FTIR Studies of the D⁺Q_A ⁻ and D⁺Q_B ⁻ 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 (Q_A and Q_B) 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 Q_A and Q_B 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 Q_A and Q_B quinones can be removed, and functionally active RC’s (containing quinone at either both the Q_A and Q_B 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⁺Q_A ⁻ and D⁺Q_B ⁻ for RC’s containing a series of quinones with vibrational modes different from UQ₁₀ (i.e. ¹³C-UQ₁₀, ¹⁸O-UQ₁₀ and duroquinone, denoted dQ).