Dichotomous Disorder versus Excitonic Splitting of the B800 Band of Allochromatium vinosum.

The LH2 antenna complex of the purple bacterium Allochromatium vinosum has a distinct double peak structure of the 800 nm band (B800). Several hypotheses were proposed to explain its origin. Recent 77 K two-dimensional electronic spectroscopy data suggested that excitonic coupling of dimerized bacteriochlorophylls (BChls) within the B800 ring is largely responsible for the B800 split [M. Schröter et al., J. Phys. Chem. Lett. 2018, 9, 1340]. Here we argue that the excitonic interactions between BChls in the B800 ring, though present, are weak and cannot explain the B800 band split. This conclusion is based on hole-burning data and modeling studies using an exciton model with dichotomous protein conformation disorder. Therefore, we uphold our earlier interpretation, first reported by Kell et al. [ J. Phys. Chem. B 2017, 121, 9999], that the two B800 sub-bands are due to different site-energies (most likely due to weakly and strongly hydrogen-bonded B800 BChls).