Electrooptical Properties of Different Redox States of Native and Modified Reaction Centers of Rhodobacter sphaeroides

Abstract

The Stark spectra of selectively and nonselectively BPheo → Pheo exchanged reaction centers are presented and compared to those of native reaction centers. It is shown that Pheo, when incorporated into the reaction center protein, shows an almost pure second-derivative shaped Stark spectrum indicating that absorbance changes are mainly caused by the interaction between the electric field and the change in permanent electric dipole moment |Δμ| upon excitation. |Δμ| is estimated to be about 1.5 D/f, which is substantially smaller than that of the native BPheo pigment. By comparing the spectra of open reaction centers to those of reaction centers in the P+ΦA state, evidence for a difference in polarizability between the ground and the excited state of the BPheo is found. Although the difference polarizability is not large enough to appreciably influence the Stark spectrum of randomly oriented molecules induced by an external electric field, it is argued that it is large enough to necessitate its inclusion in the analysis of electrochromic bandshifts induced by an internal electric field. The near-equal optical energies and electrooptical parameters of the Pheos reconstituted in the active and inactive branch, as well as their electrochromic bandshift upon P+ΦA formation (which is much smaller than for the native BPheo pigments), are discussed in terms of the interaction of the Pheos with internal electric fields from their environment.