(A) Best nonlinear least-squares fit of the single-site binding model (solid curve) to the spermine-DNA binding isotherm (data points), at 25°C; χ2 = 8.7 × 104. Note: abscissa represents [spermine]/[DNA] molar ratio. (B) ITC isotherms for spermine-DNA binding isotherms at different temperatures. Binding progresses in two stages: i), a pretransition binding phase, and ii), a second endothermic peak suggesting DNA collapse. n = 3; average standard errors are 5.5% for 25°C, 5.6% for 30°C, 5.4% for 37°C, and 19% for 45°C. Note: abscissa represents spermine/DNA (+/−) charge ratio.