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. without other accompanying Van der Waals close contacts, is a non-enthalpic process driven by the entropy increase resulting from release of tightly bound water molecules from the component polar groups. This unexpected conclusion finds a parallel in the formation of ionic bonds, for example between the amino groups of DNA binding proteins and the oxygens of DNA phosphate groups that are also non-enthalpic and entropy driven. The thermodynamic correspondence between these two types of polar non-covalent bonding implies that the non-enthalpic nature of base pairing in DNA is not particular to that specific structural circumstance.