Cyclodepsipeptides as chemical tools for studying ionic transport through membranes

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Abstract
This paper reports a study of the chemistry of valinomycin, enniatins and related membrane-active depsipeptides that increase alkali metal ion permeability of model and biological membranes. The antimicrobial activity of these compounds and their effect on membranes has been correlated with their cation-complexing ability. The complexing reaction has been studied by spectropolarimetric and conductimetric methods. Nuclear magnetic resonance, optical rotatory dispersion, and infrared spectrophotometric studies have revealed the coexistence of conformers of the cyclodepsipeptides in solution and have led to elucidation of the spatial structure of valinomycin, enniatin B and their K+ complexes. The effect of the conformational properties of the cyclodepsipeptides on their complexation efficiency and selectivity, surface-active properties and behavior towards phospholipid monolayers, bimolecular phospholipid membranes and a number of biological membrane systems has been ascertained. The studies have clearly shown the feasibility of using cyclodepsipeptides with predetermined structural and conformational parameters as chemical tools for membrane studies. it is suggested that the principle of conformation-dependent cation binding through iondipole interactions may possibly lie at the basis of the mode of action of systems governing the natural ion permeability in biological membranes.