Synthesis of galactosylated zinc bacteriochlorophyll-d analogs and their self-aggregation in an aqueous methanol solution

Chlorophyll derivatives possessing a galactosyl moiety at the 17-substituent were prepared by condensation of a carboxy group in the 17-substituent of chlorin chromophores with an amino group on the galactosides having four unprotected hydroxy moieties. Synthetic zinc 3-hydroxymethyl-131-oxo-chlorins covalently linked with the galactosyl group through a spacer of various lengths self-aggregated in an aqueous methanol solution to give a slipped cofacial dimer and large oligomers with a red-shifted Qy absorption band. The resulting oligomers were similar to J-aggregates of bacteriochlorophyll-d in main light-harvesting antennas of green photosynthetic bacteria. Electronic absorption spectral analyses of the hydrophilic chlorophyll derivatives in the aqueous solution indicated that their self-aggregation was dependent on the spacer; the shorter afforded their dimers predominantly and the longer disturbed the formation of chlorosomal large oligomers.