Energetic basis of hydrogen bond formation in aqueous solution.

The thermodynamic forces driving the formation of H-bonds in macromolecules have long been the subject of speculation, theory and experiment. Comparison of the energetic parameters of AT and GC base pairs in DNA duplexes has recently led to the realisation that formation of a 'naked' hydrogen bond, i.e.

Thermodynamic basis of the α-helix and DNA duplex.

Analysis of calorimetric and crystallographic information shows that the α-helix is maintained not only by the hydrogen bonds between its polar peptide groups, as originally supposed, but also by van der Waals interactions between tightly packed apolar groups in the interior of the helix. These apolar contacts are responsible for about 60% of the forces stabilizing the folded conformation of the α-helix and their exposure to water on unfolding results in the observed heat capacity increment, i.e.

Forces maintaining the DNA double helix.

Despite the common acceptance that the enthalpy of DNA duplex unfolding does not depend on temperature and is greater for the CG base pair held by three hydrogen bonds than for the AT base pair held by only two, direct calorimetric measurements have shown that the enthalpic and entropic contributions of both base pairs are temperature dependent and at all temperatures are greater for the AT than the CG pair. The temperature dependence results from hydration of the apolar surfaces of bases that become exposed upon duplex dissociation. The larger enthalpic and entropic contributions of the AT pair are caused by water fixed by this pair in the minor groove of DNA and released on duplex dissociation.

Thermodynamics of DNA: heat capacity changes on duplex unfolding.

The heat capacity change, ΔCp, accompanying the folding/unfolding of macromolecules reflects their changing state of hydration. Thermal denaturation of the DNA duplex is characterized by an increase in ΔCp but of much lower magnitude than observed for proteins. To understand this difference, the changes in solvent accessible surface area (ΔASA) have been determined for unfolding the B-form DNA duplex into disordered single strands.