Heat capacity changes associated with G-quadruplex unfolding

G-quadruplexes are four-stranded DNA structures that have been found in the cell and are thought to act as elements of control in genomic events. The measurements of the thermodynamic stability,Delta G, of G-quadruplexes shed light on the molecular forces involved in the stabilization of these structures. In thermodynamic studies, the differential heat capacity,Delta C-P, of the folded and unfolded states of a G-quadruplex is a fundamental property that describes the temperature dependences of the differential enthalpy,Delta H, entropy,Delta S, and free energy,Delta G. Despite its recognized importance, the Delta C-P of G-quadruplex unfolding has not been measured directly. Here, we use differential scanning calorimetry to evaluate changes in heat capacity,Delta C-P, accompanying the unfolding transitions of G-quadruplexes formed by modified DNA sequences from the promoter regions of the c-MYC, VEGF, and Bcl-2 oncogenes. The average value of Delta C-P is 0.49 +/- 0.12 kcal mol(-1) K-1. Our analysis revealed that disregarding Delta C-P leads to significant errors in extrapolated values of the differential enthalpy,.H, and entropy,Delta S, of the folded and unfolded DNA conformations. Although the compensation between Delta H and Delta S weakens the effect of.CP on the differential free energy, Delta G, neglecting Delta C-P may still result in relative errors in Delta G extrapolated to room temperature as great as 140%. We emphasize the importance of proper consideration of the effect of Delta C-P in conformational studies of guanine-rich DNA molecules.