STARK-EFFECT SPECTROSCOPY OF CAROTENOIDS IN PHOTOSYNTHETIC ANTENNA AND REACTION CENTER COMPLEXES

The effects of electric field on the absorption spectra of the carotenoids spheroidene and spheroidenone in photosynthetic antenna and reaction center complexes (wild-type and several mutants) from purple non-sulfur bacteria are compared with those for the isolated pigments in organic glasses. In general, the field effects are substantially larger for the carotenoid in the protein complexes than for the extracted pigments and larger for spheroidenone than spheroidene. Furthermore, the electrochromic effects for carotenoids in all complexes are much larger than those for the Q(x) transitions of the bacteriochlorophyll and bacteriopheophytin pigments which absorb in the 450-700 nm spectral region. The underlying mechanism responsible for the Stark effect spectra in the complexes is found to be dominated by a change in permanent dipole moment of the carotenoid upon excitation. The magnitude of this dipole moment change is found to be considerably larger in the B800-850 complex compared to the reaction center for spheroidene; it is approximately equivalent in the two complexes for spheroidenone. These results are discussed in terms of the effects of differences in the carotenoid functional groups, isomers and perturbations on the electronic structure from interactions with organized environment in the proteins. These data provide a quantitative basis for the analysis of carotenoid bandshifts which are used to measure transmembrane potential, and they highlight some of the pitfalls in making such measurements on complex membranes containing multiple populations of carotenoids. The results for spheroidenone should be useful for studies of mutant proteins, since mutant strains are often grown semi-aerobically to minimize reversion.