Photochemical Holeburning and Stark Spectroscopy of Photosynthetic Reaction Centers

Abstract

The initial step in photosynthesis involves photoexcitation of a primary electron donor and electron transfer to a primary electron acceptor. These processes take place in a membrane-protein complex called the photosynthetic reaction center (RC). The best characterized RCs have been isolated from the photosynthetic bacteria R. spheroides (R-26 mutant) and R. viridis. In both species, the complexes consist of three proteins each of approximate molecular weight 25–30kDa, four bacteriochlorophylls (BChls), two bacteriopheophytins (BPheo, BChl in which two H atoms replace the central Mg atom), two quinones and one non-heme iron [1]. In R. spheroides the pigments are a-type BChl, whereas in R. viridis the pigments are b-type. The prosthetic groups are embedded in the RC protein in vectorial fashion across the membrane, as shown by x-ray crystallography (Fig. 1) [2–4]. Combined with extensive earlier spectroscopic studies, the x-ray structure identifies the functional components with specific structural elements: the primary electron donor, often called P or the special pair, is identified as the strongly-coupled BChl dimer; the intermediate acceptor, often called I, is identified as the BPheo on the L-side; and the primary quinone (0_A, ubiquinone in R. Spheroides, menaquinone in R. Viridis) is the quinone on the L-side. The function of the BChl situated between the dimeric electron donor and the BPheo acceptor is uncertain, though it is certainly involved in mediating electron transfer from the excited state of P to I. The role of the chromophores on the M-side of the RC, which are related to those on the functional L-side by a C₂ axis running vertically through the Fe, is unknown. Recent structural work on the R. spheroides RC suggests that the gross features of its structure are very similar to those of R. viridis [5,6].