Chirality distribution in the fifth ligand coordination of chlorophylls-c in photosynthetic pigment-protein complexes.

Chlorophylls (Chls) and bacteriochlorophylls (BChls) in photosynthetic organisms are embedded in pigment-protein complexes, mostly through the coordination of peptidyl residues to their central metal atoms. An asymmetry occurs at the axially coordinated metal centers of (B)Chls. The stereochemistry of the ligand coordination toward 42 molecules of Chls-c, the fully π-conjugated porphyrin pigments lacking C17- and C18-chiralities and a long ester chain in the C17-substituent, was surveyed across 21 pigment-protein complexes registered in the Protein Data Bank. It was found that 24 of 42 Chl-c molecules in these complexes were coordinated from the β-side of the macrocyclic π-plane, revealing a tendency opposite to the preferential α-ligation observed for the structurally different (B)Chls with a (bacterio)chlorin π-system in other proteins (129/163 surveyed sites). The most common binding sites for Chl-c molecules in the proteins correspond to the a603 and a612 sites in Chl-a/b-binding light-harvesting complex II (LHCII), both of which accommodate β-epimers. In some cases, a carotenoid molecule contacts Chl-c to occupy the space where the C172-ester group of the Chl-a/b molecule is situated at the corresponding sites in LHCII, and its position of the carotenoid molecule is related to the macrocycle side of the axial ligand coordination of Chl-c. The absence or presence of chirality and bulky substituents at the C17-position of (B)Chls may be concerned with the stereochemistry of the axial coordination and the formation of these pigment-protein complexes.