ELECTRON-TRANSFER DYNAMICS OF RHODOPSEUDOMONAS-VIRIDIS REACTION CENTERS WITH A MODIFIED BINDING-SITE FOR THE ACCESSORY BACTERIOCHLOROPHYLL

Femtosecond spectroscopy in combination with site-directed mutagenesis was used to study the influence of histidine L153 in primary electron transfer in the reaction center of Rhodopseudomas viridis. Histidine was replaced by cysteine, glutamate, or leucine. The exchange to cyste ine did not lead to significant changes in the primary reaction dynami cs. In the case of the glutamate mutation, the decay of the excited el ectronic level of the special pair P is slowed by a factor of 3. The exchange to leucine caused the incorporation of a bacteriopheophytin b instead of a bacteriochlorophyll b molecule at the B-A site, As a con sequence of this chromophore exchange, the energy level of the electro n transfer state P+BA- is lowered to such an extent that repopulation from the next electron transfer intermediate state P+HA- takes place, resulting in a long-lasting P+BA- population. The observed differences in time constants are discussed in the scope of nonadiabatic electron transfer theory considering the influence of the amino acids at posit ion L153 and the chromophore exchange on the energy level of the inter mediate state P+BA-. The results show that the high efficiency of prim ary electron transfer is reduced substantially, if the energy level of P+BA- is lowered or raised by several hundred wavenumbers.