Electron paramagnetic resonance methods for measuring pH gradients, transmembrane potentials, and membrane dynamics

In this article, we have described the use of paramagnetic hydrophobic ions and secondary alkylamines to estimate pH gradients, and transmembrane and interfacial boundary potentials in membrane vesicle systems. Several features of these probes make them powerful tools for the investigation of membrane potentials. The mechanisms by which they function are well understood, thereby facilitating their use in a quantitative fashion. They have a relatively high sensitivity and are not affected by turbidity. While they are ideally suited for use in model membrane systems, they have also been used successfully in biological and reconstituted membrane systems. Finally, the synthetic flexibility of the nitroxide permits a much wider range of probes to be synthesized than has been described here. Future work will likely bring additional developments in the application and information obtainable from this methodology.