Theoretical Investigations of Reaction Center Chromophores

Abstract

The primary electron donor (P or “special pair”) in the bacterial reaction center (RC) is a dimer of bacterio-chlorophyll (BCh1). The experimental spectrum of P is red-shifted compared to BCh1 in solution. This is due to interactions between the two BChls and to interactions between P and the surrounding protein. Explanations of the special properties of P have been offered by several investigators [1–8]. Recent hole-burning [9] and electric-field [10] studies suggest that the first excited state of P has charge-transfer character. In our earlier calculations [11], using preliminary coordinates where P was planar, we could not see any localization of the orbitals on to either BChl. With the refined coordinates P was found to be non-planar. We have now investigated the effects of this non-planarity, as well as those of the axial histidine ligands, on spectra, orbital distributions and transition-dipole orientations.