FIGURE 6.

Rates of Na⁺ binding to Glt_Ph F273W. The top panels show the stopped-flow fluorescence measurements. Each trace (black) represents an average of 6–15 individual traces, fitted with a single exponential equation (gray). A, 1.5 μm F273W apoprotein was mixed with solutions of the indicated concentrations of NaCl; KCl was present so that the sum of the NaCl and KCl concentrations was 1 m. All measurements were performed at room temperature. The bottom panel shows the k_obs values plotted against the corresponding NaCl concentrations. The slope of the linear fit gives the k_on, and the intercept with the k_obs axis gives the k_off value. B, 1.5 μm F273W was mixed with 1 m NaCl at different temperatures as indicated, and the bottom panel shows the Arrhenius plot of the derived k_obs. The gray line is a linear fit of the data according to the Arrhenius equation ln(k) = B + (ln(E_a)/RT), with E_a representing the activation energy, B representing the frequency factor, T representing the absolute temperature, and R representing the gas constant. The linear fit results in an E_a = 106.8 kJ mol⁻¹.