This paper presents a theoretical study of the excitonic spectra of a hexagonal 2D lattice of trimers (the structure of bacteriorhodopsin – BR). Formalism of dielectric permittivity and of gyration tensor allows to calculate the components of both tensors near the frequency of electronic excitation of retinal with λ≈ 570 nm. The two cases of oblique and normal incidence of left- and right-circularly polarized electromagnetic waves have been considered. In accordance with the experimental data at oblique incidence a negative circular dichroism (CD) couplet appears connected with the chiral excitonic coupling. At perpendicular incidence a single CD band of Lorentz type is caused by the internal gyrotropy of the molecules.

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