Oxidation photosensitized by tetracyclines

Irradiation with 365-nm UV light of aerated aqueous solutions of tetracycline gives rise to oxygen uptake when the pH of the solution is above 7.5. The kinetics of the reaction were followed using a polarographic oxygen electrode at a range of pH values for seven currently prescribed tetracyclines. Variation of tetracycline concentration, UV light intensity, and temperature showed the characteristics normally associated with a sensitized photo-oxygenation mechanism rather than a free-radical process. Copper(II) ions inhibited the photo-oxidation of tetracycline, apparently by complex formation. The tetracyclines were tested for photosensitizing capability with oxidizable acceptors. In aqueous solution, no photosensitizing effect could be seen, but methanol solutions of 2,5-dimethylfuran and dl-limonene were oxidized at considerably increased rates when small amounts of tetracyclines were present. This observations has implications for the mechanism of in vivo photosensitivity reactions that occur when tetracyclines are taken internally.