Mechanism of Charge Separation and Stabilization of Separated Charges in Reaction Centers of Chloroflexus aurantiacus and of YM210W(L) Mutants of Rhodobacter sphaeroides Excited by 20 fs Pulses at 90 K

摘要

The nuclear wave packet formed by 20 fs excitation on the P~* potential energy surface in native and mutant (YM210W and YM210L) reaction centers (RCs) of Rhodobacter (Rb.) sphaeroides and in Chloroflexus (C.) aurantiacus RCswas found to be reversibly transferred to the P~+B_A~- surface at 120, 380, etc. fs delays (monitored by measurements of B_A~- absorption at 1020-1028 nm). The YM210W(L) mutant RCs show the most simple pattern of femtosecond oscillations with a period of 230 fs in stimulated emission from P~* and with the initial amplitude comparable to that in plant phenohytin a (Pheo)-modified Rb. sphaeroides R-26 RCs. Similar reversible oscillations are observed in the 1020 nm band of the mutants, the initial amplitude of which is smaller by a factor of ～10 with respect to Pheo-modified Rb. sphaeroides R-26 RCs. In contrast to native and Pheo-modified Rb. sphaeroides R-26 RCs, irreversible quasi-exponential stabilization of P~+B_A~- is considerably suppressed in the mutant RCs in the picosecond time domain. The water rotational mode with a frequency of 32 cm~(-1) and its overtones, described earlier (Yakovlev; et al. Biochemistry 2002, 41, 2667-2674), are decreased in the YM210W(L) mutants and strongly suppressed in dry films of the mutant RCs. In the dry film of both YM210W and YM210L RCs neither reversible nor irreversible P~+B_A~- formation monitored at 1020 nm is observed despite the preservation of fs oscillations with a frequency of 144 cm~(-1) in the 935 nm kinetics of stimulated emission from P~*. Furthermore, the 1020 nm band is not formed inside of P~*. In C. aurantiacus RCs, containing leucine instead of tyrosine at the M208 position, the P~* decay is slowed to ～5 ps at 90 K(1.5 ps in Rb. sphaeroides RCs) and characterized by fs oscillations with the amplitude comparable to that measured in native Rb. sphaeroides R-26 RCs. The B_A~- absorption band development at 1028 nm is observed at 90 K with fs oscillations similar to those described for native Rb. sphaeroides R-26 RCs at 293 K but with the amplitude being smaller by a factor of ～6. The kinetics of absorbance changes in the 1028 nm band in C. aurantiacus RCs includes the stabilization of P~+B_A~- within ～5 ps with subsequent decay due to electron transfer to H_A within ～1 ps. The mechanisms of the electron-transfer between P~* and B_A and of the stabilization of the state P~+B_A~- in bacterial RCs are discussed.