Determination of relative diffusion coefficients when three ions are present in simulations, equivalent to those of Fig. 2 . (A) Initially, 100 mM KCl is assumed to be present everywhere with K and Cl having identical diffusion coefficients. As indicated, 50 mM K(Mg)ATP is assumed to be added to the solution in front of the column, whereby the diffusion coefficient of (Mg)ATP is 2.5-fold less than that of K and Cl. The results give rise to double exponential rising and falling conductances, whereby the fitted time constants occur in ratios of 2.47 and 2.32, respectively. (B) The ratios of time constants fitted to simulations in which the diffusion coefficient of ATP was varied over a 10-fold range with respect to those of K and Cl. The fitted exponentials accurately recover the relative diffusion constants of (Mg)ATP and KCl over the entire concentration range.