Objectives: It is known that the color of Tetracycline deposited in human dentine changes by an exposure to ultraviolet rays and other factors over time. In this paper, the mechanisms of discoloration using hydrogen peroxide were investigated by nuclear magnetic resonance (NMR) and spectrophotometer. Methods: 1)1H-NMR(GSX-270, JEOL)spectra of 20mM tetracycline in D2O solution with and without hydrogen peroxide were compared during 24 hours. 2)Spectra of colors of tetracycline solution (0.1mM-20mM) in H2O with hydrogen peroxide were measured by spectrophotometer(UV-1200, Shimadzu) during 10 months. Results: In 1H-NMR spectra, a new peak was found at 2.6ppm at the time elapsed 24 hours. The new peak indicated that the nitrogen of dimethylamino group in tetracycline was oxidized by hydrogen peroxide. Absorbance intensities of photospectra of tetracycline solution (lower concentration than 1mM) decreased immediately after mixing with hydrogen peroxide. Absorbance intensities of photospectra of tetracycline solution (higher concentration than 1mM) increased until 24 hours. The absorbance intensities of 20mM solution decreased to 0 after six months. In the aliquot without hydrogen peroxide crystallization of tetracycline was observed and the color of the solution changed from yellow to brown. Conclusion: It is suggested that the color of tetracycline changed by the oxidazation. The color depended on the concentration of hydrogen peroxide desolved in the solution; i.e. the nitrogen atom of dimethylamino group of tetracycline was oxidized. In clinical point of view, hydrogen peroxide should be used carefully to decolorize teeth with high tetracycline concentration.

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