Protonation of the imidazole ring prevents the modulation by histidine of oxidative DNA degradation

When supercoiled DNA is incubated Fe(II) at pH 7 in the presence of hydrogen peroxide, the rate of nicking first increases with increasing H₂O₂ concentration to reach a maximum, then decreases and eventually increases agein. When 0.1 mM histidine is added at neutral pH at low H₂O₂ concentration (< 3 mM), it hinders the nicking of DNA; when it is added at high H₂O₂ concentrations (> 10 mM), it enhances the rate of nicking. When similar experiments are performed at slightly acidic pH (4.5) the biphasic behavior is maintained, independent of the presence of histidine. One can conclude that the protonation of imidazole (pK = 5.9) abolishes the capability of histidine to modulate the oxidative degradation of DNA. Results of electron spin resonance experiments suggest that at low H₂O₂ concentration, the protective effect of histidine could be the consequence of its capability to bind OH^. radicals.