Synthesis of chlorophyll-a homologs by C13-substitutions and their physico- and biochemical properties.

A mixture of pheophytins-a/a', metal-free forms of photosynthetically active chlorophyll(Chl)s-a/a' bearing the 13-methoxycarbonyl group, was substituted at the C13-position by bimolecular nucleophilic substitution with methyl bromoacetate or Michael addition with methyl acrylate, followed by C13-demethoxycarbonylation and magnesium insertion at the central position, to afford Chl-a/a' homologs possessing a methoxycarbonylmethyl or 2-methoxycarbonylethyl group at the C13-position, respectively. These C13-methylene- and ethylene-inserted homologs were characterized by 1D/2D H NMR spectroscopy, and the optical properties of their C13-epimerically pure samples are investigated using visible absorption, fluorescence emission, and circular dichroism spectroscopies. The stereochemistry at the C13-chiral center of these Chl-a/a' homologs was not inverted in a basic solution, and the Chl-a homologs were effective for the substrates for the chlorophyllase reaction, hydrolysis of the phytyl ester.