ABSTRACT
According to the Watson-Crick model, an essential role in stabilization of the DNA double helix is played by the hydrogen bonds between complementary bases: two between adenine and thymine and three between cytosine and guanine (Figure 11.1b). One would expect, therefore, that an increase of the CG base pair content should lead to increased stability of the double helix (Watson et al. 1987). is is just what was observed: the thermal stability of the DNA duplex does indeed increase with an increase in CG content (Marmur and Doty 1962; Wartell and Benight 1985). is experimental fact was considered a strong argument for the correctness of the double helix Watson-Crick DNA model, according to which the hydrogen bonds between the complementary bases play a dominant role in stabilization of the double helix. However, thermodynamic verication was still required, that is, direct determination of the enthalpy and entropy of formation of various DNA duplexes diering in their content of CG and AT base pairs. is required calorimetric measurement of the heat eect of DNA duplex dissociation, particularly on heating, or measurement at xed temperatures of the heats of complementary strand association into a double helix. However, realization of these experiments appeared to be far from simple, because (a) pure DNA was available in very limited amounts and (b) the process of dissociation-association
of these polymeric molecules needs to be studied in highly dilute solutions to eliminate side eects resulting from their nonspecic interactions. Calorimetric study of DNA thermodynamics, therefore, required calorimeters signicantly more sensitive than those existing at that time. Such an instrument, now called the dierential scanning calorimeter (DSC), was specially designed for measuring the heat of DNA melting (Privalov et al. 1965). e isothermal titration calorimeter (ITC) was subsequently developed to an instrument of higher sensitivity, permitting measurements of the heat of association of the DNA complementary strands. (McKinnon et al. 1984). e latest modications of these instruments, the NanoDSC and the Nano-ITC, designed at the Johns Hopkins University, are now manufactured by TA Instruments (for the evolution of microcalorimetry, see Privalov 2012).
