Photooxidative cleavage of zinc 20-substituted chlorophyll derivatives: conformationally P-helix-favored formation of regioselectively 19–20 opened linear tetrapyrroles

Abstract

Photoreaction of zinc methyl 20-substituted meso(pyro)pheophorbide-a prepared by modifying naturally occurring chlorophyll-a in the presence of oxygen molecules gave its C19–C20 oxidative cleavage (1-carbonyl-19-oxo-bilatrienes) as the major products and the regioisomeric C1–C20 cleavage (19-carbonyl-1-oxo-bilatrienes) as the minor products. The resulting zinc complexes of linear tetrapyrroles took a helical conformation and the P-conformers were preferential over the M-stereoisomers due to the presence of their 17S,18S-chiral centers. The helical conformers (diastereomers) of the corresponding nickel complexes were separated by reverse-phase or chiral HPLC and their conformational changes were observed in solution.