The stereochemistry of chlorophyll- c3 from the haptophyte Emiliania huxleyi: The (13 2R)-enantiomers of chlorophylls- c are exclusively selected as the photosynthetically active pigments in chromophyte algae

Abstract

Chlorophyll(Chl)- c pigments in algae, diatoms and some prokaryotes are characterized by the fully conjugated porphyrin π-system as well as the acrylate residue at the 17-position. The precise structural characterization of Chl- c 3 from the haptophyte Emiliania huxleyi was performed. The conformations of the π-conjugated peripheral substituents, the 3-/8-vinyl, 7-methoxycarbonyl and 17-acrylate moieties were evaluated, in a solution, using nuclear Overhauser enhancement correlations and molecular modeling calculations. The rotation of the 17-acrylate residue was considerably restricted, whereas the other three substituents readily rotated at ambient temperature. Moreover, the stereochemistry at the 13 2-position was determined by combination of chiral high-performance liquid chromatography (HPLC) with circular dichroism (CD) spectroscopy. Compared with the CD spectra of the structurally related, synthetic (13 2 R)- and (13 2 S)-protochlorophyllide(PChlide)- a, naturally occurring Chl- c 3 had exclusively the (13 2 R)-configuration. To elucidate this natural selection of a single enantiomer, we analyzed the three major Chl- c pigments (Chl- c 1, c 2 and c 3) in four phylogenetically distinct classes of Chl- c containing algae, i.e., heterokontophyta, dinophyta, cryptophyta and haptophyta using chiral HPLC. All the photosynthetic organisms contained only the (13 2 R)-enantiomerically pure Chls- c, and lacked the corresponding enantiomeric (13 2 S)-forms. Additionally, Chl- c 2 was found in all the organisms as the common Chl- c. These results throw a light on the biosynthesis as well as photosynthetic function of Chl- c pigments: Chl- c 2 is derived from 8-vinyl-PChlide- a by dehydrogenation of the 17-propionate to acrylate residues as generally proposed, and the (13 2 R)-enantiomers of Chls- c function as photosynthetically active, light-harvesting pigments together with the principal Chl- a and carotenoids.