Exciton Band Structure in Bacterial Peripheral Light-Harvesting Complexes

The variability of the exciton spectra of bacteriochlorophyll molecules in light-harvesting (LH) complexes of photosynthetic bacteria ensures the excitation energy funneling trend toward the reaction center. The decisive shift of the energies is achieved due to exciton spectra formation caused by the resonance interaction between the pigments. The possibility to resolve the upper Davydov sub-band corresponding to the B850 ring and, thus, to estimate the exciton bandwidth by analyzing the temperature dependence of the steady-state absorption spectra of the LH2 complexes is demonstrated. For this purpose a self-modeling curve resolution approach was applied for analysis of the temperature dependence of the absorption spectra of LH2 complexes from the photosynthetic bacteria <i>Rhodobacter (Rba.) sphaeroides</i> and <i>Rhodoblastus (Rbl.) acidophilus</i>. Estimations of the intradimer resonance interaction values as follows directly from obtained estimations of the exciton bandwidths at room temperature give 385 and 397 cm<sup>–1</sup> for the LH2 complexes from the photosynthetic bacteria <i>Rba. sphaeroides</i> and <i>Rhl. acidophilus</i>, respectively. At 4 K the corresponding couplings are slightly higher (391 and 435 cm<sup>–1</sup>, respectively). The retained exciton bandwidth at physiological conditions supports the decisive role of the exciton coherence determining light absorption in bacterial light-harvesting antenna complexes.