a–d, ITC for the titration of tCD81-LEL (a, b) or hCD81-LEL (c, d) into eE2 at pH 7.5 (a, c) and pH 5.0 (b, d). Thermogram (upper panel), integrated heats and error bars (middle panel), and fit residuals (lower panel) are shown for each. The measurements were performed at 20 °C and analysed with an A + B ⇄ AB heterodimer model. Error bars indicate the error of peak integration over an interpolated baseline with a 68% (1 sigma) confidence interval. Residuals are the y-axis difference between the data point and the fitted curve in kcal mol−1.