Rate-determining processes in the transport of Pr3+ ions by the ionophore A23187 across phospholipid vesicular membranes. A 1H-MR and theoretical study.

Rate constants and activation parameters have been determined for the transport of Pr3+ ions by the ionophore A23187 across dipalmitoyl phosphatidylcholine vesicular membranes. The novel method described depends on the measurement of changes in chemical shift of the 1H-NMR choline head-group signals as Pr3+ is transported from outside to inside the vesicles. The determined rates are directly proportional to A23187 concentration, suggesting that the rate-determining step involves the species [Pr(A23187)](2+). A theoretical analysis of the initial stages of Pr3+ transport leads to the conclusion that diffusion over the image potential barrier is the rate-determing step. Calculation of the form and height of this barrier for the non-equilibrium state gives results which agree well with the experimental activation energy and also correctly predict a two-fold reduction in rate of transport when 7 mol % decane is present in the bilayer.