Two Reaction Sites of a Spin Label, TEMPOL (4-Hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl), with Hydroxyl Radical

The exposure of 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL) to hydroxyl radical (·OH) led to the appearance of a new triplet ESR signal along with a reduction in the intensity of the TEMPOL signal. The observed spectrum was in good agreement with the spectrum simulated with 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPON) and TEMPOL, and the addition of sodium borohydride to the product solution led to the disappearance of the new signal. These results indicate the formation of TEMPON during the reaction. A comparison of the concentration of TEMPON formed with that of nitroxyl compounds that lost the electron spin resonance (ESR) signal showed that the rate of TEMPON formation is less than one-tenth that of the paramagnetism loss caused by ·OH in the pH range 6.5–8.5. These results indicate that TEMPOL has two reaction sites with ·OH; the nitroxyl group and the 4-position of the piperidine ring. The reaction at the 4-position should contribute to the ·OH scavenging activity of TEMPOL together with that at the nitroxyl group, although the rate of the former reaction is relatively low.