THE ENNIATIN IONOPHORES. CONFORMATION AND ION BINDING PROPERTIES

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Abstract
The ability of enniatins to bind alkali metal, alkaline earth metal and certain transition metal ions in solution has been investigated. The complexes have been shown to form in 1:1, 2:1 and 3:2 macrocycle:cation ratios; their stability constants have been determined, and the conditions favoring their formation have been established. The enniatins were shown to be complexones of very wide spectrum, displaying low metal ion selectivity in alcoholic solutions.Two basic spatial forms, the non‐symmetric N3 form (in nonpolar solvents) and the symmetric P form (in polar solvents) have been disclosed and proposals made as to their conformational parameters. The molecular structure of the enniatin complexes has been established. Independent of the complexed ion species, of the complex stoichiometry, or of the type of solvent, the enniatin backbone of the complex is in the P form. In the 1:1 complexes, the ion is accommodated in the molecular cavity, forming iondipole bonds with all the amide and ester carbonyls. The depsipeptide chain of the enniatins is flexible, permitting “adaptation” of the complexone to the size of the ion, and thus explaining the low selectivity of these complexones. In the 2:1 and 3:2 complexes the cation is sandwiched in between two molecules of the antibiotic at the symmetry axis and interacts mainly with the N‐methylamide carbonyls. In these “sandwich” complexes the cation is much better screened from the anion and solvent than in the equimolar complexes and is highly soluble in organic solvents.The data presented here may serve as basis for interpreting the dependence between the structure, metal complexing behavior and membrane‐affecting properties in the series of the naturally occurring enniatin ionophores and their synthetic analogs.