マクロライド系抗生物質の構造と機能

概要

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Macrocyclic antibiotics can bind alkali metal ions forming 1: 1 complexes, and act as cation-carriers (ionophores) passing through biological and artificial membranes. Molecular structures in crystals and conformational states in solutions are described for valinomycin and nactins, both in free and complexed states. Valinomycin molecule (oval shaped) possesses an approximate symmetry of 1 and six intramolecular H-bonds. On complexation, two H-bonds of 13-membered-ring are disrupted and new ones are formed to complete complex molecule of pseudo C3 symmetry with minor conformational change. In polar solvent, only three H-bonds are observed for K+-valinomycin complex, indicating that the molecule is widely opened to release K+ as ion-pump. In crystals, molecular shape of nonactin (‘torus’, C2 and an approximate S4 symmetry, less stable conformation) is markedly different from that of tetranactin (‘propeller’, C2 symmetry, stable), but they exhibit similar IR and NMR spectra in solutions which indicate molecular symmetry 4. Interconversion of the conformation of free and complexed nactins may be readily achieved by intramolecular motion. Energy differences in intra-ligand-steric-interaction among the complexes of nactins with alkali metal ions are rather small in comparison with those of ion-dipole interactions and hydration energies of cations.
日本結晶学会の論文