Nature of the special-pair radical cation in bacterial photosynthesis

Primary charge separation in bacterial photosynthesis occurs at the "specia l pair," a bacteriochlorophyll dimer that, on optical excitation, ejects an electron to become the special-pair radical cation. Understanding the natu re of this species is important to both the charge separation process itsel f and details of subsequent steps including charge recombination. Electron spin resonance (ESR)-based studies have led to the conclusion that the posi tive charge is delocalized over both bacteriochlorophyll monomers, the degr ee 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 electro n-transfer theory, indicate strong charge localization on just one of the b acteriochlorophylls. We present a complex computational strategy aimed at r esolving this issue through vibronic coupling analysis of the high- and low -resolution spectra using a priori computed vibrational analyses, quantum c hemical calculation of the strengths of a variety of key interactions, quan tum mechanical/molecular mechanical (QM/MM) calculation of the structure of over 20 mutant reaction centers, calculation and interpretation of the obs erved midpoint potentials, analyses relevant to ESR and related spectroscop ies, etc. The current state of progress is described, leading to a consiste nt picture that includes almost all available experimental data. (C) 2000 J ohn Wiley & Sons, Inc.