Short-Range Exciton Couplings in LH2 Photosynthetic Antenna Proteins Studied by High Hydrostatic Pressure Absorption Spectroscopy

Abstract

The effects of high hydrostatic pressure (up to 8 kbar) on bacteriochlorophyll a Qy electronic absorption bands of LH2 photosynthetic antenna complexes have been studied at ambient temperature. A variety of samples were studied, including intact membranes and isolated complexes from wild type and mutant photosynthetic bacteria Rhodobacter sphaeroides, Rhodopseudomonas acidophila, and Rhodospirillum molischianum. The spectra of the complexes universally red shift and broaden under elastic compression, while the variations of the integrated intensity remain within the experimental uncertainty. A qualitatively different slope and variation of the slope of the pressure-induced shift is observed for the B800 and B850 absorption bands of LH2 complexes belonging to quasi-monomer and aggregated pigments, respectively. For the complexes from Rhodobacter sphaeroides, e.g., the corresponding slopes are −28 ± 2 and −65 ± 2 cm-1/kbar. The shift rate of the B800 band declines with pressure, while the opposite is observed for the B850 band. The shifts show little if any correlation with hydrogen bonds. Using simple phenomenological arguments and numerical simulations of molecular exciton spectra, it is shown that the shift of the B800 band is governed by pigment−protein interactions, while in addition to that, interpigment couplings (including long-range dipolar and short-range orbital overlap interactions) are instrumental for the B850 band shift. The compressibility of the B800 bacteriochlorophyll binding sites deduced from the B800 band shift at ambient pressure is ∼0.02 kbar-1, and it decreases nonlinearly with pressure. Inter-pigment couplings are responsible for approximately one-third of both the total ambient-pressure solvent shift of the B850 absorption band and its pressure-induced growth. A slight increase with pressure of the B850 band shift due to orbital overlap couplings is predicted.