Conformational Complexity in the LH2 Antenna of the Purple Sulfur Bacterium Allochromatium vinosum Revealed by Hole-Burning Spectroscopy.

This work discusses the protein conformational complexity of the B800-850 LH2 complexes from the purple sulfur bacterium Allochromatium vinosum, focusing on the spectral characteristics of the B850 chromophores. Low-temperature B850 absorption and the split B800 band shift blue and red, respectively, at elevated temperatures, revealing isosbestic points. The latter indicates the presence of two (unresolved) conformations of B850 bacteriochlorophylls (BChls), referred to as conformations 1 and 2, and two conformations of B800 BChls, denoted as B800 and B800. The energy differences between average site energies of conformations 1 and 2, and B800 and B800 are similar (∼200 cm), suggesting weak and strong hydrogen bonds linking two major subpopulations of BChls and the protein scaffolding. Although conformations 1 and 2 of the B850 chromophores, and B800 and B800, exist in the ground state, selective excitation leads to 1 → 2 and B800 → B800 phototransformations. Different static inhomogeneous broadening is revealed for the lowest energy exciton states of B850 (fwhm ∼195 cm) and B800 (fwhm ∼140 cm). To describe the 5 K absorption spectrum and the above-mentioned conformations, we employ an exciton model with dichotomous protein conformation disorder. We show that both experimental data and the modeling study support a two-site model with strongly and weakly hydrogen-bonded B850 and B800 BChls, which under illumination undergo conformational changes, most likely caused by proton dynamics.