Intervalence Band Stark Effect of the Special Pair Radical Cation in Bacterial Photosynthetic Reaction Centers

Abstract

The Stark spectrum of the intervalence band of the special pair radical cation in bacterial photosynthetic reaction centers is presented. This spectrum, centered at 2600 cm-1, is modeled using a general treatment of intervalence band Stark effects based on a two-state vibronic coupling model of mixed-valency. The observed line shape can be predicted using values for the electronic coupling, driving force, and reorganization energy along a single antisymmetric mode as derived from previous measurements. The model can be used to estimate the difference between the electric dipole moments of the two hole-localized electronic states of a mixed-valence complex, ΔμCT, even if the dipole strength of its IVB is unknown or affected by intensity borrowing by other electronic transitions. We determine the value of ΔμCT to be 33 ± 5 D/f for the special pair radical cation in wild-type Rhodobacter sphaeroides reaction centers, where f is the scalar approximation to the local field correction tensor. This value of ΔμCT is less than would be predicted from the separation between the magnesium ions of the special pair, yet large enough to suggest that the observed dipole strength of its intervalence band is strongly affected by intensity borrowing.