SPECTRAL FORMS AND ORIENTATION OF BACTERIOCHLOROPHYLLS c AND α IN CHLOROSOMES OF THE GREEN PHOTOSYNTHETIC BACTERIUM Chloroflexus aurantiacus

Abstract

Spectral forms of bacteriochlorophyll (Bchl) in chlorosomes were analyzed by linear dichroism, circular dichroism (CD), and deconvolution of these spectra. Isolated chlorosomes were embedded in polyacrylamide gels and compressed unidirectionally (along the x-axis) while allowing the gel to stretch in another direction (along the z-axis). The chlorosomes were aligned three-dimensionally due to their Rat oblong shape; the longest axis was presumed to parallel the z-axis, its shortest axis was presumed to parallel the x-axis, and the intermediate-length axis was presumed to parallel the y-axis. Degrees of polarization (A,, -A,)/(A, + A,) of Bchl c and a measured from the y-axis with linearly polarized light were significantly different from those measured from the x-axis. Deconvolution of spectra into components revealed the presence of two major forms of Bchl c with peaks at 744 nm and 727 nm. The degrees of polarization of the 744 and 727 nm spectral forms were 0.76 and 0.59 from the y-axis and 0.48 and 0.39 from the x-axis, respectively. The degrees of polarization of Bchl a794 were -0.21 from the y-axis and 0.12 from the x-axis. These values indicate that the direction of the Q, transition moment of Bchl c744 is almost completely parallel to the longest axis of chlorosomes and that of Bchl c727 is also nearly, but slightly less so, parallel to the longest axis of the chlorosomes. The Q, transition moment of the baseplate Bchl a peak at 794 nm is nearly perpendicular to the longest axis and parallel to the shortest axis: that is, it is perpendicular to the associated membrane plane in the cell. These alignments of Bchl transition moments in chlorosomes were lost after suspending the chlorosomes in a solution saturated with 1-hexanol accompanying a shift in the peak position from 742 nm to 670 nm. The alignment recovered after the hexanol concentration was decreased. The presence of two major spectral forms of Bchl c was supported by the deconvolution of CD spectra and absorption spectra.

Presence and significance of minor antenna components in the energy transfer sequence of the green photosynthetic bacterium Chloroflexus aurantiacus

“…We found six fluorescence components in this study and suggested the presence of a short wavelength form of Bchl c. The last one may correspond to Bchl ~727 which we observed by LD spectra [6]. Besides these, Bchl a806 [24] is known to be present.…”

“…Thus, the number of spectral components has now been estimated to eight in C. aurantiacus. Two major Bchl c's (Bchl ~727 and Bchl ~744) [6] and the third fluorescence component (Bchl ~766 or F781, in this study) were assigned to be constituents of chlorosomes. Presence of two baseplate Bchl's a (F810 and F819 in this study) are consistent with our previous report [8].…”

“…It is widely accepted that Bchl c's form the selfaggregated functional structure [5,6] which corresponds to rod elements found by electron microscope observation [7]; polypeptides in the chlorosomes are not involved in the formation of the self-aggregated structure. This concept is an exceptional case for the organization of photosynthetic pigment systems where pigment-polypeptide interaction is the main force.…”

“…In our previous study [6], we identified two major Bchl c components as based on linear dichroism (LD) and circular dichroism (CD) spectra of chlorosomes. We also suggested the presence of additional Bchl c components in the longer wavelength region of major Bchl c components in chlorosomes.…”

The Application of Photosynthetic Materials and Architectures to Solar Cells

“…We refrain from an extensive discussion of all antenna types, as excellent reviews can be found elsewhere. [19] Antennas contain high concentrations of pigment molecules, including chloro- While several models have been proposed for the pigment organization in chlorosomes, a common characteristic is the existence of aggregates of bacteriochlorophyll c, either rods of 5-10 nm in diameter and 100-200 nm in length [22,23,24,25,26] or semicrystalline lamellar sheets. [20] The regular structure of Bchl c van der Waals bonded aggregates leads to strong exciton coupling and a red-shift in absorption.…”