Binding of Mg 2+ to single-stranded polynucleotides: hydration and optical studies

Abstract

The binding of Mg 2+ to single-stranded ribo- and deoxy-polynucleotides, poly(rA), poly(rU), poly(dA) and poly(dT), has been investigated in dilute aqueous solutions at pH 7.5 and 20 °C. A combination of ultrasound velocimetry, density, UV and CD spectroscopy have been employed to study hydration and spectral effects of Mg 2+ binding to the polynucleotides. Volume and compressibility effects of Mg 2+ binding to random-coiled poly(rU) and poly(dT) correspond to two coordination bonds probably between the adjacent phosphate groups. The same parameters for poly(rA)+Mg 2+ correspond to an inner-sphere complex with three–four direct contacts. However, almost no hydration effects are arising in binding to its deoxy analog, poly(dA), indicating mostly a delocalized binding mode. In agreement with hydration studies, optical investigations revealed almost no influence of Mg 2+ on poly(dA) properties, while it stabilizes and aggregates poly(rA) single-helix. The evidence presented here indicates that Mg 2+ are able to bind specifically to single-stranded polynucleotides, and recognize their composition and backbone conformation.