Chlortetracycline as a probe of membrane-associated calcium and magnesium: interaction with red cell membranes, phospholipids, and proteins monitored by fluorescence and circular dichroism.

The fluorescence emission and circular dichroism spectra of chlortetracycline (CTC) have been measured, including the effects of multivalent cations (Ca, Mg, La), of medium polarity, and of interaction with human red cell membranes, lipids, and a variety of proteins. An obligatory role of Ca in the association of CTC with membranes was demonstrated. Binding and kinetic constants for the CTC-Ca chelate interaction with membranes and phospholipids were determined. The results suggest that the CTC-Ca chelate fluorescence is greatly enhanced in the vicinity of membrane phospholipid head groups. The circular dichroism spectra indicate a number of distinct CTC conformations corresponding to chelation of specific cations, to interaction with membranes and phospholipids, and to medium polarity. The high quantum yield CTC-Ca conformation associated with membranes or phospholipids was identified by its characteristic circular dichroism spectrum and is different from the CTC-Ca conformation in nonpolar media (80% methanol).