Interactions of Photosynthetic Reaction Centers with Bc 1 Complexes from Rhodobacter Sphaeroides Studied Using SEIRAS on a Nano-Structured Gold Surface

Inter-protein reactions of photosynthetic reaction centers (RCs) with bc 1 complexes from R. sphaeroides , have been investigated using surface-enhanced Infrared-absorption spectroscopy (SEIRAS). Surface enhancement was achieved by a nano-structured gold surface. The proteins were immobilized via his-tags attached to the P side of the RC and the C-terminal end of the cytochrome (cyt) b subunit and co-reconstituted into a lipid bilayer by in-situ dialysis. In this configuration, the cyt c binding site of the two proteins is located on opposite sides of the membrane. Light-minus-dark absorbance spectra under continuous illumination in the absence of an electron donor indicated a slow quinone/semiquinone exchange, allowing release of ubiquinol (QH 2 ) into the membrane. The interaction of the bc 1 with QH 2 was indicated by the stationary state obtained but only in the presence of cyt c . The interaction is discussed in terms of a semiquinone species formed in the course of the Q cycle mechanism of the bc 1 .