Nuclear magnetic resonance studies of cation transport across vesicle bilayer membranes

We analyze an increasingly popular NMR method analogous to the black lipid membrane (BLM) isotopic tracer experiment for the study of mediated cation transport but involving the preparation of vesicles with an environment asymmetric in that paramagnetic metal ions are present only outside the vesicles. This asymmetry is manifest in the NMR spectrum as two distinct resonances for magnetic nuclei in outside and inside lipid headgroups. As mediated transport begins and for the paramagnetic metal ions enter the vesicles, the inner headgroup resonance line shifts and changes shape with a time course containing much information on the actual ion transport mechanism. Processes by which the ions enter the vesicles one or a few at a time (such as via a diffusive carrier) are easily distinguishable from those by which the ions enter in large bursts (such as by pore activation). The limiting case where intervesicular mediator exchange is slow relative to cation transport (the situation for integral membrane proteins) is treated analytically. Computer simulated curves indicate conditions necessary for certain changes in the line shape which are analogous to the "current jumps" observed in BLM conductance studies. The theory derived allows estimates of the average number of ions entering the first few bursts, how often the bursts occur, and how they depend on the concentration of the mediating species in the vesicular membrane. Preliminary experimental spectra illustrating some of the various possible line shape behaviors are presented.