Kinetics and mechanism of the reduction of Cr^VI to Cr^III by D-ribose and 2-deoxy-D-ribose

Abstract

The oxidation of D-ribose and 2-deoxy-D-ribose by CrVI yields the aldonic acid and Cr3+ as final products when an excess of sugar over CrVI is used. The redox reaction occurs through CrVI–>CrIII and CrVI–>CrV–>CrIII paths. The complete rate laws for the CrVI oxidation reactions are expressed by -d[CrVI]/dt = kH[H+]2 [ribose][CrVI], where kH = (5.9 ± 0.1) × 10-2 mol-3 dm9 s-1, and -d[CrVI]/dt = (k0 + kH'[H+]2) [2-deoxyribose][CrVI], where k0 = (1.3 ± 0.5) × 10-3 mol-1 dm3 s-1 and kH' = (4.2 ± 0.1) × 10-2 mol-3 dm9 s-1, at 33°C. An intermediate sugar alkoxide radical could be trapped with DMPO and observed by EPR as a multiline signal at g = 2.003. CrV is formed in a rapid step by reaction of the sugar radical with CrVI. CrV reacts with the substrate faster than CrVI does. The EPR spectra show that five- and six-coordinate oxochromate(V) intermediates are formed, and the distribution of these CrV species in the reaction mixture essentially depends on the solution acidity.Key words: ribose, 2-deoxyribose, chromium, redox, mechanism, kinetics.

Role of manganese(II), micelles, and inorganic salts on the kinetics of the redox reaction of L‐sorbose and chromium(VI)

“…2a, Set B). These results indicate that under the kinetic experimental conditions, L-sorbose or its oxidation product(s) does not complex with chromium(III) because the spectrum of the reaction mixture remains identical to that of authentic chromium(III) solution ( max = 410 and 570 nm) [18]. The agreement shows that the aquachromium(III) is the final reaction product under any of the conditions used herein.…”

Kinetics and mechanism of the oxidation of disaccharides by CrVI

“…[2]). This is based on the mechanism reported for aldoses, which are also selectively oxidized by Cr VI at the hemiacetalic group with an initial two-electron transfer slow step (21)(22)(23)(24)(25)(26)(27)(28). After the slow redox step, Cr IV is predicted to react with excess saccharide to yield Cr III and a disaccharide radical in a fast step (eq.…”

“…Our studies on the reduction of Cr VI and the intermediate Cr V by aldoses (21)(22)(23)(24)(25)(26), deoxyaldoses (27)(28), sugar acids (31,32,35), and methyl glycosides (29)(30) showed that the relative redox reactivities of these saccharides toward chromate are based on the relative rates of the oxidation vs. complexation processes. In view of some preliminary results obtained on the milk-Cr VI system suggesting the formation of long lived lactose-Cr V species generated by direct reaction of Cr VI with lactose (5), we decided to evaluate the kinetics of the reaction of Cr VI with four disaccharides: D-lactose (Lac), D-maltose (Mal), D-cellobiose (Cel), and D-melibiose (Mel), to determine the influence of the glycoside moiety attached to the C 6 or C 4 of glucose (glc) on the chelating and (or) redox reactivity of the saccharide with high oxidation states of chromium.…”

“…The EPR spectral parameters, g iso and A iso , together with the proton superhyperfine (shf) coupling, are useful in determining the binding modes of sugars to the Cr V center (1,22,24,26,27,57,58). An empirical relationship between the nature and number of donor groups and the EPR spectral parameters of Cr V complexes has been established (1,57).…”

“…We are studying the possible fate of Cr VI and Cr V in biological systems by examining reactions of Cr VI with low-molecular-weight molecules (21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37). Our studies on the reduction of Cr VI and the intermediate Cr V by aldoses (21)(22)(23)(24)(25)(26), deoxyaldoses (27)(28), sugar acids (31,32,35), and methyl glycosides (29)(30) showed that the relative redox reactivities of these saccharides toward chromate are based on the relative rates of the oxidation vs. complexation processes.…”