Hydration differences between the major and minor grooves of DNA revealed from heat capacity measurements

The nature of water on the surface of a macromolecule is reflected in the temperature dependence of the heat effect, i.e., the heat capacity change, ΔCp, that accompanies its removal on forming a complex. The relationship between ΔCp and the nature of the surface dehydrated cannot be modeled for DNA by the use of small molecules, as previously done for proteins, since the contiguous surfaces of the grooves cannot be treated as the sum of small component molecules such as nucleotides. An alternative approach is used here in which ΔCp is measured for the formation of several protein/DNA complexes and the calculated contribution from protein dehydration subtracted to yield the heat capacity change attributable to dehydration of the DNA. The polar and apolar surface areas of the DNA dehydrated on complex formation were calculated from the known structures of the complexes, allowing heat capacity coefficients to be derived representing dehydration of unit surface area of polar and apolar surface in both grooves. Dehydration of apolar surfaces in both grooves is essentially identical and accompanied by a reduction in ΔCp by about 3 J K^-1 mol^-1 (Ų)^-1, a value of somewhat greater magnitude than observed for proteins {ΔCp = - 1.79 J K^-1 mol^-1 (Ų)^-1}. In contrast, dehydration of polar surfaces is very different in the two grooves: in the minor groove ΔCp increases by 2.7 J K^-1 mol^-1 (Ų)^-1, but in the major groove, although ΔCp is also positive, it is low in value: + 0.4 J K^-1 mol^-1 (Ų)^-1. Physical explanations for the magnitudes of ΔCp are discussed.