Effect of pH on the Reactivity of the Carbonate Radical in Aqueous Solution

The rate constants for the reaction of <tex-math>${\rm CO}_{3}{}^{-}$</tex-math> radicals, generated in aqueous solution by the flash photolysis of <tex-math>${\rm Co}({\rm NH}_{3})_{4}{\rm CO}_{3}{}^{+}$</tex-math>, with some sulfur-containing compounds, some amino acids and enzymes, and nitromethane have been measured. The radical is significantly more reactive with the deprotonated sulfhydryl group than with the protonated form. The presence of an amino group causes the rate constant to diminish upon deprotonation. The radical is more reactive with the deprotonated imidazole ring in histidine than with the protonated form. Ribonuclease A and tyrosine show the same reactivity pattern with the radical; deprotonation of the phenolic groups in tyrosine increases the value of the rate constant. Lysozyme and α-chymotrypsin show pH-independent values. Finally, <tex-math>${\rm CO}_{3}{}^{-}$</tex-math> radicals are more reactive with the aci anionic form of nitromethane than with the unionized form.