ELECTRON-TRANSFER AND PROTON UPTAKE OF PHOTOSYNTHETIC BACTERIAL REACTION CENTERS RECONSTITUTED IN PHOSPHOLIPID-VESICLES

Flash-induced electron transfer, proton binding and the relationship between them were studied in reaction centre protein from the photosynthetic purple bacteria Rhodobacter sphaeroides incorporated into artificial lipid (phosphatidylcholine) membrane vesicles; the results obtained were compared with the same parameters measured in detergents and chromatophores. The reaction centres were embedded randomly in the unilamellar vesicles which were permeable to pH indicator dyes and neutral electron donors (ferrocene, diaminodurene), but not to cytochrome c2+. The lipid environment did not affect the redox properties of the primary donor and acceptor or the pK values of the protonatable amino acid residues directly involved in the protonation of the reaction centre. However, it led to a decrease in the free energy of the secondary quinone and an increase in the reorganization energy of the interquinone electron transfer by 20 meV at neutral pH compared with the values measured in detergents. The functional activity of the acceptor quinones was reconstituted better in vesicles than in detergents and was close to that observed in the chromatophore membrane. The results are discussed in terms of competitive lipid-detergent binding, possible heterogeneity of the secondary quinone binding site and the protonation states of the reaction centre.