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Biophys J, September 2000, p. 1561-1572, Vol. 79, No. 3

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

H.-M. Wu,* M. Rätsep,dagger C. S. Young,* R. Jankowiak,dagger R. E. Blankenship,* and G. J. Smalldagger

 *Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287, and  dagger Ames Laboratory- U.S. Department of Energy and Department of Chemistry, Iowa State University, Ames, Iowa 50011 USA

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 Qy-band at 100 K and the lowest Qy-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, f Delta µ = 0.35 D, where Delta µ is the dipole moment change for the Qy transition and f  is the local field correction factor. The data establish that Delta µ is dominated by the matrix-induced contribution. The change in polarizability (Delta alpha ) for the Qy transition of the BChl c monomer is estimated at 19 Å3, 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 105 V/cm). Possible explanations for this are given, and these include consideration of structural models for the chlorosome BChl c aggregates.

Biophys J, September 2000, p. 1561-1572, Vol. 79, No. 3
© 2000 by the Biophysical Society   0006-3495/00/09/1561/12  $2.00


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