Reaction center mutants that affect the amount and pathway of B-side electron transfer in purple nonsulfur bacteria

Abstract

Absorption of blue (390 nm) light by wild type reaction centers of Rhodobacter sphaeroides results in substantial B-side electron transfer (see abstract by Lin et al.). We have isolated a number of different reaction center mutants that show B-side electron transfer using longer wavelength excitation. In a series of reaction centers in which the P/P+ potential is progressively lowered, we have found that the lowest potential mutant that we were able to measure (the double mutant L168 Hisà Glu + L170Asnà Asp which is 127 mV below wild type) undergoes substantial B-side electron transfer upon excitation into either the monomer bacteriochlorophyll band at 800 nm or into the bacteriopheophytin band at 740 or 760 nm. In contrast, excitation of P at 860 nm results in only A-side electron transfer. The B-side charge separated state formed in this mutant upon excitation at or below 800 nm is stable for hundreds of picoseconds. We have also generated mutations at the amino acid that serves as the A-side monomer bacteriochlorophyll ligand resulting in loss of the 860 nm transition of P (both L153 Hisà Leu and L153 Hisà Gln have this property). These mutants show rapid and exclusive electron transfer along the B-branch upon excitation at 800 nm (monomer bacteriochlorophyll band). The kinetics and thermodynamics of charge separation along the B-branch will be discussed.