Transport of competing Na and K ions by (222) C10-cryptand, an ionizable mobile carrier: effects of pH and temperature.

The kinetics of the electroneutral exchange of competing sodium and potassium 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)C10-cryptand and FCCP (proton carrier) at various temperatures. The aim of the present work was to quantify the pH-dependent enthalpies of an ionizable mobile carrier affinities for competing alkali cations, and to focus on the effects of pH and temperature on the competitive transport selectivity of the carrier for K+ over Na+ ions. At any given temperature and pH, the apparent pH-dependent affinity of (222)C10 was higher for K+ than for Na+. The enthalpy of this affinity for K+ was significantly lower than that for Na+, whereas it varied similarly with the pH (delta H(KpHmK) = 32.8 and 37.0 kJ/mol, and delta H(KpHmNa) = 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(KmK) = 37.9 kJ/mol and delta H(KmNa) = 53.9 kJ/mol). The pH-dependence of the delta H(KpHm) of the cations could therefore theoretically be shown to arise from the temperature-induced changes in the ionization of the buffer dissolved in the aqueous phases and of the amine groups of the binding cavity of the carrier. The K/Na competitive transport selectivity (Sc(K/Na)) of (222)C10 increased linearly with the K+ concentration. It decreased hyperbolically with increasing concentration of Na+ while being independent of pH at any given temperature. In equimolecular ionic mixtures, Sc(K/Na) varied from 2.2 to 3.0 when temperature rose from 20 degrees C to 35 degrees C (delta H(Sc(K/Na)) = 15.6 +/- 0.5 kJ/mol). The results are discussed in terms of the structural, physico-chemical and electrical characteristics of carriers and complexes.