Binding of alkaline cations to the double-helical form of gramicidin

Gramicidin is a polypeptide antibiotic that forms monovalent cation-specific channels in membrane environments. In organic solvents and in lipids containing unsaturated fatty acid chains, it forms a double-helical "pore" structure, in which two monomers are intertwined. This form of gramicidin can bind two cations inside its lumen, and the crystal structures of both an ion complex and an ion-free form have been determined. In this study, we have used circular dichroism (CD) spectroscopy to examine the binding mechanism and the binding constants (K1 and K2) of cations to gramicidin in the double helical form in methanol solution. The dramatic change in optical rotation in the far-ultraviolet CD spectrum of gramicidin provides a useful tool for monitoring the binding. The binding mechanism appears to involve a large conformation change associated with the binding of ions to the first of the two sites. The calculated values for the K1 binding constants for alkaline cations are considerably smaller than the K2 binding constants. The order of binding affinity for alkaline cations is similar to that for the helical dimer "channel" form of gramicidin, i.e., Cs+ approximately Rb+ > > K+ > Li+, but in comparison to the helical dimer form, the binding to double-helical dimers is dominated by a cation size-dependent conformational change in the gramicidin structure.