Conformation and reactivity of DNA: III. Circular dichroism studies of the effects of aqueous concentrated univalent salt solutions upon helix conformation

Abstract

CD measurements of (G+C)-rich DNA (72 mole % G+C ) in concentrated salt solutions of various alkali metal ions show that salt-induced conformational changes of the DNA double helix, already previously reported, are dependent on the nature of the monovalent ion. Lithium salts are most effective in changing the CD spectrum of DNA compared to Cs +, Na +, K + or NH 4 +. Up to 7.5 M LiCl the CD spectrum of (G+C)-rich DNA exhibits a larger variation of the amplitude at 282 nm and smaller changes at 267 nm. The positive CD band at 270 nm observed at low salt concentrations splits into two CD maxima at extremely high molarities of lithium salts: one negative around 282 nm and one positive at 267 nm. The amplitudes at 282 and 267 nm are lowered monotonically with increasing salt molarity as demonstrated for various lithium salts and CsCl. Differences in the characteristic depression of the positive CD maximum of DNA have been found between the salt systems LiCl, LiClO 4 and lithium acetate. This anionic effect on DNA conformation is larger in the systems of NaCl, NaClO 4 and sodium acetate. The results are interpreted in terms of changes in the DNA helix conformation from B to a C-like structure, involving alterations in winding angle between base pairs. The order of effectivity is not according to the sequence of ionic radii of the alkali ions. The hydration tendencies of the univalent cations are considered to affect the hydration of the DNA conformation to different extents. These alterations cannot simply be explained by the lowering of the water activity. It is suggested that electrostatic neutralization of phosphate charges primarily influences the hydration of the helix at low salt molarity. At higher salt molarities decreasing water activity will also appreciably influence the properties of the bulk water of the hydrated DNA.