NA K COMPETITIVE TRANSPORT SELECTIVITY OF (221)C-10-CRYPTAND - EFFECTOF TEMPERATURE/

The kinetics of the competitive transport of Na+ and K+ ions across th e membrane of large unilamellar vesicles (LUV) were determined when tr ansport was induced by (221)C-10-cryptand at various temperatures in o rder to quantify the temperature-dependence of the Na/K competitive tr ansport selectivity of this ionizable mobile carrier. At any given tem perature, the apparent affinity of (221)C-10 for Na+ was higher and le ss dependent on the concentration of the other competing ion than that for K+. Its enthalpy for Na+ (Delta H(K-mNa) = 50.6 kJ/mol) was not s ignificantly different from that for K+ (Delta H(K-mK) = 52.7 kJ/mol). The Na/K competitive transport selectivity (S-C(Na/K)) of (221)C-10 i ncreased linearly with the Na+ concentrations and decreased hyperbolic ally with increasing those of K+. When the cation concentrations were equal, this competitive selectivity amounted to about 2 at any given t emperature. Equations were established to describe the variations of t he competitive transport selectivity (S-C) of cryptands, and for compa rison of their noncompetitive selectivity (S-NC), with the ionic conce ntrations and the Michaelis parameters of the cations. It is theoretic ally demonstrated that the ratio between the competitive and noncompet itive transport selectivities, i.e., S-C/S-NC, of mobile carriers does not depend on the J(max) of the competing ions and that its value amo unts to 1 when the specific concentrations (C-S/K-m) of the ions are e qual. Under these conditions, the transport selectivity of any given m obile carrier has the same value whether determined from competition o r seperated experiments. The reaction order in Na+ (n(Na)) increased s ignificantly as the temperature rose and decreased significantly as th e K+ concentration increased. The results are discussed in terms of th e structural, physicochemical and electrical characteristics of carrie rs and complexes.