Femtosecond charge separation in dry films of reaction centers of Rhodobacter sphaeroides and Chloroflexus aurantiacus

In this work, the influence of the crystallographic water on electron transfer between primary donor P and acceptor B A was studied in reaction centers (RCs) of the purple bacterium Rhodobacter sphaeroides and the green bacterium Chloroflexus aurantiacus . For this purpose, time constants and oscillations of charge separation kinetics are compared between dry film RCs and RCs in glycerol-water buffer at 90 K. A common result of the drying of Rba. sphaeroides and Cfx. aurantiacus RCs is slowing of the charge separation process, decrease in amplitude of the oscillatory components of the kinetics, and the depletion of its spectrum. Thus, the major time constant of stimulated emission decay of P* bacteriochlorophyll dimer at 940 nm is increased from 1.1 psec for water-containing Rba. sphaeroides RCs to 1.9 psec for dry films of Rba. sphaeroides RCs. An analogous increase from 3.5 to 4.2 psec takes place in Cfx. aurantiacus RCs. In dry films of Rba. sphaeroides RCs, the amplitude of coherent oscillations of the absorption band of monomeric bacteriochlorophyll B A − at 1020 nm is 1.8 times less for the 130-cm −1 component and 2.3 times less for the 32-cm −1 component than the analogous amplitudes for water-containing RCs. Measurements in the analogous band of Cfx. aurantiacus RCs show that strong decrease (∼5-10 times) of the B A − absorption band and strong slowing (from ∼0.8 to ∼3 psec) of B A − accumulation together with ∼3-fold decrease in oscillation amplitude occurs on drying of these RCs. The overtones of the 32-cm −1 component disappeared from the oscillations of the kinetics at 940 and 1020–1028 nm after drying of the Rba. sphaeroides and Cfx. aurantiacus RCs. The results are in agreement with the results for GM203L mutant of Rba. sphaeroides , in which the HOH55 water molecule is sterically removed, and with the results for dry films of pheophytin-modified RCs of Rba. sphaeroides R-26 and for YM210W and YM210L Rba. sphaeroides mutant RCs. The data are discussed in terms of the influence (or participation) of the HOH55 water molecule on electron transfer along the chain of polar atomic groups N-Mg(P B )-N-C-N(HisM202)-HOH55-O=(B A ) connecting P B and B A in Rba. sphaeroides RCs.