Synthesis of methyl C132-alkylated pheophorbides a/a′ and their stereochemical characterization

Intramolecular axial α/β-coordination of the 132-terminal pyridyl group to the central zinc atom in chlorophyll-a derivatives

Chlorophyll(Chl)-a derivatives possessing a zinc center and a C13²-alkanoate residue with a terminal pyridyl group were synthesized. Their C13²-epimerically pure products were isolated by preparative reverse phase HPLC. The C13²-stereochemistry resulted in two directed terminal pyridinyl groups that coordinate with the central zinc atom in each stereoisomeric molecule: α/β-intramolecular axial coordination. The asymmetric axial coordinations mimic the immobilization manner of Chl-a in photosynthetically active proteins. The diastereomerically dependent conformers in solution were characterized by 1D/2D NMR, UV-visible absorption, and fluorescence emission as well as circular dichroism (CD) spectroscopy. The ¹H NMR and DOSY spectra revealed that the stereoselectively intramolecular coordination occurred in less coordinative deuterated chloroform, whereas a highly coordinative deuterated pyridine molecule replaced the terminal pyridine moiety as the axial ligand to form a mixture of α/β-coordinated species. Their optical spectra in pyridine were nearly independent of the C13²-stereochemistry and the linkers in the C13²-substituents. The CD bands of β-coordinated species in chloroform were more intense than those of the corresponding α-coordinated stereoisomers, indicating that the former had a larger distortion of the chlorin π-plane than the latter. Therefore, the α-coordinated Chl complex is more conformationally stable than the β-complex.

In vitro C132-dealkoxycarbonylations of zinc chlorophyll a derivatives including C132-substitutes by a BciC enzyme

Chlorosomes in the green photosynthetic bacteria are the largest and most efficient light-harvesting antenna systems of all phototrophs. The core part of chlorosomes consists of bacteriochlorophyll c, d, e, or f molecules. In their biosynthetic pathway, a BciC enzyme catalyzes the removal of the C13²-methoxycarbonyl group of chlorophyllide a. In this study, in vitro C13²-dealkoxycarbonylations of zinc chlorophyll a derivatives bearing a methyl-, ethyl- or propyl-esterifying group and its methyl ester analogs with additional alkyl and hydroxy groups at the C13²-position were examined using the BciC enzyme. The BciC-catalyzed reaction activity for the C13²-methoxycarbonylated substrate was comparable to that for the ethoxycarbonylated compound; however, depropoxycarbonylation did not proceed. The BciC enzymatic demethoxycarbonylation of zinc methyl C13²-alkylated pheophorbides a was gradually suppressed with the elongation of the alkyl chain and finally became inactive for the propyl substrate. The reaction of the C13²-hydroxylated substrate (allomer) was accelerated compared to that of the C13²-methyl analog possessing a similar steric size, and gave the corresponding C13²-oxo product via further air-oxidation. All of the abovementioned enzymatic reactions occurred for one of the C13²-epimers with the same configuration as in chlorophyllide a. The above substrate specificities and product distributions indicated the stereochemistry and size of the BciC enzymatic active site (pocket).