[Oxidation-reduction properties of compounds containing the alpha-ketoendiolic system in the molecule]

The performed investigations covered the reducing-oxidizing properties of five selected analogues of L-ascorbic acid (those of acids: D-arabo-, L-arabo-, D-oxylo-, D-gluco and D-galactoascorbic). Due to the fact that alpha-ketonendiolic system appears in their molecules, the character of these compounds is that of medium strength acids, and they concurrently show distinct reducing-oxidizing properties. The studied compounds were obtained by resorting to synthesis. Their reducing properties were estimated on the basis of determined static and kinetic parameters. In the first group of tests, concentration-activity dissociation constant of first and second steps k'1,2 as well as formal normal oxidation-reduction potentials E'2, corresponding to the conditions of total ionization of the studied systems, were determined for all the acids. The example of L-ascorbic acid was used to define the dependence of dissociation constant values on temperature and ionic strength of the medium. Calculation was made to establish effective diameters of ion reactions occurring in the medium as well as standard enthalpies for both dissociation steps. In the other group of tests the second order rate constants k"0 in reaction of potassium ferricyanide and cytochrome c reduction were determined for all the studied compounds. Ionic strength effect on kinetics of both reactions was investigated, using L-ascorbic acid as a model system. In consequence of the performed studies it has been ascertained that, under conditions almost similar to physiological ones (pH, ionic strength, temperature) the dinegative ions of all the tested acids are the main, reduction-active, form. Their absolute reduction power is as follows, acid: L-ascorbic greater than D-iso- greater than D-gluco- greater than D-galactoascorbic. The results of the accomplished studies confirm the close association of reducing-oxidizing properties of the studied compounds with their acidic properties. The acids, more readily undergoing total dissociation, are characterized by greater durability of reduced forms. In the pH range from 4 to 7, both of ascorbinate ions take parallel part in the reactions. However, the principal share of dinegative ions is characteristic of reactions proceeding in media with pH 7.