The mechanisms and kinetics of oxidation of ascorbate, AH-, by Ni(III)Liaq and by LiNi(III) (HPO4)2- complexes (L1 = meso-(5,12)-7,7,14,14-hex-amethyl-1,4,8,11-tetraazacyclotetradecane; L2 = 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane) in neutral aqueous solutions have been investigated. The oxidation of ascorbate by the LiNi(III) (HPO4)2- and Ni(III)L1aq proceeds via two consecutive reactions well separated in time. The products of the first reaction are the A.- radical anion and the corresponding Ni(II) complex. The oxidations by the LiNi(III)(HPO4)2- complexes proceed via the outer sphere mechanism, whereas the detailed mechanism of reaction of Ni(III)L1aq cannot be determined. The rate of reaction decreases with the increase in the concentration of phosphate, thus indicating that LiNi(III)(HPO4)(H2O)+ and LiNi(III)OH2+ are stronger oxidizing agents than LiNi(III)(HPO4)2-. The oxidation of ascorbate by Ni(III)L2aq proceeds via three consecutive reactions which are well separated in time. Thus the results clearly point out that this process occurs via the inner sphere mechanism. The first transient observed is tentatively identified as L2(H2O)Ni(II)(A.-)2+, i.e., an unexpected complex of the ascorbate anion radical. Also in this process the last transient observed is the A.- anion radical. The stabilization of the ascorbyl radical in a transient complex might be of biological significance.
|Number of pages||8|
|State||Published - 2001|
Bibliographical noteFunding Information:
We express our sincere gratitude for Professor Dr. Joachim Sieler (Leipzig University, Germany) for his active interest in our work. The present study was in part supported by a visiting Stipendium from the Sächsische Staatsministerium für Wissenschaft und Kunst (KVD), which is gratefully acknowledged.
- Ascorbyl radial