Self-Aggregates of Synthetic Zinc 20-Substituted Chlorins as Models for Chlorosomal Photosynthetic Antennae

Abstract

Green photosynthetic bacteria possess unique extramembranous light-harvesting antenna systems (=chlorosomes). In naturally occurring chlorosomes, major chlorophyllous pigments self-aggregate to form oligomers which are characteristic of red-shifted absorption bands compared with the corresponding monomers [1]. The pigments are several homologues of magnesium complexes of 3¹-hydroxy-13¹-oxochlorins (=bacteriochlorophyll(=BChl)s-c and d, see Figure 1). The variation of the substituents at the 20-position affects the supramolecular structures of the self-aggregates and also energy migration in the oligomer. Here we report on the synthesis of zinc 20-substituted chlorins and the visible spectra of the monomers and oligomers. Synthetic zinc chlorins self-aggregated in non-polar organic solvents to form similar oligomeric supramolecules with 20-unsubstituted and methylated compounds, indicating that the in vitro aggregate are good structural models for chlorosomal BChl-cld aggregates.