FIGURE 4.

(A) Representative data for ITC of DNA duplexes. A 75 μM solution of D-I S2 was titrated into 1.4 mL of a 5 μM solution of D-I S1. Solutions were equilibrated at 45 °C. Both DNAs were in 10 mM NaHEPES, pH 7.5, and 1 M added NaCl. The upper panel shows the raw thermogram. The lower panel shows integrated injection data (solid squares) and a least-squares fit of the data to a one-site binding model (solid line), yielding the following parameters: ΔH = −84.4 kcal mol⁻¹; ΔS = −128 cal mol⁻¹ K⁻¹; K_A = 1.1 × 10⁸ M⁻¹; n = 0.95; c = 522. Values of K_A and ΔS have relatively large errors due to the high c-value. Data collection parameters were designed to optimize the accuracy of ΔH and ΔC_p to facilitate the comparisons relevant to this study. (B) Plot of ΔH values observed by ITC for formation of duplex D-I at 0.1 M added NaCl (solid circles), 0.4 M added NaCl (solid squares), and 1.0 M added NaCl (solid triangles). Dashed lines represent linear least-squares fits of each data series; the slope of each fit corresponds to a linear approximation of the observed ΔC_p. (C) Plot of ΔH values observed by ITC for formation of duplex D-V. All conventions are the same as in (B).