ELECTROOPTICAL PROPERTIES OF DIFFERENT REDOX STATES OF NATIVE AND MODIFIED REACTION CENTERS OF RHODOBACTER-SPHAEROIDES

The Stark spectra of selectively and nonselectively BPheo --> Pheo exc hanged reaction centers are presented and compared to those of native reaction centers. It is shown that Pheo, when incorporated into the re action center protein, shows an almost pure second-derivative shaped S tark spectrum indicating that absorbance changes are mainly caused by the interaction between the,electric field and the change in permanent electric dipole moment \Delta mu\ upon excitation. \Delta mu\ is esti mated 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 c enters to those of reaction centers in the p(+)Phi(A)Q(A)(-), state, e vidence for a difference in polarizability between the ground and the excited state of the BPheo is found. Although the difference polarizab ility 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 t he analysis of electrochromic bandshifts induced by an internal electr ic field. The near-equal optical energies and electrooptical parameter s of the Pheos reconstituted in the active and inactive branch, as wel l as their electrochromic bandshift upon P(+)Phi(A)Q(A)(-), formation (which is much smaller than for the native BPheo pigments), are discus sed in terms of the interaction of the Pheos with internal electric fi elds from their environment.