Manifestation of protein conformations in the B850 absorption band of light-harvesting complex LH2

- A modified (dichotomous) exciton model of two conformational states is developed.
- The equilibrium between conformational states satisfies the Boltzmann distribution.
- The modified exciton model describes the peak shift of absorption spectra of LH2.

As experimentally established the absorption bands corresponding to the B850 ring of LH2 complexes from Rhodoblastus acidophilus and Rhodobacter sphaeroides broaden and shift by changing the temperature. The peak shift with the temperature cannot be explained in terms of the conventional disordered exciton model, therefore the modified (dichotomous) exciton model is formulated. The modified exciton model, which assumes the presence of two conformational states for every bacteriochlorophyll molecule constituting the B850 ring, is postulated. Since the conformational states are characterized by their free energies, the population ratio of the conformational states is temperature dependent. As the result, the disorder becomes also temperature dependent and, thus, both the broadening and the shift of the B850 absorption bands are explained within the frame of the modified exciton model.