Coherent Electron Transfer in Reaction Centers of YM210L and YM210L/HL168L Mutants of Rba. Sphaeroides

Abstract

A role of tyrosine M210 in the charge separation and stabilization of separated charges was studied by analyzing of the femtosecond oscillations in the kinetics of decay of stimulated emission from P* and of a population of the primary charge separated state P⁺B_A ⁻ in the YM210L and YM210L/HL168L mutant reaction centers (RCs) of Rhodobacter (Rba.) sphaeroides in comparison with those in native RCs of Rba. sphaeroides. Kinetics of P* decay at 940 nm of the both mutants show a significant slowing-down of the primary charge separation reaction in comparison with native RCs. Distinct damped oscillations in these kinetics with main frequency bands in the range of 90–150 cm⁻¹ reflect mostly nuclear motions inside the dimer P. Formation of a very small absorption band of B_A ⁻ at 1020 nm is registered in RCs of both mutants. The formation of the B_A ⁻ band is accompanied by damped oscillations with main frequencies from 10 to 150 cm⁻¹. Only a partial stabilization of the P⁺B_A ⁻ state is seen in the YM210L/HL168L mutant in the form of a small non-oscillating background of the1020-nm kinetics. Similar charge stabilization is absent in the YM210L mutant. A model of oscillatory reorientation of the OH-group of TyrM210 in the electric fields of P⁺ and B_A ⁻ was proposed to explain a rapid stabilization of the P⁺B_A ⁻ state in native RCs. A conclusion was done that, probably, the absence of TyrM210 can not be compensated by lowering of the P⁺B_A ⁻ free energy that is expected for the double YM210L/HL168L mutant.