Temperature-dependent chlorosomal self-aggregation of bacteriochlorophyll-d analogs with a branched alkyl chain in a single 1-chlorooctane solvent

Recently, the supramolecular polymerization of chlorophyll pigments mimicking a natural light-harvesting apparatus (chlorosome) was demonstrated in low-polar organic solvents or aqueous solutions. To obtain the most aggregation models, a concentrated solution of the pigments in a polar organic solvent was diluted with a large amount of a nonpolar organic solvent or water. Here, bacteriochlorophyll-d analogs possessing branched alkyl chains of different lengths at the peripheral 17-propionate residue on the core chlorin pi-system were prepared and their highly soluble chlorosomal supramolecules were produced in single 1-chlorooctane as a low-polar solvent. Temperature-dependent electronic absorption and circular dichroism spectroscopies were employed to analyze their self-aggregation and disaggregation mechanisms. The synthetic analogs were monomeric at high temperatures and self-aggregated during cooling of the hot solution through a non-sigmoidal pathway. The obtained chlorosomal self-aggregates were reversibly monomerized by heating the homogeneous solution. The disaggregation pathway was fitted to an isodesmic model whose melting points depended on the alkyl-chain lengths. Graphical Abstract In a single 1-chlorooctane solvent, synthetic branched BChl-d analogs self-aggregate and their self-aggregation behaviors were analyzed by using temperature-dependent circular dichroism spectra. The cooling aggregation was fitted to a non-sigmoidal pathway with an elongation temperature and thermodynamic parameters that were independent of the alkyl-chain lengths. The heating disaggregation was fitted to a sigmoidal pathway with melting temperatures dependent on the alkyl-chain length.