High-Pressure and Stark Hole-Burning Studies of Chlorosome Antennas from Chlorobium tepidum

Abstract

Results from high-pressure and Stark hole-burning experiments on isolated chlorosomes from the green sulfur bacterium Chlorobium tepidum are presented, as well as Stark hole-burning data for bacteriochlorophyll c (BChl c) monomers in a poly(vinyl butyral) copolymer film. Large linear pressure shift rates of −0.44 and −0.54 cm −1/MPa were observed for the chlorosome BChl c Q y-band at 100 K and the lowest Q y-exciton level at 12 K, respectively. It is argued that approximately half of the latter shift rate is due to electron exchange coupling between BChl c molecules. The similarity between the above shift rates and those observed for the B875 and B850 BChl a rings of the light-harvesting complexes of purple bacteria is emphasized. For BChl c monomer, ƒΔ μ = 0.35 D, where Δ μ is the dipole moment change for the Q y transition and ƒ is the local field correction factor. The data establish that Δ μ is dominated by the matrix-induced contribution. The change in polarizability (Δ α) for the Q y transition of the BChl c monomer is estimated at 19 Å 3, which is essentially identical to that of the Chl a monomer. Interestingly, no Stark effects were observed for the lowest exciton level of the chlorosomes (maximum Stark field of 10 5 V/cm). Possible explanations for this are given, and these include consideration of structural models for the chlorosome BChl c aggregates.