Model Systems for the Investigation of the Opsin Shift in Bacteriorhodopsin

Donor-acceptor substituted styrenes and phenylbutadienes with substituents varying in donor and acceptor strength and as reconstituted chromophore-protein complexes were investigated as model compounds for the protonated Schiff base chromophore in bacteriorhodopsin (bR) both experimentally and theoretically. Charge distribution, donor-acceptor strength, and the shift of the absorption energy are correlated. The effect of the external electrostatic field was tested with a compound carrying an additional nonconjugated charge. The concept of overpolarization by the external charge, that is, the reversal of the relative importance of the two main resonance structures in S(0) and S(1), has been emphasized and related to a simple qualitative 2 x 2 interaction model. The variable donor approach is a new way for a better understanding of the Opsin shift in Bacteriorhodopsin.