scholarly journals Antioxidative Reactivity of L-Ascorbic Acid and D-Isoascorbic Acid Species towards Reduction of Hexachloroiridate (IV)

2021 ◽  
Vol 2021 ◽  
pp. 1-9
Author(s):  
Han Zhang ◽  
Yanqing Xia ◽  
Peng Zhang ◽  
Liqian Hou ◽  
Ying Sun ◽  
...  
Keyword(s):  
Ascorbic Acid ◽  
Rate Constants ◽  
Dehydroascorbic Acid ◽  
Outer Sphere ◽  
Spectrophotometric Titration ◽  
Second Order ◽  
Oxidation Products ◽  
Isoascorbic Acid ◽  
Parallel Reactions ◽  
Rate Determining Steps

The pair [IrCl6]2–/[IrCl6]3– has been demonstrated to be a good redox probe in biological systems while L-ascorbic acid (AA) is one of the most important antioxidants. D-isoascorbic acid (IAA) is an epimer of AA and is widely used as an antioxidant in various foods, beverages, meat, and fisher products. Reductions of [IrCl6]2– by AA and IAA have been analyzed kinetically and mechanistically in this work. The reductions strictly follow overall second-order kinetics and the observed second-order rate constants were collected in the pH region of 0 ≤ pH ≤ 2.33 at 25.0°C. Spectrophotometric titration experiments revealed a well-defined 1 : 2 stoichiometry, namely Δ[AA] : Δ[Ir(IV)] or Δ[IAA] : Δ[Ir(IV)] = 1 : 2, indicating that L-dehydroascorbic acid (DHA) and D-dehydroisoascorbic acid (DHIA) were the oxidation products of AA and IAA, respectively. A reaction mechanism is suggested involving parallel reactions of [IrCl6]2– with three protolysis species of AA/IAA (fully protonated, monoanionic, and dianionic forms) as the rate-determining steps and formation of ascorbic/isoascorbic and ascorbate/isoascorbate radicals; in each of the steps, [IrCl6]2– acquires an electron via an outer-sphere electron transfer mode. Rate constants of the rate-determining steps have been derived or estimated. The fully protonated forms of AA and IAA display virtually identical reactivity whereas ascorbate and isoascorbate monoanions have a significant reactivity difference. The ascorbate and isoascorbate dianions are extremely reactive and their reactions with [IrCl6]2– proceed with the diffusion-controlled rate. The species versus pH and the species reactivity versus pH distribution diagrams were constructed endowing that the ascorbate/isoascorbate monoanionic form dominated the total reactivity at physiological pH. In addition, the value of pKa1 = 3.74 ± 0.05 for IAA at 25.0°C and 1.0 M ionic strength was determined in this work.

scholarly journals The mechanism of reduction of single-site redox proteins by ascorbic acid

1979 ◽  
Vol 177 (2) ◽  
pp. 641-648 ◽  
Author(s):  
A I Al-Ayash ◽  
M T Wilson
Keyword(s):  
Ascorbic Acid ◽  
Rate Constants ◽  
Activation Parameters ◽  
Outer Sphere ◽  
Reducing Agent ◽  
Second Order ◽  
Single Site ◽  
Redox Proteins ◽  
Order Rate ◽  
Temperature Dependences

The reduction of single-site haem and copper redox proteins by ascorbic acid was studied as a function of pH. Evidence is presented that indicates that the double-deprotonated ascorbate anion, ascorbate2-, is the reducing agent, and the pH-independent second-order rate constants for reduction by this species are given. Investigation of the temperature dependences of these rate constants have yielded the values of the activation parameters (delta H++ and delta S++) for reduction. These values, together with ligand-replacement studies, suggest that ascorbate2- acts as an outer-sphere reductant for these proteins. Reasons to account for the apparent inability of ascorbic acid to reduce the alkaline conformer of mammalian ferricytochrome c are suggested.

Determination of ascorbic acid, dehydroascorbic acid, and isoascorbic acid in blood

1986 ◽  
Vol 325 (5) ◽  
pp. 473-475 ◽  
Author(s):  
Jörg Schreiber ◽  
Wolfgang Lohmann ◽  
Dietrich Unverzagt ◽  
Albert Otten
Keyword(s):  
Ascorbic Acid ◽  
Dehydroascorbic Acid ◽  
Isoascorbic Acid

scholarly journals Oxidations of Benzhydrazide and Phenylacetic Hydrazide by Hexachloroiridate(IV): Reaction Mechanism and Structure–Reactivity Relationship

Molecules ◽  
2020 ◽  
Vol 25 (2) ◽  
pp. 308 ◽  
Author(s):  
Xiaolai Zhang
Keyword(s):  
Reaction Mechanism ◽  
Rational Design ◽  
Outer Sphere ◽  
Order Rate Constant ◽  
Second Order ◽  
Spectroscopic Techniques ◽  
Oxidation Reactions ◽  
Product Analysis ◽  
Pka Values ◽  
Rate Determining Steps

Benz(o)hydrazide (BH) is the basic aryl hydrazide; aryl hydrazides have been pursued in the course of drug discovery. Oxidations of BH and phenylacetic hydrazide (PAH) by hexachloroiridate(IV) ([IrCl6]2−) were investigated by use of stopped-flow spectral, rapid spectral scan, RP-HPLC and NMR spectroscopic techniques. The oxidation reactions followed well-defined second-order kinetics and the observed second-order rate constant k′ versus pH profiles were established over a wide pH range. Product analysis revealed that BH and PAH were cleanly oxidized to benzoic acid and phenylacetic acid, respectively. A reaction mechanism was proposed, resembling those suggested previously for the oxidations of isoniazid (INH) and nicotinic hydrazide (NH) by [IrCl6]2−. Rate constants of the rate-determining steps were evaluated, confirming a huge reactivity span of the protolysis species observed previously. The enolate species of BH is extremely reactive towards reduction of [IrCl6]2−. The determined middle-ranged negative values of activation entropies together with rapid scan spectra manifest that an outer-sphere electron transfer is probably taking place in the rate-determining steps. The reactivity of neutral species of hydrazides is clearly not correlated to the corresponding pKa values of the hydrazides. On the other hand, a linear correlation, logkenolate = (0.16 ± 0.07)pKenol + (6.1 ± 0.8), is found for the aryl hydrazides studied so far. The big intercept and the small slope of this correlation may pave a way for a rational design of new antioxidants based on aryl hydrazides. The present work also provides the pKa values for BH and PAH at 25.0 °C and 1.0 M ionic strength which were not reported before.

scholarly journals The oxidation of dehydroascorbic acid and 2,3-diketogulonate by distinct reactive oxygen species

2018 ◽  
Vol 475 (21) ◽  
pp. 3451-3470 ◽  
Author(s):  
Rebecca A. Dewhirst ◽  
Stephen C. Fry
Keyword(s):  
Reactive Oxygen Species ◽  
Ascorbic Acid ◽  
Hydroxyl Radicals ◽  
Reactive Oxygen ◽  
Dehydroascorbic Acid ◽  
Oxidation Products ◽  
Oxygen Species ◽  
H2o2 Treatment ◽  
Fold Excess

l-Ascorbate, dehydro-l-ascorbic acid (DHA), and 2,3-diketo-l-gulonate (DKG) can all quench reactive oxygen species (ROS) in plants and animals. The vitamin C oxidation products thereby formed are investigated here. DHA and DKG were incubated aerobically at pH 4.7 with peroxide (H2O2), ‘superoxide’ (a ∼50 : 50 mixture of and ), hydroxyl radicals (•OH, formed in Fenton mixtures), and illuminated riboflavin (generating singlet oxygen, 1O2). Products were monitored electrophoretically. DHA quenched H2O2 far more effectively than superoxide, but the main products in both cases were 4-O-oxalyl-l-threonate (4-OxT) and smaller amounts of 3-OxT and OxA + threonate. H2O2, but not superoxide, also yielded cyclic-OxT. Dilute Fenton mixture almost completely oxidised a 50-fold excess of DHA, indicating that it generated oxidant(s) greatly exceeding the theoretical •OH yield; it yielded oxalate, threonate, and OxT. 1O2 had no effect on DHA. DKG was oxidatively decarboxylated by H2O2, Fenton mixture, and 1O2, forming a newly characterised product, 2-oxo-l-threo-pentonate (OTP; ‘2-keto-l-xylonate’). Superoxide yielded negligible OTP. Prolonged H2O2 treatment oxidatively decarboxylated OTP to threonate. Oxidation of DKG by H2O2, Fenton mixture, or 1O2 also gave traces of 4-OxT but no detectable 3-OxT or cyclic-OxT. In conclusion, DHA and DKG yield different oxidation products when attacked by different ROS. DHA is more readily oxidised by H2O2 and superoxide; DKG more readily by 1O2. The diverse products are potential signals, enabling organisms to respond appropriately to diverse stresses. Also, the reaction-product ‘fingerprints’ are analytically useful, indicating which ROS are acting in vivo.

Oxidation of tetrahydropterins by azide radical and the spectra of trihydropterin radicals

1990 ◽  
Vol 68 (11) ◽  
pp. 1974-1978 ◽  
Author(s):  
David A. Armstrong ◽  
Mohsen Farahani ◽  
Parminder S. Surdhar
Keyword(s):  
Rate Constants ◽  
Spectrophotometric Titration ◽  
Second Order ◽  
Strong Peak ◽  
Order Rate ◽  
Equivalent Point ◽  
Electron Equivalent

Spectrophotometric titration of tetrahydropterins with incremental amounts of •N3 showed that 6,7-dimethyl-7,8-dihydropterin was an intermediate in the oxidation of 6,7-dimethyltetrahydropterin at the two electron-equivalent oxidation point. The eventual end product at four electron equivalents was 6,7-dimethylpterin. In the case of unsubstituted tetrahydropterin, the dihydro form was clearly not an exclusive product at the two electron equivalent point, and this was attributed to the disproportionation of different forms of dihydropterin to tetrahydropterin and pterin.The azide radical oxidized tetrahydropterins to trihydropterin radicals, •PnH3, with overall second-order rate constants at pH 7 of 4.1, 3.8, and 2.9 × 109 M−1 s−1 for tetrahydropterin, 6,7-dimethyltetrahydropterin, and 6-carboxylated tetrahydropterin, respectively. At pH 10 the rate constants are slightly larger due to the presence of the deprotonated enolate forms of tetrahydropterins. Spectra vary somewhat with substitution in the pterin molecule, but all species have a strong peak (ε ~9000 M−1 cm−1) near 320 nm and lower absorption above this, with a tail extending to 580 nm. Keywords: tetrahydropterin oxidation, tetrahydropterin, trihydropterin radical, azide radical, trihydropterin spectrum.

scholarly journals Absorption of vitamin C from the human buccal cavity

1979 ◽  
Vol 42 (1) ◽  
pp. 15-20 ◽  
Author(s):  
F. Sadoogh-Abasian ◽  
D. F. Evered
Keyword(s):  
Ascorbic Acid ◽  
Vitamin C ◽  
Calcium Ions ◽  
Buccal Mucosa ◽  
Buccal Cavity ◽  
Dehydroascorbic Acid ◽  
Secondary Effect ◽  
Sodium Ions ◽  
Acid Absorption ◽  
Isoascorbic Acid

1. Ascorbic acid was absorbed across the mucosa of the human mounth.2. Omission of sodium ions from the medium decreased the absorption of ascorbic acid.3. The presence of D-glucose, or 3-O-methyl-D-glucose, incrased the absorption of ascorbic acid but D-fructose had little effect and D-mannitol had no effect.4. Calcium ions also increased ascorbic acid absorption probably by a secondary effect on Na+ fluxes.5. Buccal mucosa was also premeable to dehydroascorbic acid and D-isoascorbic. acid

scholarly journals The pH-dependence of second-order rate constants of enzyme modification may provide free-reactant pKa values

1977 ◽  
Vol 167 (3) ◽  
pp. 859-862 ◽  
Author(s):  
K Brocklehurst ◽  
H B F Dixon
Keyword(s):  
Rate Constant ◽  
Rate Constants ◽  
Ph Dependence ◽  
Order Rate Constant ◽  
Second Order ◽  
Quasi Equilibrium ◽  
Pka Values ◽  
First Order ◽  
Order Rate ◽  
Parallel Reactions

1. Reactions of enzymes with site-specific reagents may involve intermediate adsorptive complexes formed by parallel reactions in several protonic states. Accordingly, a profile of the apparent second-order rate constant for the modification reaction (Kobs., the observed rate constant under conditions where the reagent concentration is low enough for the reaction to be first-order in reagent) against pH can, in general, reflect free-reactant-state molecular pKa values only if a quasi-equilibrium condition exists around the reactive protonic state (EHR) of the adsorptive complex. 2. Usually the condition for quasi-equilibrium is expressed in terms of the rate constants around EHR: (formula: see text) i.e. k mod. less than k-2. This often cannot be assessed directly, particularly if it is not possible to determine kmod. 3. It is shown that kmod. must be much less than k-2, however, if kobs. (the pH-independent value of kobs.) less than k+2. 4. Since probable values of k+2 greater than 10(6)M-1.S-1 and since values of kobs. for many modification reactions less than 10(6)M-1.S-1, the equilibrium assumption should be valid, and kinetic study of such reactions should provide reactant-state pKa values. 5. This may not apply to catalyses, because for them the value of kcat./Km may exceed 5 X 10(5)M-1.S-1. 6. The conditions under which the formation of an intermediate complex by parallel pathways may come to quasi-equilibrium are discussed in the Appendix.

Kinetics and mechanisms of the oxidation of ascorbic acid and benzene diols by nickel(III)tetraazamacrocycles in aqueous perchloric acid

1983 ◽  
Vol 61 (6) ◽  
pp. 1120-1125 ◽  
Author(s):  
A. McAuley ◽  
T. Oswald ◽  
R. I. Haines
Keyword(s):  
Ascorbic Acid ◽  
Perchloric Acid ◽  
Rate Constants ◽  
Cross Correlation ◽  
Outer Sphere ◽  
Transfer Reactions ◽  
Aqueous Perchloric Acid ◽  
Kinetics Of Reaction ◽  
Kinetics Of ◽  
Sphere Mechanism

The kinetics of reaction of ascorbic acid, hydroquinone, and catechol (H2A) with nickel(III) macrocycles (NiL3+) (L = cyclam, meso-(5,12)-7,7,14,14-Me6-14-ane-1,4,8,11-N4 (tet-a), and rac-(5,14)-7,7,12,12-Me6-14-ane-1,4,8,11-N4 (tet-c):[Formula: see text]have been investigated in aqueous perchloric acid solutions using the stopped-flow technique. The data are consistent with a rate-determining one-electron transfer reactions:[Formula: see text]followed by a rapid oxidation of the radical formed. In the reaction with ascorbic acid, for Ni(cyclam)3+, k1 = 250 M−1 s−1 (25 °C), k4Ka = 680 s−1; Ni(tet-c)3+, k1 = 2.52 × 103 M−1 s−1, k4Ka = 1.06 × 104 s−1; and Ni(tet-a)3+, k1 = 2.85 × 103 M−1 s−1 (21.95 °C), k4Ka = 1.26 × 104 s−1. With catechol, k1 = 6.98 × 102 M−1 s−1, 1.73 × 104 M−1 s−1, and 3.3 × 104 M−1 s−1 respectively in reactions with Ni(cyclam)3+, Ni(tet-c)3+, and Ni(tet-a)3+. With hydroquinone, k1 = 1.09 × 104 M−1s−1 (Ni(cyclam)3+) and 2.49 × 105 M−1 s−1 (20.9 °C) (Ni(tet-c)3+). The reactions are considered to take place via an outer-sphere mechanism and rate constants are discussed in terms of the Marcus cross correlation. Use has been made of predicted rate constants to identify reaction pathways in the hydrogen-ion dependent systems.

Kinetics and mechanism of the oxidation of benzenediols and ascorbic acid by bis(1,4,7-triazacyciononane)nickel(III) in aqueous perchlorate media

1985 ◽  
Vol 63 (6) ◽  
pp. 1198-1203 ◽  
Author(s):  
A. McAuley ◽  
Lee Spencer ◽  
P. R. West
Keyword(s):  
Electron Transfer ◽  
Ascorbic Acid ◽  
Exchange Rate ◽  
Rate Constants ◽  
Outer Sphere ◽  
Kinetics And Mechanism ◽  
Hydrogen Ion ◽  
Transfer Reactions ◽  
Radical Ions ◽  
Electron Transfer Reactions

The reactions of the outer-sphere electron transfer reagent, Ni(9-aneN3)23+, (bis(1,4,7-triazacyclononane)nickel(III) ion) with ascorbic acid, hydroquinone, catechol, and resorcinol have been investigated. The absence of any proton related equilibria with the oxidant provides a means of ascribing the observed inverse hydrogen ion dependences to reactions of the dissociated ascorbate or quinolate ions, (HA−). The data are consistent with the rate-determining one-electron transfer reactions:[Formula: see text]followed by rapid oxidation of the radical ions formed. In the reaction with ascorbic acid, k1 ~ 0 and k2 (T = 25° C) = 5.2 × 106 M−1 s−1 (ΔH≠ = 10.1 ± 2.5 kcal mol−1, ΔS≠ = 5.7 ± 5.1 cal mol−1 K−1). For hydroquinone, catechol, and resorcinol, k1 = 2.9 × 103, 2.8 × 102, and ~0 M−1 s−1and k2 = 6.9 × 109, 4.1 × 109, and 2.8 × 108 M−1 s−1, respectively. These data have been combined with those from other similar reactions leading, by use of a Marcus correlation, to self-exchange rate constants for the HAsc−/HAsc• couple of 3.5 × 105 M−1 s−1 and for the H2Quin0/+ and H2cat0/+ systems of 5 × 107 and 2 × 107 M−1 s−1, respectively. The importance of the effect of bond-reorganisation on electron transfer is discussed.

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