7Li and 23Na nuclear magnetic resonance studies of transport and diffusion in liposomes. Comparison of transport rate constants estimated using pulsed field gradient and magnetization-transfer procedures

Abstract

The ‘two-region approximation’ to pulsed field gradient (PFG) NMR studies of heterogeneous systems has been applied to the estimation of the rates of ionophore-mediated transmembrane exchange of Li⁺ in liposomes. Transport modulation of restricted diffusion, originally observed for hypophosphite in human erythrocytes, was also seen in the suspensions of liposomes with Na⁺ and Li⁺ transport. There was good agreement between the transmembrane exchange rate constants estimated from PFG and magnetization-transfer ⁷Li NMR experiments. Thus PFG experiments are likely to be a viable alternative to ‘dynamic’ NMR methods for measuring rapid transport rates in those biological systems which do not display a significant difference in chemical shift, or relaxation rate, of the transmembrane-distributed species.