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

Abstract

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 RCs was found to be reversibly transferred to the P+BA- surface at 120, 380, etc. fs delays (monitored by measurements of BA- 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 pheophytin 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+BA- 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+BA- 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 BA- 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+BA- within ∼5 ps with subsequent decay due to electron transfer to HA within ∼1 ps. The mechanisms of the electron-transfer between P* and BA and of the stabilization of the state P+BA- in bacterial RCs are discussed.