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

• Published in: Biophysical journal 2000-09, Vol.79 (3), p.1561-1572
• Main Authors: Wu, H.-M., Rätsep, M., Young, C.S., Jankowiak, R., Blankenship, R.E., Small, G.J.
• Format: Article
• Language: English
• Subjects: Bacteria; Bacterial Proteins - chemistry; Bacteriochlorophylls; Biochemistry; Chlorobi - physiology; Chlorobium tepidum; Light-Harvesting Protein Complexes; Organelles - physiology; Photosynthetic Reaction Center Complex Proteins - chemistry; Photosynthetic Reaction Center Complex Proteins - metabolism; Polyvinyls; Pressure; Space life sciences; Spectrophotometry
• ISSN: 0006-3495; 1542-0086
• DOI: 10.1016/S0006-3495(00)76407-1

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.