Described in this work is the kinetics of the oxidation of a series of thiols, viz. 2-mercaptoeth... more Described in this work is the kinetics of the oxidation of a series of thiols, viz. 2-mercaptoethanol (mercap), thioglycolic acid (tga) and L-cysteine (cys), a paradigm for aliphatic thiols, by a tetranuclear Mn oxidant, [Mn 4 (l-O) 6 (bipy) 6 ] 4+ (bipy = 2,2 0-bipyridine), in weakly acidic aqueous media (pH 2.0-4.0) in the presence of added bipy (60.0-80.0 mM). The thiols were quantitatively oxidized to their respective disulfides and the Mn IV 4 species was reduced to Mn II. Excess bipy present in the reaction mixture removes any possibility of Cu 2+ catalysis in these reactions. The reactions were acid-catalyzed and the oxo-bridged protonated Mn oxidant, [Mn 4 (l-O) 5 (l-OH)(bipy) 6 ] 5+ , was found to be a kinetically far superior oxidant that its deprotonated analog, [Mn 4 (l-O) 6 (bipy) 6 ] 4+. The reactions show kinetic isotope effects. The tga oxidation rate was increased in media enriched with D 2 O whereas the rate of oxidation of the other two thiols was increased in H 2 O. However, the increase/decrease in rate in H 2 O-D 2 O mixed media varied linearly with the D 2 O content of the media, which might be indicative of transfer of a single proton in the rate step of the reactions, at least for cys and mercap. The increase in rate of tga oxidation in D 2 O media could be explained by the relative abundance of the reactive species in D 2 O in comparison to that in H 2 O.
In aqueous solution [Fe 2 (l-O)(phen) 4 (H 2 O) 2 ] 4+ (1, phen = 1,10-phenanthroline) equilibrat... more In aqueous solution [Fe 2 (l-O)(phen) 4 (H 2 O) 2 ] 4+ (1, phen = 1,10-phenanthroline) equilibrates with its conjugate bases [Fe 2 (l-O)(phen) 4 (H 2 O)(OH)] 3+ (2) and [Fe 2 (l-O)(phen) 4 (OH) 2 ] 2+ (3). In the presence of excess phen and in the pH range 2.5-5.5, the dimer quantitatively oxidizes pyruvic acid to acetic acid and carbon dioxide, the end iron species being ferroin, [Fe(phen) 3 ] 2+. The observed reaction rate shows a bell-shaped curve as pH increases, but is independent of added phen. Kinetic analysis shows that (3) is non-reactive and (1) has much higher reactivity than (2) in oxidizing pyruvic acid. The basicity of the bridging oxygen increases with deprotonation of the aqua ligands. The reaction rate decreases significantly in media enriched with D 2 O in comparison to that in H 2 O, with a greater retardation at higher pH, suggesting the occurrence of proton coupled electron transfer (PCET; 1e, 1H +), which possibly drags the energetically unfavorable reaction to completion in presence of excess phen.
In aqueous media (pH 2.5-6.0), the Mn IV tetramer [Mn 4 (m-O) 6 A C H T U N G T R E N N U N G (bi... more In aqueous media (pH 2.5-6.0), the Mn IV tetramer [Mn 4 (m-O) 6 A C H T U N G T R E N N U N G (bipy) 6 ] 4+ (1 4+ ; bipy = 2,2'-bipyridine) oxidizes both glyoxylic and pyruvic acid to formic and acetic acid, respectively, under formation of CO 2. Kinetics studies suggest that the species 1 4+ , its oxo-bridge protonated form [1H] 5+ , i.e., [Mn 4 (m-O) 5 A C H T U N G T R E N N U N G (m-OH)(bipy) 6 ] 5+ , the reducing acids (RH) and their conjugate bases (R À) all take part in the reaction. The oxo-bridge protonated oxidant [1H] 5+ was found to react much faster than 1 4+. Thereby, the gemdiol forms of the a-oxo acids (especially in the case of glyoxylic acid) are the possible reductants. A one-electron/one-proton electroprotic mechanism operates in the rate-determining step. 1) The hydration constants of the reducing species as found in [25] are as follows: glyoxylic acid, 1.0 10 2-1.4 10 3 ; glyoxylate, 6.5-260; pyruvic acid, 0.61-3.1; pyruvate, 6.4 10 À2 À 0.21.
In aqueous media, the trinuclear Mn IV complex, [Mn IV 3 (l-O) 4 (phen) 4 (H 2 O) 2 ] 4+ (phen = ... more In aqueous media, the trinuclear Mn IV complex, [Mn IV 3 (l-O) 4 (phen) 4 (H 2 O) 2 ] 4+ (phen = 1,10-phenanthroline) (1) behaves like a monobasic acid, equilibrates with its deprotonated conjugate base (2) (1 2 + H + , pK a = 4.00 (± 0.15) at 25.0°C, I = 1.0 M, maintained with NaNO 3). An aqueous solution of 1 in the pH range 2.2-4.0 smoothly oxidizes 2-mercaptoethanol (mercap) and L-cysteine (cys) to their respective disulfides; itself being reduced to [Mn(phen) 3 ] 2+ in presence of excess 1,10-phenanthroline which also masks Cu 2+ , if any, present in the reaction media as impurity as well as acting as a buffer that controls pH range during the reaction. The observed rate constants in H 2 O media were found to be considerably lowered in media enriched with D 2 O and a proton-coupled single electron transfer rate step is proposed. Interestingly, we found that deprotonated oxidant (2) reacts faster than the protonated species (1) in oxidizing both the thiols.
Described in this work is the kinetics of the oxidation of a series of thiols, viz. 2-mercaptoeth... more Described in this work is the kinetics of the oxidation of a series of thiols, viz. 2-mercaptoethanol (mercap), thioglycolic acid (tga) and L-cysteine (cys), a paradigm for aliphatic thiols, by a tetranuclear Mn oxidant, [Mn 4 (l-O) 6 (bipy) 6 ] 4+ (bipy = 2,2 0-bipyridine), in weakly acidic aqueous media (pH 2.0-4.0) in the presence of added bipy (60.0-80.0 mM). The thiols were quantitatively oxidized to their respective disulfides and the Mn IV 4 species was reduced to Mn II. Excess bipy present in the reaction mixture removes any possibility of Cu 2+ catalysis in these reactions. The reactions were acid-catalyzed and the oxo-bridged protonated Mn oxidant, [Mn 4 (l-O) 5 (l-OH)(bipy) 6 ] 5+ , was found to be a kinetically far superior oxidant that its deprotonated analog, [Mn 4 (l-O) 6 (bipy) 6 ] 4+. The reactions show kinetic isotope effects. The tga oxidation rate was increased in media enriched with D 2 O whereas the rate of oxidation of the other two thiols was increased in H 2 O. However, the increase/decrease in rate in H 2 O-D 2 O mixed media varied linearly with the D 2 O content of the media, which might be indicative of transfer of a single proton in the rate step of the reactions, at least for cys and mercap. The increase in rate of tga oxidation in D 2 O media could be explained by the relative abundance of the reactive species in D 2 O in comparison to that in H 2 O.
In aqueous solution [Fe 2 (l-O)(phen) 4 (H 2 O) 2 ] 4+ (1, phen = 1,10-phenanthroline) equilibrat... more In aqueous solution [Fe 2 (l-O)(phen) 4 (H 2 O) 2 ] 4+ (1, phen = 1,10-phenanthroline) equilibrates with its conjugate bases [Fe 2 (l-O)(phen) 4 (H 2 O)(OH)] 3+ (2) and [Fe 2 (l-O)(phen) 4 (OH) 2 ] 2+ (3). In the presence of excess phen and in the pH range 2.5-5.5, the dimer quantitatively oxidizes pyruvic acid to acetic acid and carbon dioxide, the end iron species being ferroin, [Fe(phen) 3 ] 2+. The observed reaction rate shows a bell-shaped curve as pH increases, but is independent of added phen. Kinetic analysis shows that (3) is non-reactive and (1) has much higher reactivity than (2) in oxidizing pyruvic acid. The basicity of the bridging oxygen increases with deprotonation of the aqua ligands. The reaction rate decreases significantly in media enriched with D 2 O in comparison to that in H 2 O, with a greater retardation at higher pH, suggesting the occurrence of proton coupled electron transfer (PCET; 1e, 1H +), which possibly drags the energetically unfavorable reaction to completion in presence of excess phen.
In aqueous media (pH 2.5-6.0), the Mn IV tetramer [Mn 4 (m-O) 6 A C H T U N G T R E N N U N G (bi... more In aqueous media (pH 2.5-6.0), the Mn IV tetramer [Mn 4 (m-O) 6 A C H T U N G T R E N N U N G (bipy) 6 ] 4+ (1 4+ ; bipy = 2,2'-bipyridine) oxidizes both glyoxylic and pyruvic acid to formic and acetic acid, respectively, under formation of CO 2. Kinetics studies suggest that the species 1 4+ , its oxo-bridge protonated form [1H] 5+ , i.e., [Mn 4 (m-O) 5 A C H T U N G T R E N N U N G (m-OH)(bipy) 6 ] 5+ , the reducing acids (RH) and their conjugate bases (R À) all take part in the reaction. The oxo-bridge protonated oxidant [1H] 5+ was found to react much faster than 1 4+. Thereby, the gemdiol forms of the a-oxo acids (especially in the case of glyoxylic acid) are the possible reductants. A one-electron/one-proton electroprotic mechanism operates in the rate-determining step. 1) The hydration constants of the reducing species as found in [25] are as follows: glyoxylic acid, 1.0 10 2-1.4 10 3 ; glyoxylate, 6.5-260; pyruvic acid, 0.61-3.1; pyruvate, 6.4 10 À2 À 0.21.
In aqueous media, the trinuclear Mn IV complex, [Mn IV 3 (l-O) 4 (phen) 4 (H 2 O) 2 ] 4+ (phen = ... more In aqueous media, the trinuclear Mn IV complex, [Mn IV 3 (l-O) 4 (phen) 4 (H 2 O) 2 ] 4+ (phen = 1,10-phenanthroline) (1) behaves like a monobasic acid, equilibrates with its deprotonated conjugate base (2) (1 2 + H + , pK a = 4.00 (± 0.15) at 25.0°C, I = 1.0 M, maintained with NaNO 3). An aqueous solution of 1 in the pH range 2.2-4.0 smoothly oxidizes 2-mercaptoethanol (mercap) and L-cysteine (cys) to their respective disulfides; itself being reduced to [Mn(phen) 3 ] 2+ in presence of excess 1,10-phenanthroline which also masks Cu 2+ , if any, present in the reaction media as impurity as well as acting as a buffer that controls pH range during the reaction. The observed rate constants in H 2 O media were found to be considerably lowered in media enriched with D 2 O and a proton-coupled single electron transfer rate step is proposed. Interestingly, we found that deprotonated oxidant (2) reacts faster than the protonated species (1) in oxidizing both the thiols.
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