Intramolecular axial α/β-coordination of the 13 2 -terminal pyridyl group to the central zinc atom in chlorophyll- a derivatives.

Chlorophyll(Chl)- a derivatives possessing a zinc center and a C13 2 -alkanoate residue with a terminal pyridyl group were synthesized. Their C13 2 -epimerically pure products were isolated by preparative reverse phase HPLC. The C13 2 -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 1 H 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 C13 2 -stereochemistry and the linkers in the C13 2 -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.