Physicochemical and biochemical properties of synthetic zinc 131-(un)substituted chlorophyll-a derivatives

Abstract

Zinc methyl pheophorbides a / a ’, which possess oxo and methoxycarbonyl groups at the C13 1 - and C13 2 -positions, respectively, on the exo -five membered ring fusing the cyclic tetrapyrrole of their chlorin π-skeletons, were prepared via chemical modifications of chlorophyll a produced in phototrophs. The C13 1 -hydroxylated and unsubstituted analogues were synthesized by reduction of the ketone moiety. The C13 1 - and C13 2 -stereochemistry of these semi-synthetic compounds was confirmed by 1 H NMR and circular dichroism spectroscopies. A methoxycarbonyl group at the chiral C13 2 -position in the ketone was stereo-inverted under basic conditions, whereas those in the corresponding alcoholic and dihydro-forms were not. The in vitro BciC enzymatic reaction of the (13 2 R )-ketone was found to stereoselectively hydrolyze the C13 2 -methoxycarbonyl group, followed by spontaneous decarboxylation. Similarly, the (13 2 R )-COOCH 3 group in the (13 1 S )-alcohol was hydrolyzed, whereas the (13 2 R )-COOCH 3 group in the (13 1 R )-alcohol was not. After the enzymatic reaction of the C13 1 -dihydro-compound, its (13 2 S )-demethoxycarbonylated and hydroxylated product was observed. • Zinc 13 1 -hydroxylated and unsubstituted chlorophyll- a derivatives were synthesized. • The 13 1 - and 13 2 -stereochemistry was confirmed by 1 H NMR and CD spectroscopies. • The 13 1 -oxo analog was 13 2 -epimerized, but 13 1 -hydroxy and dihydro forms could not. • A recombinant BciC enzyme stereospecifically hydrolyzed the 13 2 -COOMe in vitro.