Nature of the special-pair radical cation in bacterial photosynthesis

Abstract

Primary charge separation in bacterial photosynthesis occurs at the `special pair,' a bacteriochlorophyll dimer that, on optical excitation, ejects an electron to become the special-pair radical cation. Understanding the nature of this species is important to both the charge separation process itself and details of subsequent steps including charge recombination. Electron spin resonance (ESR)-based studies have led to the conclusion that the positive charge is delocalized over both bacteriochlorophyll monomers, the degree of delocalization being affected by site-directed mutagenesis. However, Breton et al. have observed charge-transfer electronic absorption spectra (centred at ca. 2500 cm-1), which, when interpreted using standard electron-transfer theory, indicate strong charge localization on just one of the bacteriochlorophylls. We present a complex computational strategy aimed at resolving this issue through vibronic coupling analysis of the high- and low-resolution spectra using a priori computed vibrational analyses, quantum chemical calculation of the strengths of a variety of key interactions, quantum mechanical/molecular mechanical (QM/MM) calculation of the structure of over 20 mutant reaction centers, calculation and interpretation of the observed midpoint potentials, analyses relevant to ESR and related spectroscopies, etc. The current state of progress is described, leading to a consistent picture that includes almost all available experimental data.