- Titanium(IV) and vitamin C: Aqueous complexes of a bioactive form of Ti(IV)
-
Ascorbic acid is among the biorelevant ligands that render Ti(IV) stable in aqueous solution. A series of pH-dependent titanium(IV) coordination complexes of l-ascorbic acid is described. Directed by spectropotentiometric methods, important aspects of the aqueous interactions in this system are investigated, including ligand binding mode, pH-dependent metal-ligand stoichiometry, and the importance of metal ion-promoted hydrolysis and the binding of hydroxide. Stability constants are determined for all metal ion-ligand-proton complexes by a process of model optimization and nonlinear least-squares fitting of the combined spectropotentiometric titration data to the log βMLH values in the model. A speciation diagram is generated from the set of stability constants described in the model. In the range pH 3-10, the aqueous speciation is characterized by the sequential appearance of the following complexes as a function of added base: Ti(asc)20 → Ti(asc) 32- → Ti(asc)2(OH)2 2- → Ti(asc)(OH)42-. These species dominate the speciation at pH 10, respectively, with minimum log stability constants (β values) of 25.70, 36.91, 16.43, and -6.91. Results from electrospray mass spectrometry, metal-ligand binding experiments, and kinetics measurements support the speciation, which is characterized by bidentate chelation of the ascorbate dianion to the titanium(IV) ion via proton displacement, and a pH-dependent metal-ligand binding motif of ligand addition followed by metal ion-promoted hydrolysis, stepwise ligand dissociation, and the concomitant binding of hydroxide ion. Additionally, the kinetics of ligand exchange of titanium ascorbate with citrate are reported to understand better the possible fate of titanium ascorbate under biologically relevant conditions.
- Buettner, Katherine M.,Collins, Joseph M.,Valentine, Ann M.
-
p. 11030 - 11039,10
(2012)
-
Read Online
- Reactions of thiyl radicals with phenothiazines and other antioxidants in aqueous solutions. A pulse radiolysis study.
-
Reaction rate constants of thiyl radicals with phenothiazines, promethazine (PMZ), chlorpromazine (CPZ), prochlorperazine (PPZ), trimeperazine tarterate (TPZ), and other antioxidants, e. g., ascorbic acid and 2,2-azinobis(3-ethyl benzthiazoline-6 sulphonate) (ABTS) have been estimated using pulse radiolysis technique.In general they are quite high (108-109 M-1 s-1).The k value of cysteine, cysteamine, mercaptoethanol, and mercaptopropionic acid with similar structures and molecular weights are more or less similar for PMZ, CPZ, and PPZ.Rate constants are lower in value for PenS., DT.T, DT.E and GuS. radicals of phenothiazines showing that they are dependent upon the structure and the molecular weight of the compound. Rate constants are the same whether the thiyl radicals are generated from thiols or their corresponding disulphides.The k values for chlorine-containing phenothiazines, chlorpromazine, and prochlorperazine are higher than those of promothazine.
- Lal, Manohar,Mahal, H. S.
-
p. 1376 - 1382
(2007/10/02)
-
- Solvent Effects in the Reactions of Peroxyl Radicals with Organic Reductants. Evidence for Proton-Transfer-Mediated Electron Transfer
-
Absolute rate constants for the reaction of substituted methylperoxyl radicals with ascorbate, urate, trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) and TMPD (N,N,N',N'-tetramethyl-p-phenylenediamine) have been determined by pulse radiolysis in different solvents.In water-alcohol or water-dioxane solutions, the rate constants for trihalomethylperoxyl radicals generally increase with increasing water content.The rate constant for the reaction of CCl3O2* radicals with trolox was measured in water, MeOH, i-PrOH, t-BuOH, ethylene glycol, diethyl ether, dioxane, acetone, acetonitrile, formamide, dimethylformamide, pyridie, and CCl4.The rate constants were found to correlate well with a two-parameter equation that includes the dielectric constant of the solvent and the coordinate covalency parameter, a measure of the proton-transfer basicity of the solvent.Kinetic isotope effects in H2O/D2O of about 2 and the activation entropies of about -10 eu for reduction of RO2* by the organic reductants indicate that electron transfer to the peroxyl radical is concerted with the transfer of proton from the solvent to the incipient hydroperoxide anion.
- Neta, P.,Huie, R. E.,Maruthamuthu, P.,Steenken, S.
-
p. 7654 - 7659
(2007/10/02)
-
- Kinetics and Mechanism of the Reactions of Superoxide Ion in Solution. Part 5. Kinetics and Mechanism of the Interaction of Superoxide Ion with Vitamin E and Ascorbic Acid
-
The reactions of electrochemically generated O2-. with α-tocopherol and α-tocopheryl acetate have been studied in acetonitrile and with ascorbic acid in acetonitrile and mixed acetonitrile-water solvents.It was found that both α-tocopherol and ascorbic acid react with superoxide ion via a deprotonation mechanism to form the corresponding anions.On subsequent steps of the process these anions are oxidized by O2 to form O2-..The mechanism of the oxidation of ascorbate anion depended on the solvent composition: in aqueous solution the reaction proceeded only in the presence of Fe3+ ion, but in acetonitrile a direct one-electron transfer from the ascorbate anion to dioxygen was observed.Both mechanisms were suppressed to a considerable extent in mixed solvents.
- Afanas'ev, Igor B.,Grabovetskii, Vadim V.,Kuprianova, Natalia S.
-
p. 281 - 286
(2007/10/02)
-
- Oxidation of Ascorbic Acid and Dehydroascorbic Acid by Superoxide Ion in Aprotic Media
-
In dimethylformamide, superoxide ion (O2-) oxidizes ascorbic acid (H2A) to dehydroascorbic acid (A).The rate-limiting step is first order for each reactant and has a second-order rate constant (k) of 2.8 x 104 M-1 s-1; the overall stoichiometry is 3H2A molecules per 2O2- ions.Addition of O2- to dehydroascorbic acid (A) results in its rapid oxidation (k = 3.3 x 104 M-1 s-1), with an overall stoichiometry of 2O2- per A.The major products of the process are oxalate ion and the anion of threonic acid.On the basis of the reaction stoichiometries, kinetics, and products, self-consistent mechanisms are proposed for the oxidation of ascorbic acid and dehydroascorbic acid.
- Sawyer, Donald T.,Chiericato, Glaico,Tsuchiya, Tohru
-
p. 6273 - 6278
(2007/10/02)
-