Stacking of Macrocycles in Bacteriochlorophyll c Aggregates and in Chlorosomes

Abstract

Green photosynthetic bacteria have an antenna complex called ‘chlorosomes’ that contain bacteriochlorophyll c (BChl c) as the major component, and the self-assembly of the BChl c molecules is supposed to play a key role in forming the chlorosome structure [1]. One of the strategies to be chosen for the elucidation of the assembly of BChl c in chlorosomes is structural determination of better-defined in vitro aggregates followed by structural correlation with the real chlorosomes on the basis of spectral comparison. In vitro aggregate forms of BChl c have been classified by the wavelength of the Qy absorption [2,3]: (1) a dimer absorbing around 675 nm, (2) a ‘lower aggregate’ absorbing around 705 nm and (3) a ‘higher aggregate’ absorbing above 740 nm. Here, we note that no definitive information has been available concerning the size of these aggregates except for the dimer. Hereafter, we will call them ‘the B675 dimer’, ‘the B705 aggregate’ and ‘the B745 aggregate’ according to the Q_y absorption. The composition of these aggregate forms in a BChl c suspension depends on the stereoisomeric configuration of the hydroxyethyl group and the sidechains attached to 8 and 12 carbons of the BChl c molecule, and the BChl c concentration and the solvent in the suspension.