Vaitiekunas Paulius, Crane-Robinson Colyn, Privalov Peter L
Department of Biology, Johns Hopkins University, Baltimore, MD 21218, USA.
Department of Biology, Johns Hopkins University, Baltimore, MD 21218, USA
Nucleic Acids Res. 2015 Sep 30;43(17):8577-89. doi: 10.1093/nar/gkv812. Epub 2015 Aug 24.
Microcalorimetric studies of DNA duplexes and their component single strands showed that association enthalpies of unfolded complementary strands into completely folded duplexes increase linearly with temperature and do not depend on salt concentration, i.e. duplex formation results in a constant heat capacity decrement, identical for CG and AT pairs. Although duplex thermostability increases with CG content, the enthalpic and entropic contributions of an AT pair to duplex formation exceed that of a CG pair when compared at the same temperature. The reduced contribution of AT pairs to duplex stabilization comes not from their lower enthalpy, as previously supposed, but from their larger entropy contribution. This larger enthalpy and particularly the greater entropy results from water fixed by the AT pair in the minor groove. As the increased entropy of an AT pair exceeds that of melting ice, the water molecule fixed by this pair must affect those of its neighbors. Water in the minor groove is, thus, orchestrated by the arrangement of AT groups, i.e. is context dependent. In contrast, water hydrating exposed nonpolar surfaces of bases is responsible for the heat capacity increment on dissociation and, therefore, for the temperature dependence of all thermodynamic characteristics of the double helix.
对DNA双链及其单链组成部分的微量量热研究表明，未折叠的互补链形成完全折叠的双链时的缔合焓随温度呈线性增加，且不依赖于盐浓度，即双链形成导致恒定的热容降低，CG对和AT对相同。虽然双链的热稳定性随CG含量增加，但在相同温度下比较时，AT对形成双链的焓和熵贡献超过CG对。AT对双链稳定性贡献降低并非如先前假设的那样源于其较低的焓，而是源于其较大的熵贡献。这种较大的焓，尤其是更大的熵是由AT对在小沟中固定的水导致的。由于AT对增加的熵超过了冰融化的熵，该对固定的水分子必定会影响其相邻水分子。因此，小沟中的水由AT基团的排列编排，即取决于上下文。相比之下，为碱基暴露的非极性表面提供水合作用的水负责解离时的热容增加，因此也负责双螺旋所有热力学特性的温度依赖性。
