Na/K competitive transport selectivity of (221) C10-cryptand: effect of temperature.

The kinetics of the competitive transport of Na+ and K+ ions across the membrane of large unilamellar vesicles (LUV) were determined when transport was induced by (221)C10-cryptand at various temperatures in order to quantify the temperature-dependence of the Na/K competitive transport selectivity of this ionizable mobile carrier. At any given temperature, the apparent affinity of (221)C10 for Na+ was higher and less dependent on the concentration of the other competing ion than that for K+. Its enthalpy for Na+ (delta H(KmNa) = 50.6 kJ/mol) was not significantly different from that for K+ (delta H(KmK) = 52.7 kJ/mol). The Na/K competitive transport selectivity (SC(Na/K)) of (221)C10 increased linearly with the Na+ concentrations and decreased hyperbolically with increasing those of K+. When the cation concentrations were equal, this competitive selectivity amounted to about 2 at any given temperature. Equations were established to describe the variations of the competitive transport selectivity (SC) of cryptands, and for comparison of their noncompetitive selectivity (SNC), with the ionic concentrations and the Michaelis parameters of the cations. It is theoretically demonstrated that the ratio between the competitive and noncompetitive transport selectivities, i.e., SC/SNC, of mobile carriers does not depend on the Jmax of the competing ions and that its value amounts to 1 when the specific concentrations (C'S/Km) of the ions are equal. Under these conditions, the transport selectivity of any given mobile carrier has the same value whether determined from competition or separated experiments. The reaction order in Na+ (n(Na)) increased significantly as the temperature rose and decreased significantly as the K+ concentration increased. The results are discussed in terms of the structural, physicochemical and electrical characteristics of carriers and complexes.