WATER-POLARON MODEL IN BACTERIAL PHOTOSYNTHESIS .1. FUNCTIONAL-MODEL OF REACTION CENTERS

A model was developed for the functioning of purple bacteria reaction centers (RC). It predicts that appearance of electron excitation in th e special RC pair (P-2) causes polarization of the nearest water molec ules. The most important features are that the new state (i) evolves i n (0.5-5). 10(-13) sec, i.e., time an order of magnitude shorter than that needed for the onset of the primary electron transfer reaction, a nd (ii) involves a loss of electron energy about 0.03-0.12 eV, i.e., a n order of magnitude greater than the characteristic values for Franck -Condon rearrangement in monomeric aromatic molecules. The predicted p olaron state of excited RC ensures primary trapping of electron excita tions, and precedes primary electron donation to the transfer chain. G eneration of a polaron state in RC explains a number of heretofore obs cure phenomena, in particular, (a) the idleness of one of the two elec tron transfer chains in bacterial RC, (b) the higher energy position o f absorption peaks for P-2 in Rhodospirillum rubrum relative to the an tennal bacteriochlorophylls; (c) the anomalously large Stokes' shift f or BChl of some RC as compared with their core BChl.