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ABSTRACT The bacteriochlorophyll (Bchl) c content and organization was determined for Chlorobium (Cb.) tepidum chlorosomes, the light-harvesting complexes from green photosynthetic bacteria, using fluorescence correlation spectroscopy and atomic force microscopy. Single-chlorosome fluorescence data was analyzed in terms of the correlation of the fluorescence intensity with time. Using this technique, known as fluorescence correlation spectroscopy, chlorosomes were shown to have a hydrodynamic radius (Rh) of 25 + or - 3.2 nm. This technique was also used to determine the concentration of chlorosomes in a sample, and pigment extraction and quantitation was used to determine the molar concentration of Bchl c present. From these data, a number of ~215,000 + or - 80,000 Bchl c per chlorosome was determined. Homogeneity of the sample was further characterized by dynamic light scattering, giving a single population of particles with a hydrodynamic radius of 26.8 + or - 3.7 nm in the sample. Tapping-mode atomic force microscopy (TMAFM) was used to determine the x,y,z dimensions of chlorosomes present in the sample. The results of the TMAFM studies indicated that the average chlorosome dimensions for Cb. tepidum was 174 + or - 8.3 x 91.4 + or - 7.7 x 10.9 + or - 2.71 nm and an overall average volume 90,800 nm^sup 3^ for the chlorosomes was determined. The data collected from these experiments as well as a model for Bchl c aggregate dimensions was used to determine possible arrangements of Bchl c oligomers in the chlorosomes. The results obtained in this study have significant implications on chlorosome structure and architecture, and will allow a more thorough investigation of the energetics of photosynthetic light harvesting in green bacteria.
INTRODUCTION
Green bacteria contain complexes known as chlorosomes as their main light-harvesting complex, but unlike most light-harvesting complexes, proteins are of secondary importance in determining pigment geometry in the chlorosome (Blankenship et al., 1995; Griebenow and Holzwarth, 1989; Prokhorenko et al., 2000). Chlorosomes utilize a pigment-pigment architecture instead of pigment-protein scaffolding to create large oligomers that fill the interior of the chlorosome (Blankenship et al., 1995; Brune et al., 1987; Griebenow and Holzwarth, 1989). Along the base of the chlorosome is a pigment-protein complex known as the baseplate. It serves as an intermediate in energy transfer to the reaction center. On...