Synthesis and Self-Assembly of Zinc Methyl Bacteriopheophorbide- f and its Homolog

Abstract

Zinc methyl bacteriopheophorbide- f, zinc 3-(1-hydroxyethyl)-7-formyl-chlorin, and its 3 1-demethyl derivative were prepared by modification of naturally occurring chlorophyll- b. The former complex was a 3 1-epimeric mixture and separated to pure stereoisomers by HPLC, while such a separation was unnecessary for the latter possessing the 3-hydroxymethyl group. The absolute configuration at the 3 1-position of the separated samples was determined by transformation of the 7-formyl to the 7-methyl group because the 3 1-stereochemistry of zinc methyl bacteriopheophorbide- d (3-(1-hydroxyethyl)-7-methyl-chlorin) has been confirmed. All the synthetic zinc chlorins self-aggregated in 0.5% (v/v) THF–hexane as well as in 6% (v/v) THF–water to form oligomers which absorbed longer-wavelength lights by the J-aggregation than the monomers in THF. Diastereomeric control in the self-aggregation of the 3 1-epimers was observed. The 3 1-demethyl compound interacted more tightly in the self-aggregates than the 3 1-epimers. Comparing with the corresponding 7-methyl analogs, zinc 7-formyl-chlorins have red-shifted Soret and blue-shifted Q y bands in both the monomeric and aggregated states. Visible, circular dichroism and resonance Raman spectra showed that the supramolecular structures of self-aggregates of the 7-formyl and 7-methyl derivatives were similar and highly ordered, and were built-up by 13-CO⋯H(3-CHR)O⋯Zn and π–π interaction of the chlorin chromophores. The coordinatable 7-formyl group did not disturb the special bondings of the composite molecules in the self-aggregates.