FTIR Studies of the D+QA − and D+QB − States in Reaction Centers from Rb. sphaeroides

Abstract

In bacterial reaction centers (RC’s), molecular changes associated with the charge separation between the primary electron donor (D) and the quinone acceptors (QA and QB) have been investigated by Fourier transform infrared (FTIR) difference spectroscopy. From these studies, specific changes in the IR absorption of bacteriochlorophyll carbonyl groups of D have been characterized, and the absence of any large conformational change of the RC protein backbone has been demonstrated (1). However, the contribution of QA and QB to the light-induced spectra has not been characterized. In this work we utilize the fact that in RC’s from Rb. sphaeroides both the QA and QB quinones can be removed, and functionally active RC’s (containing quinone at either both the QA and QB sites or only the QA site) can be reconstituted by addition of either the native quinone (UQ10). or a number of other quinones. In this way, we have obtained the light-induced IR difference spectra between the relaxed, charge-unseparated state, and the two charge-separated states, D+QA − and D+QB − for RC’s containing a series of quinones with vibrational modes different from UQ10 (i.e. 13C-UQ10, 18O-UQ10 and duroquinone, denoted dQ).