Effects of positional disorder on optical absorption spectra of light-harvesting antenna complexes in photosynthetic bacteria

We study theoretically the influence of both diagonal and off-diagonal disorder on the absorption spectra of the light-harvesting antenna complex LH2 consisting of two circular aggregates, B850 and B800, of bacteriochlorophyll pigments in photosynthetic purple bacteria. Off-diagonal disorder, i.e., randomness in excitonic couplings between molecules, is introduced by a model of disorder in the position of each pigment molecule embedded in proteins. We demonstrate that a large contribution of positional disorder provides a natural explanation for the experimental fact that the excitonic B850 absorption peak is broader than that of monomeric B800 in spite of motional narrowing.