Resolution of electron and proton transfer events in the electrochromism associated with quinone reduction in bacterial reaction centers

Abstract

We have measured the electrochromic response of the bacteriopheophytin, BPh, and bacteriochlorophyll, BChl, cofactors during the QA −QB → QAQB − electron transfer in chromatophores of Rhodobacter (Rb.) capsulatus and Rb. sphaeroides. The electrochromic response rises faster in chromatophores and is more clearly biexponential than it is in isolated reaction centers. The chromatophore spectra can be interpreted in terms of a clear kinetic separation between fast electron transfer and slower non-electron transfer events such as proton transfer or protein relaxation. The electrochromic response to electron transfer exhibits rise times of about 4 µs (70%) and 40 µs (30%) in Rb. capsulatus and 4 µs (60%) and 80 µs (40%) in Rb. sphaeroides. The BPh absorption band is shifted to nearly equivalent positions in the QA − and nascent QB − states, indicating that the electrochromic perturbation of BPh absorption from the newly formed QB − state is comparable to that of QA − . Subsequently, partial attenuation of the QB − electrochromism occurs with a time constant on the order of 200 µs. This can be attributed to partial charge compensation by H+ (or other counter ion) movement into the QB pocket. Electron transfer events were found to be slower in detergent isolated RCs than in chromatophores, more nearly monoexponential, and overlap H+ transfer, suggesting that a change in rate-limiting step has occurred upon detergent solubilization.