Excitonic model and circular dichroism spectrum of bacteriorhodopsin

Abstract

This paper presents a theoretical study of the excitonic spectra of a hexagonal two-dimensional lattice of trimers of bacteriorhodopsin (BR). The investigations follow the treating of the electronic excitations in molecular and organic solids. Formalism of dielectric permittivity and of gyration tensor allows to calculate their components near the frequency of electronic excitation of chromophore of retinal with λ ≈ 570 nm. The excitonic spectra of noninteracting trimers and their demonstration in the linear absorption and circular dichroism (CD) spectra represent the model of the spectra of hexagonal 2D lattice with excitonic inter-trimer transfers between the nearest neighbors. The two cases of oblique and perpendicular incidence of left- and right-circularly polarized electromagnetic waves have been considered and corresponding CD signals have been calculated. In accordance with the experimental data a negative CD couplet appears (at oblique incidence) connected with the chiral excitonic coupling whereas at perpendicular incidence a single CD band of Lorentz-type is caused by the intrinsic gyrotropy of the molecules.