Peroxynitrite-mediated oxidation of dichlorodihydrofluorescein and dihydrorhodamine.

The oxidations of dichlorodihydrofluorescein and dihydrorhodamine by peroxynitrite are zero-order in the indicator between pH 3 and 10. The yield of the oxidized products, dichlorofluorescein and rhodamine, significantly increased at pH values>7, and the maximal molar yields were 0.47 +/- 0.04 mol rhodamine and 0.54 +/- 0.06 mol, dichlorofluorescein per mol peroxynitrite at pH 8.5. The increase in yield of oxidized products as a function of pH indicates that the peroxynitrite anion may form an adduct with the indicator, followed by protonation and oxidation of the indicator. Carbon dioxide decreased the yield of fluorescent products to about 5%, relative to peroxynitrite, and the rate of product formation is again zero-order in the indicator. Given this yield, it is proposed that nitrogen dioxide and trioxocarbonate (*1-) are the reactive species that oxidize the indicators.