Self-aggregation of synthetic zinc methyl 20-substituted 3-hydroxymethyl-pyropheophorbides as models of bacteriochlorophyll-c

Zinc 3-hydroxymethyl-13(1)-oxo-chlorins bearing a variety of primary alkyl groups at the 20-position were prepared as models of bacteriochlorophyll-c by chemical modification of naturally occurring chlorophyll-a. The synthetic chlorophyll-a derivatives self-aggregated in an aqueous Triton X-100 solution to afford large oligomers whose Soret and Qy bands were red-shifted and broadened, compared with the bands of their monomers in tetrahydrofuran. The oligomeric bands are similar to those of bacteriochlorophyll-c self-aggregates in chlorosomes, the main light-harvesting antennae of photosynthetic green bacteria. The 20-alkylation led to bathochromic shifts of the visible Soret maxima in J-type self-aggregates of the synthetic models, while elongation of the 20-alkyl group decreased the chlorosomal Qy maxima due to an increase in steric hindrance. Considering the light-harvesting and energy-transferring processes in a chlorosome, the 20-methylation in bacteriochlorophyll-c would be more suitable for efficient culturing of green bacteria than the 20-ethylation and propylation as well as the 20-unsubstitution in bacteriochlorophyll-d.