TRANSPORT OF COMPETING NA AND K IONS BY (222)C-10-CRYPTAND, AN IONIZABLE MOBILE CARRIER - EFFECTS OF PH AND TEMPERATURE

The kinetics of the electroneutral exchange of competing sodium and po tassium with protons across the membrane of large unilamellar vesicles (LUV) were determined at two pH values when transport was induced by the simultaneous presence of (222)C-10-cryptand and FCCP (proton carri er) at various temperatures. The aim of the present work was to quanti fy the pH-dependent enthalpies of an ionizable mobile carrier affiniti es for competing alkali cations, and to focus on the effects of pH and temperature on the competitive transport selectivity of the carrier f or K+ over Na+ ions. At any given temperature and pH, the apparent pH- dependent affinity of (222)C-10 was higher for K+ than for Na+. The en thalpy of this affinity for K+ was significantly lower than that for N a+, whereas it varied similarly with the pH(Delta H(K-mK(pH)) = 32.8 a nd 37.0 kJ/mol, and Delta H(K-mNa(pH)) = 47.9 and 52.9 kJ/mol at pH 7. 8 and 8.8, respectively). When using a kinetic model, the pH effect on these parameters was discriminated (Delta H(K-mK) = 37.9 kJ/mol and D elta H(K-mNa) = 53.9 kJ/mol). The pH-dependence of the Delta H(K-m(pH) of the cations could therefore theoretically be shown to arise from t he temperature-induced changes in the ionization of the buffer dissolv ed in the aqueous phases and of the amine groups of the binding cavity of the carrier. The K/Na competitive transport selectivity (S-c(K/Na) ) of (222)C-10 increased linearly with the K+ concentration. It decrea sed hyperbolically with increasing concentration of Na+ while being in dependent of pH at any given temperature. In equimolecular ionic mixtu res, S-c(K/Na) varied from 2.2 to 3.0 when temperature rose from 20 de grees C to 35 degrees C (Delta H(S-c(K/Na)) = 15.6 +/- 0.5 kJ/mo]). Th e results are discussed in terms of the structural, physico-chemical a nd electrical characteristics of carriers and complexes.