An Alternative Pathway of Light-Induced Transmembrane Electron Transfer in Photosynthetic Reaction Centers of Rhodobacter sphaeroides.

Abstract

In the absorption spectrum of Rhodobacter sphaeroides reaction centers, a minor absorption band was found with a maximum at 1053nm. The amplitude of this band is ~10,000 times less and its half-width is comparable to that of the long-wavelength absorption band of the primary electron donor P870. When the primary electron donor is excited by femtosecond light pulses at 870nm, the absorption band at 1053nm is increased manifold during the earliest stages of charge separation. The growth of this absorption band in difference absorption spectra precedes the appearance of stimulated emission at 935nm and the appearance of the absorption band of anion-radical BA- at 1020nm, reported earlier by several researchers. When reaction centers are illuminated with 1064nm light, the absorption spectrum undergoes changes indicating reduction of the primary electron acceptor QA, with the primary electron donor P870 remaining neutral. These photoinduced absorption changes reflect the formation of the long-lived radical state PBAHAQA-.