TABLE 3.

Thermodynamic parameters of the gp120-sCD4 interaction at 37°C^a

Proteins	kd (nM)	ΔG (kcal/mol of sCD4)	ΔHobs (kcal/mol of sCD4)	−TΔS (kcal/mol of sCD4)
Wild-type YU2 gp120 + sCD4	38 ± 2	−10.52 ± 0.03	−52.1 ± 0.2	41.6 ± 0.2
375 S/W gp120 + sCD4	6.4 ± 0.9	−11.62 ± 0.08	−35.5 ± 0.2	23.9 ± 0.2

^aIsothermal titration calorimetry was performed with sCD4 and either the wild-type HIV-1 YU2 gp120 or the 375 S/W YU2 gp120 mutant, as described in Materials and Methods. The results, shown in Fig. 4, allowed determination of the k_d values and the observed binding enthalpy changes (ΔH^obs). The free energy changes (ΔG) and entropy changes (−TΔS) were calculated based on the ΔG and ΔH^obs values. The molar ratios of sCD4 to gp120 were 1.037 ± 0.003 for the wild-type YU2 gp120 and 0.694 ± 0.002 for the 375 S/W mutant.