The reaction between L-ascorbic acid (AAH,) and OH radical has been investigated theoretically. The addition site on AAHz and AAH-is found to be the olefinic carbon adjacent to the carbonyl group, with much more distinct selectivity for AAH-. Dehydration is found to occur readily not from the neutral 'OH adduct AAH,OH', but from the anion adduct, AAH0H'-, which leads to formation of the most probable key intermediate radical 17 (ascorbate anion radical). The parent molecule of AAH,, triose reductone TR H, (2,3-dihydroxyprop-2-enal), undergoes 'OH addition and dehydration in a similar manner to AAH,. The addition site has been confirmed by comparison of M O energies for four possible transition states of the TRH, (ab initio and MNDO) and AAH, (MNDO) systems. The importance of the conjugate bases, AAH-and TRH-, for yielding the key radicals is suggested.Redox and radiation-induced reactions of aqueous solutions of L-ascorbic acid (Vitamin C, called here AAH,, 10) have been extensively studied.' AAH, is readily oxidized to give a stable and unreactive radical AAH'. Owing to this characteristic, AAH2 is relatively non-toxic and is useful as an anti-oxidant in biological systems. In radiation chemistry, formation of AAH20H' has also been proposed. Through the use of pulse radiolysis and EPR spectroscopy (or spectrophotometry), the early stage of the radiation-induced reaction of AAH2 has been in~estigated.,,~ The experiment showed that the OH-radical adduct (AAH,OH') as well as AAH' was generated.The parent molecule with the enediol group is triose reductone (2,3-dihydroxyprop-2-enal, TRH,, 1). In our previous oxidation reactions of TRH, were studied, and reactivity similar to that of AAH, was found. Pulse radiolysis of TRH, was also carried out, and the intermediate radical was traced by spectroph~tometry.~ The observed radical Paper 2/02959F