Electron spin resonance measurements of the effect of ionophores on the transmembrane pH gradient of an acidophilic bacterium

The delta pH in ionophore-treated cells of an acidophile has been determined by electron spin resonance spectroscopy. The values obtained were comparable to those obtained using the more conventional techniques involving radiolabeled probes. No binding of the spin-labeled probe was observed as determined by two independent control experiments and by the characteristics of the probe signal. These results led us to conclude that the delta pH measured in protonophore/ionophore-treated cells is a result of a Donnan potential, which may be a physical property of all intact bacterial cells at low pH values.