Figure 3.

Isothermal titration calorimetry of PS I with cyt c₆. A, thermogram for exemplary background measurements (top, red) and oxidized (middle) and reduced (bottom) proteins. B, integrated heats of titrations after background subtraction in the presence (red; reduced) or absence (black; oxidized) of 5 mm ascorbate. The fit of the data for reduced cyt c₆ is shown for one set of binding sites with n = 1.0. Parameters obtained from the model are shown in Table 2. After substraction of the heat of dilution, the data for oxidized cyt c₆ converge to negative values at high cyt c₆:PS I ratio (−0.1 kcal/mol; not shown) and are thus not analyzed by a model. Measurements were performed at 20 °C in 25 mm Tricine-NaOH, pH 8.0, with 25 mm NaCl and 0.02% DDM. Each titration step consisted of a 5-μl injected volume from 1 mm cyt c₆.