QUANTUM CHEMICAL INTERPRETATION OF THE REACTION BETWEEN L-ASCORBIC ACID AND FORMALDEHYDE AND ITS BIOLOGICAL SIGNIFICANCE

Abstract

We have recognized a new nucleophilic addition reaction between L-ascorbic acid and formaldehyde in biological circumstances (pH 7.4, T = 300 K). UV spectroscopic investi- gation showed that the C2=C3 double bond of ascorbic acid rapidly disappears during the reaction, verifying that the addition breaks the double bond. On the other hand, 13C NMR studies have proved that on atom C2 an additional C-C single bond is formed. Ex- act interpretation of the reaction mechanism is possible only by using quantum chemical argumentations. Ab initio calculations show that in ionized (anionic) form of L-ascorbic acid in water solution the Mulliken excess atomic charge is strongly negative on atom C2 while C1 and C3 exhibit significant positive net charges. Coulombic attraction arranges formaldehyde's C=O double bond over the C2=C3 double bond of ascorbic acid and the interaction of the two loose π-electron systems causes forming of a new σ C-C bond. The described reaction plays a protective role in biological systems, since in this way the toxic carcinogenic formaldehyde can be eliminated.