes, identity and population kinetics of redox intermediates participating in the process of electron transfer and finally the quantum mechanical character of some of the participating electronic states have been examined. Time dependent change of the RC between 1560 and 1960 cm^{-1} have been measured between 0 and 4 ns with up to 400 fs time resolution. Improvements in the time and frequency resolution allows the examination of quantum uncertainty limited evolution of protein vibrational changes. The resultant time-bandwidth coupling of the evolving spectra is exploited to simultaneously measure the decay constants associated with vibrational dephasing (i.e. the vibrational bandwidths) and the population kinetics of different electronic states. Kinetic signatures of all transient intermediates participating in the primary steps of photosynthesis could be identified in the difference spectrum. Observation of the vibrational signature of a transient intermediate provides strong evidence for the existence of the reduced accessory bacteriochlorophyll as a real intermediate in the electron transfer process. An analysis of the overall difference spectrum yields no evidence for a large scale conformational change of the protein from 400 fs to 4 ns. Distinct differences in the vibrational dynamics of proteins with different humidity, associated with a difference in the electron transfer kinetics, were observed. The kinetics at >1700 cm^{-1} identified the existence of low energy electronic transitions of the P^* and P^+^ecies. A simple theoretical model developed to explain the observed kinetics and anisotropy of these transitions implies a substantial charge resonance character of the P^* photoexcited state.