CUTANEOUS PHOTOTOXICITY OF TETRACYCLINE ANTIBIOTICS: GENERATION OF FREE RADICALS and SINGLET OXYGEN DURING PHOTOLYSIS AS MEASURED BY SPIN‐TRAPPING and THE PHOSPHORESCENCE OF SINGLET MOLECULAR OXYGEN

Chlortetracycline (CTC) generated an aryl radical in aqueous buffer (pH 7.4) during near UV irradiation, as evidenced by the formation of 2‐methyl‐2‐nitrosopropane spin adducts. The radical was produced via dechlorination, a photoprocess not previously reported for tetracyclines. Demeclocyc‐line (DEM), another chlorinated tetracycline, did not produce detectable aryl radicals. Relative 1 O 2 yields obtained by direct luminescence measurements at 1268 nm for five tetracyclines in alkaline ethanol (demeclocycline · tetracycline · chlortetracycline · doxycycline · minocycline) showed that DEM produced approximately three times as much singlet oxygen as CTC. This constitutes direct evidence that tetracyclines sensitize both Type I and Type II photoreactions.