Table 1.

Thermodynamic Parameters of Ion Binding.

Two Class Model a	Class 1	Class 2
Stoichiometry	2.1 ± 0.4	15.9 ± 2.0
Ka (M−1)	40,180 ± 13,710 b	25,000 ± 3000
∆H (kcal/mol)	−14.9 ± 0.6 b	1.9 ± 0.5
∆S (cal/(°·mol))	−31.5 ± 0.2 b	26.8 ± 1.2

^a The ITC data shown in Figure 2 required two classes of binding site to describe both the exothermic and endothermic components. The χ² parameter for this fit was 385 compared to 5216 for a single class of binding site. Single component fits to the data are shown in Figure S3 (Supplementary Material); ^b Determined from FRET experiments at different temperatures (Supplementary Material, Figure S4) and a van’t Hoff analysis (see Figure 3) as described in the text.