Figure 6.

ρ₀ and ΔΔG ⁰ for MTSEA and MTSEH as a function of ring charge. (A) The ratio of the rate constants for the reactions with the receptor was divided by the ratio of the rate constants for the reactions of MTSEA and of MTSEH with 2-mercaptoethanol in solution, thereby normalizing for the difference in the intrinsic reactivities of the two reagents (Pascual and Karlin 1998). This ratio of ratios, ρ₀, is plotted versus the ring charge. Rate constants for the reactions of the mutant receptors in the presence (unfilled symbols) and absence (filled symbols) of ACh, at zero transmembrane potential, are from the data in Fig. 5. The symbols correspond to the same combinations of subunits as in Fig. 2. (B) ΔΔG ⁰ was calculated as −RTln(ρ₀) and zψ_S was calculated as ΔΔG ⁰/F.