Increased binding of a hydrophobic, photolabile probe to Escherichia coli inversely correlates to membrane potential but not adenosine 5'-triphosphate levels.

We describe conditions for a quantitative determination of azidopyrene binding to Escherichia coli cells. In addition, we define conditions whereby irradiation of azidopyrene in the presence of cells leads to irreversible association of probe with cells. This is presumably due to the light-dependent generation of reactive nitrenes and subsequent incorporation of nitrenopyrene moieties into cellular components. These methods allowed us to determine that the amount of azidopyrene bound to cells was inversely correlated with the magnitude of the cellular membrane potential, but was not correlated with high or low adenosine 5-triphosphate levels per se. Cells bound more azidopyrene if the delta psi was low. Cell-bound azidopyrene was found to be entirely associated with the inner and outer membrane. We suggest that the decreased association of hydrophobic probes upon energization of whole cells reflects a rapid transition in structural properties of the cell envelope.