SHIFTING THE EQUILIBRIUM MIXTURE OF GRAMICIDIN DOUBLE HELICES TOWARD A SINGLE CONFORMATION WITH MULTIVALENT CATIONIC SALTS

The conformation of the polypeptide antibiotic gramicidin is greatly i nfluenced by its environment. In methanol, it exists as an equilibrium mixture of four interwound double-helical conformers that differ in t heir handedness, chain orientation, and alignment. Upon the addition o f multivalent cationic salts, there is a shift in the equilibrium to a single conformer, which was monitored in this study by circular dichr oism spectroscopy. With increasing concentrations of multivalent catio ns, both the magnitude of the entire spectrum and the ratio of the 229 -nm to the 210-nm peak were increased. The spectral change is not rela ted to the charge on the cation, but appears to be related to the cati onic radius, with the maximum change in ellipticity occurring for cati ons with a radius of similar to 1 Angstrom. The effect requires the pr esence of an anion whose radius is greater than that of a fluoride ion , but is otherwise not a function of anion type. It is postulated that multivalent cations interact with a binding site in one of the confor mers, known as species 1 (a left-handed, parallel, no stagger double h elix), stabilizing a modified form of this type of structure.