The exciton origin of the visible circular dichroism spectrum of bacteriorhodopsin

The visible CD spectrum of bacteriorhodpsin (bR) in purple membrane has a negative CD band at ~600 nm and a positive band at ~530 nm and has been variously interpreted as resulting from exciton coupling within the bR trimer, heterogeneity in protein conformation, or the presence of two distinct low-energy electronic transitions in bR. We have performed time-dependent density functional theory (TDDFT) calculations on the protonated Schiff base of retinal (retPSB) in bR to predict the intrinsic CD. The resulting spectroscopic parameters have been used to predict the long-wavelength CD spectrum of retPSB trimers. TDDFT, exciton theory, and classical polarizability (DeVoe) predict a strong negative couplet centered near 570 nm, with a magnitude in good agreement with experiment. Coupling of the retPSB chromophore with aromatic and peptide chromophores has been considered by means of perturbation theory and is responsible for the net positive CD of the 570 nm band. The visible CD spectrum of bR is dominated by exciton interactions.