Structural aspects of the reaction center of photosynthetic bacteria calculated from triplet state zero-field splittings

Abstract

Since the original observation of photoexcited triplet-state EPR spectra in reaction centers of photosynthetic bacteria by Leigh and Dutton [l-4], there has been a considerable amount of magnetic resonance data accumulated on the triplet states of the pigment molecules, reaction centers and whole cell systems of photosynthetic bacteria [5-91. Zero-field splittings have been measured for the photoexcited triplet state of bacteriochlorophyll a and bacteriochlorophyll b and for a variety of bacterial systems containing these molecules [7,9]. The triplet state data for the bacterial systems have been interpreted as arising from a bacteriochlorophyll dimer within the reaction center observed when the primary electron acceptor has been reduced [4,6]. It has been shown in previous work [6,8,10], that the triplet state properties of a dimer in which the triplet excitation is shared between the two molecules may be calculated from the properties of the monomer and the geometry of the pair. Therefore, since the bacteriochlorophyll triplet state properties are known, the photoexcited triplet state can be used as a probe to investigate the orientation of the pigment molecules in the special pair [6] present in the reaction center of bacterial systems. Since the zero-field splittings are available for a large number of bacteria, we have utilized these data, along with the most recently published bacteriochlorophyll pigment zero-field splittings [8,9] to determine the relative orientation of the pair molecules in the reaction center for the presently