A theoretical analysis of absorption spectra of photosynthetic reaction centers: Mechanism of temperature dependent peak shift

Detailed analysis of the absorption spectra of the R26.Phe-a mutant RCs at various temperatures has been presented constructing a model Hamiltonian based on the previous spectral analysis results. The temperature dependence of the electronic asymmetry and the coupling between the Bchls in the special pair can be obtained by using the thermal expansion model. The major contributions to the energy gaps between the exciton states of the special pair result from the protein-induced electronic asymmetry Delta and the intra-dimer coupling beta. The values of Delta and beta are obtained to be 147 and 405.19 cm(-1) at room temperature, respectively, and they increase with a decrease in temperature. Peak intensity ratios of each electronic state to the lowest electronic state at various temperatures are calculated using the delocalized basis set. The absorption spectra at 77 and 278 K have been theoretically predicted by using the model presented in this paper. Anisotropy is also studied using both delocalized and dimer basis set. The anisotropy values are found to depend on a choice of basis sets.