Conserved electronic structure of the primary donor in reaction centres of sulphur and non-sulphur purple bacteria

摘要

In all purple bacteria the photosynthetic apparatus is located in an intracytoplasmic membrane, which can be isolated and investigated by spectroscopic techniques. The lightinduced charge separation process catalysed by the RC starts from the excited singlet state ~1P~* of the primary electron donor, which at least for some species is known to be a bacteriochlorophyll (BChl) dimer (Fig. 1). Information about the electronic structure of P is crucial for an understanding of its functional properties.In the P~(.+) state, which can be generated by illumination of membranes or RC, the un-paired electron is distributed over the dimer and interacts magnetically with protons of both BChl moieties, P_L and P_M. Therefore, the so-called highest occupied molecular orbital (HOMO) of P can be studied by using ENDOR spectroscopy and related techniques. Experiments performed on RC single crystals revealed an unequal distribution of the electron with 68 percent of the spin density on P_L (Lendzian et al. 1993).This has important implications for the optical properties of P (see Discussion). The same asymmetry of P~(.+) has been found in membranes from various species of non-sulphur purple bacteria (Rautter et al. 1994), whereas for isolated RC an influence ofthe detergent used to solubilise the membrane protein has been recognized (Rautter et al. 1994, Muh et al. 1997). In order to elucidate the significance of this asymmetry, we extended the earlier ENDOR studies of membranes to a number of sulphur and non-sulphur purple bacteria that contain BChl a. The spectra are analysed by comparison with spectra of isolated mutant RC of Rhodobacter (Rba.) sphaeroides that were designed on the basis of sequence alignments (Komiya et al. 1988, Ivancich et al. 1997).