Pure and scrambled self-aggregates prepared with zinc analogues of bacteriochlorophylls c and d.

Abstract

Zinc analogues of bacteriochlorophylls c and d self-assembled in aqueous media with phospholipids. A methanol solution of zinc chlorin and alpha-lecithin was put in a cellulose tube and the inner methanol solvent was gradually replaced with water by dialysis to form the self-assembled oligomers. Visible absorption spectra of the aqueous solution showed that zinc chlorins formed J-aggregates within the hydrophobic core of alpha-lecithin assemblies and that the supramolecular structure of the aggregates depended upon the stereochemistry at the 3(1)-position and the alkyl substituents at the 8-, 12-, and 17(4)-positions of the zinc chlorin. When the aqueous aggregates were prepared with a mixture of 3(1)-epimers and/or 8-, 12-, or 17(4)-homologues of zinc 3(1)-hydroxy-13(1)-oxochlorins, the structurally distinct components coaggregated to make scrambled oligomers. However, during the dialysis, zinc 3(1)-hydroxy- and 7(1)-hydroxy-13(1)-oxochlorins slowly individually aggregated to give two structurally different oligomer units in the cellulose tube. In contrast, if the two zinc chlorin components rapidly self-assembled in an aqueous medium, these components coaggregated to form scrambled oligomers. The present study shows that both the molecular structure of the pigments and the speed of the oligomerization determine the molecular arrangement in chlorosome-type self-assembled oligomers.