Dual fluorophore-nitroxide probes for analysis of vitamin C in biological liquids

A new method for quantitative analysis of vitamin C in biological and chemi cal liquids was proposed. The method is based on the use of dual molecule c onsisting of a fluorescent chromophore and a nitroxide radical. In the dual molecule, the nitroxide acts as a quencher of the fluorescence of the chro mophore fragment. Reduction of the nitroxide fragment by ascorbic acid resu lts in decay of ESR signal and enhancement of the fluorescence. By performi ng the series of pseudo-first-order reactions between the dual molecule and ascorbic acid and consequent plotting rate constants versus ascorbic acid concentrations the calibration curves for the vitamin C analysis were obtai ned. Variations of chemical structure of fluorophore and nitroxide fragment s allow to regulate fluorescent properties and redox potentials of the dual molecules. The proposed fluorophore-nitroxide hybrids retain all features of the spin labels and fluorescence probes gaining new advantages for monit oring redox reactions and radical processes by two independent techniques: ESR and steady-state fluorescent spectroscopy. The method was applied to th e vitamin C analysis in commercial fruit juices. (C) 1999 Elsevier Science B.V. All rights reserved.