Isopycnic sedimentation of DNA in metrizamide: the effect of low concentrations of ions on buoyant density and hydration.

Metrizamide, an inert, non-ionic organic compound, dissolves in water to give a dense solution in which DNA bands isopycnically at a density corresponding to that of fully hydrated DNA. Density-gradient centrifugation in solutions of metrizamide has been used to determine the effects of very dilute solutions of salts on the buoyant density of native and denatured DNA. It has been shown that the buoyant density of DNA is dependent on both the counter-cation and the anion present. Interpretation of the data in terms of the degree of hydration of the macromolecule indicates that (i), NaDNA is more highly hydrated than CsDNA; and (ii), the hydration of NaDNA varies with anion in the order sulphate< fluoride< chloride< bromide< iodide. It is suggested that isopycnic centrifugation in metrizamide is a simple method for determining the effects of salts (and other small molecules) on the hydration of nucleic acids under conditions of high ratios of salt to DNA (> 5 x 10(3) moles/mole) while high (0.999) water activity is maintained.