Triplet states of monomeric bacteriochlorophyll in vitro and of bacteriochlorophyll dimers in antenna and reaction center complexes

Abstract

Triplet states of monomeric bacteriochlorophyll in solution, and of dimeric bacteriochlorophyll in photosynthetic reaction centers and antenna complexes isolated from Rhodopseudomonas sphaeroides and Rps. viridis were generated by excitation with short flashes. The triplet states were characterized by measurements of optical absorption spectra, decay kinetics as functions of temperature, quenching by O 2, and delayed fluorescence. The triplet state of the antenna dimer can be described as one molecule in the ground (singlet) state and one in the excited (triplet) state. Formation of the triplet disrupts exciton interactions between the two molecules, and the complex acquires a new absorption band due to the molecule in the ground state. In reaction centers, the formation of the triplet causes bleaching of the main absorption band of P, the reactive bacteriochlorophyll complex (870 nm in Rps. sphaeroides; 960 nm in Rps. viridis). Other absorption bands that have been considered to be exciton bands of P (810 nm in Rps. sphaeroides; 850 nm in Rps. viridis) appear to undergo shifts, but not to bleach. The latter bands probably are due to neighboring bacteriochlorophylls, not to P. The decay kinetics of the triplet state in reaction centers are strongly influenced by interactions with the anionic radical of the quinone that serves as an early electron acceptor. A new transient (20 ns) state of the reaction center was found when reaction centers were excited with supersaturating flashes.