DNA hydration studied by pressure perturbation scanning microcalorimetry.

Pressure perturbation differential scanning calorimetry was used to determine thermal expansion coefficients and thus temperature-induced volume changes of DNA duplexes differing in their GC/AT content. It was shown that the temperature-induced unfolding of the DNA duplexes proceeds with a significant increase of the thermal expansion coefficient and the partial volume of the DNA. Unusually, large temperature-induced changes in the partial volume were observed for an AT-rich dodecamer, a finding consistent with previous crystallographic studies showing the presence of highly ordered water molecules hydrating the minor groove of such duplexes. The data show that the density of this ordered water is substantially higher than that of the bulk water. This ordered water cannot, therefore, be equated to ice at normal pressures but it thermodynamically resembles ice formed at high pressures.