A Kinetic Study of the Electron-Transfer Reactions of Nickel(III,II) Tripeptide Complexes with Cyano Complexes of Molybdenum, Tungsten, and Iron.

Experimentally measured rate constants, k12obsd, for the reductions of [Ni(III)tripeptides(H2O)2] with Fe(CN)64-, Mo(CN)84-, and W(CN)84- are 102 to 105 times faster than the calculated rate constants with the Marcus theory for outer-sphere electron-transfer processes, k12calc, even when work terms are considered. This gives rise to a kinetic advantage of k12obsd/k12calc = 102-105, which is consistent with an inner-sphere electron-transfer mechanism via a bridged intermediate. In addition, k12obsd values are nearly independent of the electrochemical driving force of the reactions. This is consistent with one of the two axial water ligands coordinated to [Ni(III)tripeptides(H2O)2] being substituted in the rate-limiting step to form bridged intermediates of the type [(CN)5or7M-(CN)-NiIII(tripeptide)(H2O)]4- with M = FeII, MoIV, or WIV. A limiting rate constant of H2O replacement from [Ni(III)tripeptides(H2O)2] of (5 ± 2) × 107 M-1 s-1 at 25.0 °C is observed. Electron paramagnetic resonance spectra of Ni(III) peptide complexes in the presence of Fe(CN)63-, Mo(CN)83-, or IrCl63- provide evidence for the cyanide-bridged intermediates. Substitution-limited electron-transfer reactions could serve as an additional criterion for inner-sphere pathways when atom or group transfer does not occur during electron-transfer and when precursor and successor complexes cannot be observed directly.

Kinetics of Hypobromous Acid Disproportionation.

The kinetics of aqueous hypobromous acid disproportionation are measured at 25.0 degrees C from p[H(+)] 0.2 to 10.2. The reactions are second order in HOBr with a maximum rate at pH 3-8. The rate of disproportionation decreases significantly above pH 8 as OBr(-) forms. Another suppression observed below pH 3 is attributed to the reversibility of initial steps in the decomposition. The rate expression is given by -d[Br(I)]/dt = n{(c/(c + [H(+)])k(1a) + k(B)[B])[HOBr](2) + k(1b)[OBr(-)](2)}, where k(1a) = 2 x 10(-)(3) M(-)(1) s(-)(1), k(B)[B] is a general-base-assisted pathway, k(1b) = 6 x 10(-)(7) M(-)(1) s(-)(1), n is a stoichiometric factor that ranges from 2 to 5, and c is a ratio of rate constants that is equal to 0.03 M. Decomposition is catalyzed by HPO(4)(2)(-) (k(B) = 0.05 M(-)(2) s(-)(1)) and by CO(3)(2)(-) (k(B) = 0.33 M(-)(2) s(-)(1)). Above pH 8, the first observable product is BrO(2)(-) (initially n = 2). Below pH 4, n = 5 due to Br(2) and BrO(3)(-) formation. From pH 4 to 7, n varies from 5 to 3. A detailed mechanism is presented.

Mechanism of Chlorine Dioxide and Chlorate Ion Formation from the Reaction of Hypobromous Acid and Chlorite Ion.

The rate of oxidation of ClO(2)(-) by HOBr is first-order in each reactant and is general-acid-assisted in the presence of phosphate or carbonate buffers. The products are ClO(2) and ClO(3)(-), where the relative yield depends on the concentration ratio of ClO(2)(-)/OH(-). The kinetic dependence indicates the presence of a steady-state intermediate, HOBrOClO(-) (or HOBrClO(2)(-)), that undergoes general-acid-assisted reactions to generate a metastable intermediate, BrOClO (or BrClO(2)). This intermediate reacts very rapidly by two competing pathways: in one path ClO(2)(-) reacts to form 2ClO(2) and Br(-), and in the other path OH(-) (or H(2)O) reacts to form ClO(3)(-) and Br(-). Competition between these pathways determines the yield of ClO(2) but does not affect the rate of loss of HOBr. The reactions are followed by the formation of ClO(2) in the presence of excess ClO(2)(-). The rate expression for the loss of HOBr is k(1)[ClO(2)(-)][HOBr] summation operator(k(HA)[HA])/(k(-)(1) + summation operator(k(HA)[HA])), where k(1) (for the formation of the intermediate) is 97 M(-)(1) s(-)(1) and k(HA)/k(-)(1) (M(-)(1)) values, which depend on the acid (HA) strength, are 3.1 x 10(5) for H(3)O(+), 8.3 for H(2)PO(4)(-), and 0.064 for HCO(3)(-) (25.0 degrees C, &mgr; = 1.0 M). Reactions between HOBr and ClO(2)(-) are much faster than those between HOCl and ClO(2)(-).

Equilibrium and Kinetics of Bromine Hydrolysis.

Equilibrium constants for bromine hydrolysis, K(1) = [HOBr][H(+)][Br(-)]/[Br(2)(aq)], are determined as a function of ionic strength (&mgr;) at 25.0 degrees C and as a function of temperature at &mgr; approximately 0 M. At &mgr; approximately 0 M and 25.0 degrees C, K(1) = (3.5 +/- 0.1) x 10(-)(9) M(2) and DeltaH degrees = 62 +/- 1 kJ mol(-)(1). At &mgr; = 0.50 M and 25.0 degrees C, K(1) = (6.1 +/- 0.1) x 10(-)(9) M(2) and the rate constant (k(-)(1)) for the reverse reaction of HOBr + H(+) + Br(-) equals (1.6 +/- 0.2) x 10(10) M(-)(2) s(-)(1). This reaction is general-acid-assisted with a Brønsted alpha value of 0.2. The corresponding Br(2)(aq) hydrolysis rate constant, k(1), equals 97 s(-)(1), and the reaction is general-base-assisted (beta = 0.8).

Disproportionation Kinetics of Hypoiodous Acid As Catalyzed and Suppressed by Acetic Acid-Acetate Buffer.

The kinetics of the disproportionation of hypoiodous acid to give iodine and iodate ion (5HOI right harpoon over left harpoon 2I(2) + IO(3)(-) + H(+) + 2H(2)O) are investigated in aqueous acetic acid-sodium acetate buffer. The rate of iodine formation is followed photometrically at -log [H(+)] = 3.50, 4.00, 4.50, and 5.00, &mgr; = 0.50 M (NaClO(4)), and 25.0 degrees C. Both catalytic and inhibitory buffer effects are observed. The first process is proposed to be a disproportionation of iodine(I) to give HOIO and I(-); the iodide then reacts with HOI to give I(2). The reactive species (acetato-O)iodine(I), CH(3)CO(2)I, is postulated to increase the rate by assisting in the formation of I(2)O, a steady-state species that hydrolyzes to give HOIO and I(2). Inhibition is postulated to result from the formation of the stable ion bis(acetato-O)iodate(I), (CH(3)CO(2))(2)I(-), as buffer concentration is increased. This species is observed spectrophotometrically with a UV absorption shoulder (lambda = 266 nm; epsilon = 530 M(-)(1) cm(-)(1)). The second process is proposed to be a disproportionation of HOIO to give IO(3)(-) and I(2). Above 1 M total buffer, the reaction becomes reversible with less than 90% I(2) formation. Rate and equilibrium constants are resolved and reported for the proposed mechanism.

Characterization of Copper(III)-Tetrapeptide Complexes with Histidine as the Third Residue.

Copper(III) complexes of Gly(2)HisGly and Aib(2)HisGly are characterized, where Gly is glycine, His is L-histidine, and Aib is alpha-aminoisobutyric acid. Their respective reduction potentials (V vs NHE) are 0.978 and 0.826. Both Cu(III) complexes undergo amine deprotonation with pK(a) values of 8.79 and 8.81, respectively. The influence of the 5-5-6 membered chelate rings on the E degrees ' and pK(a) values is examined relative to the Cu(III)-tripeptide complexes without histidine that have 5-5-6, 5-6-5, 6-5-5, and 5-5-5 membered linked-consecutive rings. The presence of a six-membered ring in the third peptide residue causes a decrease of approximately 1.0 pK(a) unit relative to a 5-5-5 membered ring system. Imidazole coordination from histidine compared to carboxylate coordination causes an additional decrease of 1.3 pK(a) units. Decompositions of Cu(III)(H(-2)Gly(2)HisGly) and Cu(III)(H(-2)Aib(2)HisGly) complexes are measured over the pH range 0.3-4.7. The kinetics and the reaction products show that abstraction of a proton at the alpha carbon of the histidyl residue is the rate-determining step. The initial decomposition product of Gly(2)HisGly is a tetrapeptide with an alpha-hydroxyhistidyl residue that dehydrates to give an alkene with an alpha,beta-dehydrohistidyl residue. UV-vis spectral properties are reported for the alkene as well as the Cu(III) complexes of Gly(2)HisGly and Aib(2)HisGly.

Chlorine dioxide oxidation of dihydronicotinamide adenine dinucleotide (NADH).

The oxidation of dihydronicotinamide adenine dinucleotide (NADH) by chlorine dioxide in phosphate buffered solutions (pH 6-8) is very rapid with a second-order rate constant of 3.9 x 10(6) M(-1) s(-1) at 24.6 degrees C. The overall reaction stoichiometry is 2ClO2(*) per NADH. In contrast to many oxidants where NADH reacts by hydride transfer, the proposed mechanism is a rate-limiting transfer of an electron from NADH to ClO2(*). Subsequent sequential fast reactions with H(+) transfer to H2O and transfer of an electron to a second ClO2(*) give 2ClO2(-), H3O(+), and NAD(+) as products. The electrode potential of 0.936 V for the ClO2(*)/ClO2(-) couple is so large that even 0.1 M of added ClO2(-) (a 10(3) excess over the initial ClO2(*) concentration) fails to suppress the reaction rate.

Kinetics and mechanisms of chlorine dioxide oxidation of tryptophan.

The reactions of aqueous ClO2 (*) and tryptophan (Trp) are investigated by stopped-flow kinetics, and the products are identified by high-performance liquid chromatography (HPLC) coupled with electrospray ionization mass spectrometry and by ion chromatography. The rates of ClO2 (*) loss increase from pH 3 to 5, are essentially constant from pH 5 to 7, and increase from pH 7 to 10. The reactions are first-order in Trp with variable order in ClO2 (*). Below pH 5.0, the reactions are second- or mixed-order in [ClO2 (*)], depending on the chlorite concentration. Above pH 5.0, the reactions are first-order in [ClO2 (*)] in the absence of added chlorite. At pH 7.0, the Trp reaction with ClO2 (*) is first-order in each reactant with a second-order rate constant of 3.4 x 10(4) M(-1) s(-1) at 25.0 degrees C. In the proposed mechanism, the initial reaction is a one-electron oxidation to form a tryptophyl radical cation and chlorite ion. The radical cation deprotonates to form a neutral tryptophyl radical that combines rapidly with a second ClO 2 (*) to give an observable, short-lived adduct ( k obs = 48 s(-1)) with proposed C(H)-OClO bonding. This adduct decays to give HOCl in a three-electron oxidation. The overall reaction consumes two ClO2 (*) per Trp and forms ClO2- and HOCl. This corresponds to a four-electron oxidation. Decay of the tryptophyl-OClO adduct at pH 6.4 gives five initial products that are observed after 2 min and are separated by HPLC with elution times that vary from 4 to 17 min (with an eluent of 6.3% CH 3OH and 0.1% CH 3COOH). Each of these products is characterized by mass spectrometry and UV-vis spectroscopy. One initial product with a molecular weight of 236 decays within 47 min to yield the most stable product, N-formylkynurenine (NFK), which also has a molecular weight of 236. Other products also are observed and examined.

Chlorine dioxide oxidation of guanosine 5'-monophosphate.

The reactions between aqueous ClO2 and guanosine 5'-monophosphate (5'-GMP) are investigated from pH 5.96 to 8.30. The decay of ClO2 follows mixed first-order and second-order kinetics. The addition of chlorite (0.01-0.05 M) to the reaction mixture suppresses the reaction rate and changes the observed decay of ClO2 to second-order. The reaction rates increase greatly with pH to give oxidized products. The second-order rate constant for the guanosine anion is 4.7 x 10(5 )M-1 s-1 and comprises a mixture of rate constants, k1k2/k-1. The ratio k1/k-1, with a calculated value of 2.4 x 10(-4), corresponds to the reversible reaction between ClO2 and the guanosine anion to generate ClO2- and the guanosyl radical. To determine k1/k-1 and k2, E values for guanosine and ClO2 are used as well as acid dissociation constants for guanosine and its radical. The value of k1 (1.1 x 10(5) M-1 s-1) represents the reaction between ClO2 and the guanosine anion as determined by initial rates. The second-order rate constant k2, with a value of 1.8 x 10(9 )M-1 s-1, represents the reaction between the guanosyl radical with a second molecule of ClO2 to generate a guanosyl-OClO adduct. The consumption of two mol of ClO2 per mol of 5'-GMP corresponds to a four-electron oxidation that gives ClO(2- )in the first step and HOCl in the second step. The 2',3',5'-tri-O-acetylated derivative of guanosine is used to more easily separate guanosine from its ClO2 oxidation products. Imidazolone and monochlorinated imidazolone are identified as products of the reaction between ClO2 and guanosine.

Kinetics and mechanisms of chlorine dioxide and chlorite oxidations of cysteine and glutathione.

Chlorine dioxide oxidation of cysteine (CSH) is investigated under pseudo-first-order conditions (with excess CSH) in buffered aqueous solutions, p[H+] 2.7-9.5 at 25.0 degrees C. The rates of chlorine dioxide decay are first order in both ClO2 and CSH concentrations and increase rapidly as the pH increases. The proposed mechanism is an electron transfer from CS- to ClO2 (1.03 x 10(8) M(-1) s(-1)) with a subsequent rapid reaction of the CS* radical and a second ClO2 to form a cysteinyl-ClO2 adduct (CSOClO). This highly reactive adduct decays via two pathways. In acidic solutions, it hydrolyzes to give CSO(2)H (sulfinic acid) and HOCl, which in turn rapidly react to form CSO3H (cysteic acid) and Cl-. As the pH increases, the (CSOClO) adduct reacts with CS- by a second pathway to form cystine (CSSC) and chlorite ion (ClO2-). The reaction stoichiometry changes from 6 ClO2:5 CSH at low pH to 2 ClO2:10 CSH at high pH. The ClO2 oxidation of glutathione anion (GS-) is also rapid with a second-order rate constant of 1.40 x 10(8) M(-1) s(-1). The reaction of ClO2 with CSSC is 7 orders of magnitude slower than the corresponding reaction with cysteinyl anion (CS-) at pH 6.7. Chlorite ion reacts with CSH; however, at p[H+] 6.7, the observed rate of this reaction is slower than the ClO2/CSH reaction by 6 orders of magnitude. Chlorite ion oxidizes CSH while being reduced to HOCl, which in turn reacts rapidly with CSH to form Cl-. The reaction products are CSSC and CSO3H with a pH-dependent distribution similar to the ClO2/CSH system.

Chlorine dioxide oxidations of tyrosine, N-acetyltyrosine, and dopa.

The reactions of aqueous ClO2 with tyrosine, N-acetyltyrosine, and dopa (3,4-dihydroxyphenylalanine) are investigated from pH 4 to 7. The reaction rates increase greatly with pH to give a series of oxidized products. Tyrosine and N-acetyltyrosine have similar reactivities with second-order rate constants (25.0 degrees C) for their phenoxide forms equal to 1.8x10(8) and 7.6x10(7) M-1 s-1, respectively. Both species generate phenoxyl radicals that react rapidly with a second ClO2 at the 3 position to give observable but short-lived adducts with proposed C(H)OClO bonding. The decay of these phenoxyl-ClO2 adducts also is rapid and is base-assisted to form dopaquinone (from tyrosine) and N-acetyldopaquinone (from N-acetyltyrosine) as initial products. The consumption of two ClO2 molecules corresponds to a four-electron oxidation that gives ClO2- in the first step and HOCl in the second step. The reaction between ClO2 and the deprotoned-catechol form of dopa is extremely fast (2.8x10(9) M-1 s-1). Dopa consumes two ClO2 to give dopaquinone and 2ClO2- as products. Above pH 4, dopaquinone cyclizes to give cyclodopa, which in turn is rapidly oxidized to dopachrome. A resolved first-order rate constant of 249 s-1 is evaluated for the cyclization of the basic form of dopaquinone that leads to dopachrome as a product with strong absorption bands at 305 and 485 nm.

Kinetics and mechanisms of aqueous chlorine reactions with chlorite ion in the presence of chloride ion and acetic acid/acetate buffer.

The kinetics and mechanism of the reaction between Cl(2) and ClO(2)(-) are studied in acetate buffer by stopped-flow spectrometric observation of ClO(2) formation. The reaction is first-order in [Cl(2)] and [ClO(2)(-)], with a rate constant of k(1) = (5.7 +/- 0.2) x 10(5) M(-)(1) s(-)(1) at 25.0 degrees C. Nucleophilic attack by ClO(2)(-) on Cl(2), with Cl(+) transfer to form ClOClO and Cl(-), is proposed as the rate-determining step. A possible two-step electron-transfer mechanism for Cl(2) and ClO(2)(-) is refuted by the lack of ClO(2) suppression. The yield of ClO(2) is much less than 100%, due to the rapid reactions of the metastable ClOClO intermediate via two competing pathways. In one path, ClOClO reacts with ClO(2)(-) to form 2ClO(2) and Cl(-), while in the other path it hydrolyzes to give ClO(3)(-) and Cl(-). The observed rate constant also is affected by acetate-assisted hydrolysis of Cl(2). The rate of Cl(2) loss is suppressed as the concentration of Cl(-) increases, due to the formation of Cl(3)(-). In excess ClO(2)(-), a much slower formation of ClO(2) is observed after the initial Cl(2) reaction, due to the presence of HOCl, which reacts with H(+) and Cl(-) to re-form steady-state levels of Cl(2).

Hypohalite ion catalysis of the disproportionation of chlorine dioxide.

The disproportionation of chlorine dioxide in basic solution to give ClO2- and ClO3- is catalyzed by OBr- and OCl-. The reactions have a first-order dependence in both [ClO2] and [OX-] (X = Br, Cl) when the ClO2- concentrations are low. However, the reactions become second-order in [ClO2] with the addition of excess ClO2-, and the observed rates become inversely proportional to [ClO2-]. In the proposed mechanisms, electron transfer from OX- to ClO2(k1OBr- = 2.05 +/- 0.03 M(-1) x s(-1) for OBr(-)/ClO2 and k1OCl-= 0.91 +/- 0.04 M(-1) x s(-1) for OCl-/ClO2) occurs in the first step to give OX and ClO2-. This reversible step (k1OBr-/k(-1)OBr = 1.3 x 10(-7) for OBr-/ClO2, / = 5.1 x 10(-10) for OCl-/ClO2) accounts for the observed suppression by ClO2-. The second step is the reaction between two free radicals (XO and ClO2) to form XOClO2. These rate constants are = 1.0 x 10(8) M(-1) x s(-1) for OBr/ClO2 and = 7 x 10(9) M(-1) x s(-1) for OCl/ClO2. The XOClO2 adduct hydrolyzes rapidly in the basic solution to give ClO3- and to regenerate OX-. The activation parameters for the first step are DeltaH1(++) = 55 +/- 1 kJ x mol(-1), DeltaS1(++) = - 49 +/- 2 J x mol(-1) x K(-1) for the OBr-/ClO2 reaction and DeltaH1(++) = 61 +/- 3 kJ x mol(-1), DeltaS1(++) = - 43 +/- 2 J x mol(-1) x K(-1) for the OCl-/ClO2 reaction.

Role of halogen(I) cation-transfer mechanisms in water chlorination in the presence of bromide ion.

Bromide ion is rapidly converted to HOBr via BrCl by reaction with HOCl. The subsequent slow reactions of (HOCl, OCl-)/(HOBr, OBr-) mixtures are monitored directly by multiwavelength UV-vis absorbance methods and simultaneously by ion chromatographic measurement of ClO2-, ClO3-, and BrO3- (p[H+] 5.6-7.6). A first-order loss of HOCl is observed which is catalyzed by trace concentrations of Br- and BrCl. Chlorite ion forms first and is subsequently oxidized to ClO3-. The loss of HOBr is slower and is second-order in HOBr, so that BrO3- formation takes longer than ClO3- formation. Under the conditions of this work, the relative yield of BrO3- increases with increase in pH. The decomposition of HOCI by bromide proceeds primarily by a series of halogen(I) cation-transfer reactions with subsequent halide release. The presence of HOCI increases the BrO3- yield three-fold from HOBr decay alone.

Nucleophile assistance of electron-transfer reactions between nitrogen dioxide and chlorine dioxide concurrent with the nitrogen dioxide disproportionation.

The reaction of chlorine dioxide with excess NO(2)(-) to form ClO(2)(-) and NO(3)(-) in the presence of a large concentration of ClO(2)(-) is followed via stopped-flow spectroscopy. Concentrations are set to establish a preequilibrium among ClO(2), NO(2)(-), ClO(2)(-), and an intermediate, NO(2). Studies are conducted at pH 12.0 to avoid complications due to the ClO(2)(-)/NO(2)(-) reaction. These conditions enable the kinetic study of the ClO(2) reaction with nitrogen dioxide as well as the NO(2) disproportionation reaction. The rate of the NO(2)/ClO(2) electron-transfer reaction is accelerated by different nucleophiles (NO(2)(-) > Br(-) > OH(-) > CO(3)(2-) > PO(4)(3-) > ClO(2)(-) > H(2)O). The third-order rate constants for the nucleophile-assisted reactions between NO(2) and ClO(2) (k(Nu), M(-2) s(-1)) at 25.0 degrees C vary from 4.4 x 10(6) for NO(2-) to 2.0 x 10(3) when H(2)O is the nucleophile. The nucleophile is found to associate with NO(2) and not with ClO(2) in the rate-determining step to give NuNO(2)(+) + ClO(2)(-). The concurrent NO(2) disproportionation reaction exhibits no nucleophilic effect and has a rate constant of 4.8 x 10(7) M(-1) s(-1). The ClO(2)/NO(2)/nucleophile reaction is another example of a system that exhibits general nucleophilic acceleration of electron transfer. This system also represents an alternative way to study the rate of NO(2) disproportionation.

Cu(II)Gly2HisGly oxidation by H2O2/ascorbic acid to the CuIII complex and its subsequent decay to alkene peptides.

Protonation and stability constants for Gly2HisGly and its Cu(II) complexes (beta(mhl)), determined at 25.0 degrees C and mu = 0.10 M (NaClO(4)), have values of log beta(011) = 7.90, log beta(021) = 14.51, log beta(031) = 17.55, log beta(101) = 7.82, log beta(1-21) = -0.80, and log beta(1-31) = -12.7 (where the subscripts refer to the number of metal ions, protons, and ligands, respectively). The reaction of CuII(H(-2)Gly(2)HisGly)- with l-ascorbic acid and H(2)O(2) at p[H(+)] 6.6 rapidly generates Cu(III)(H(-2)Gly(2)HisGly) with lambda(max) at 260 and 396 nm, which is separated chromatographically. The Cu(III) complex decomposes to give alkene peptide isomers of glycylglycyl-alpha,beta-dehydrohistidylglycine that are separated chromatographically and characterized. These alkene peptide species are present as geometric isomers with imidazole tautomers that have distinct spectral and chemical characteristics. The proposed major isomeric form (94%), Z-N(tau)-H, has a hydrogen on the tau nitrogen of the imidazole ring and three conjugated double bonds. It has absorption bands at 295 and 360 nm at p[H+] 6.6 in the absence of copper. The proposed minor isomeric form (6%) is E-N(pi)-H with a hydrogen on the pi nitrogen of the imidazole ring. This isomer has four conjugated double bonds and strongly binds Cu(II) to give a complex with intense absorption bands at 434 and 460 nm.

Chlorine dioxide reduction by aqueous iron(II) through outer-sphere and inner-sphere electron-transfer pathways.

The reduction of ClO(2) to ClO(2)(-) by aqueous iron(II) in 0.5 M HClO(4) proceeds by both outer-sphere (86%) and inner-sphere (14%) electron-transfer pathways. The second-order rate constant for the outer-sphere reaction is 1.3 x 10(6) M(-1) s(-1). The inner-sphere electron-transfer reaction takes place via the formation of FeClO(2)(2+) that is observed as an intermediate. The rate constant for the inner-sphere path (2.0 x 10(5) M(-1) s(-1)) is controlled by ClO(2) substitution of a coordinated water to give an inner-sphere complex between ClO(2) and Fe(II) that very rapidly transfers an electron to give (Fe(III)(ClO(2)(-))(H(2)O)(5)(2+))(IS). The composite activation parameters for the ClO(2)/Fe(aq)(2+) reaction (inner-sphere + outer-sphere) are the following: DeltaH(r)++ = 40 kJ mol(-1); DeltaS(r)++ = 1.7 J mol(-1) K(-1). The Fe(III)ClO(2)(2+) inner-sphere complex dissociates to give Fe(aq)(3+) and ClO(2)(-) (39.3 s(-1)). The activation parameters for the dissociation of this complex are the following: DeltaH(d)++= 76 kJ mol(-1); DeltaS(d)++= 32 J K(-1) mol(-1). The reaction of Fe(aq)(2+) with ClO(2)(-) is first order in each species with a second-order rate constant of k(ClO2)- = 2.0 x 10(3) M(-1) s(-1) that is five times larger than the rate constant for the Fe(aq)(2+) reaction with HClO(2) in H(2)SO(4) medium ([H(+)] = 0.01-0.13 M). The composite activation parameters for the Fe(aq)(2+)/Cl(III) reaction in H(2)SO(4) are DeltaH(Cl(III))++ = 41 kJ mol(-1) and DeltaS(Cl(III))++ = 48 J mol(-1) K(-1).

Nickel(III) oxidation of its glycylglycylhistamine complex.

The doubly-deprotonated Ni(III) complex of Gly(2)Ha (where Ha is histamine) undergoes base-assisted oxidative self-decomposition of the peptide. At

New pathways for chlorine dioxide decomposition in basic solution.

The product distribution from the decay of chlorine dioxide in basic solution changes as the ClO(2) concentration decreases. While disproportionation reactions that give equal amounts of ClO(2)(-) and ClO(3)(-) dominate the stoichiometry at millimolar or higher levels of ClO(2), the ratio of ClO(2)(-) to ClO(3)(-) formed increases significantly at micromolar ClO(2) levels. Kinetic evidence shows three concurrent pathways that all exhibit a first-order dependence in [OH(-)] but have variable order in [ClO(2)]. Pathway 1 is a disproportionation reaction that is first order in [ClO(2)]. Pathway 2, a previously unknown reaction, is also first order in [ClO(2)] but forms ClO(2)(-) as the only chlorine-containing product. Pathway 3 is second order in [ClO(2)] and generates equal amounts of ClO(2)(-) and ClO(3)(-). A Cl(2)O(4) intermediate is proposed for this path. At high concentrations of ClO(2), pathway 3 causes the overall ClO(3)(-) yield to approach the overall yield of ClO(2)(-). Pathway 2 is attributed to OH(-) attack on an oxygen atom of ClO(2) that leads to peroxide intermediates and yields ClO(2)(-) and O(2) as products. This pathway is important at low levels of ClO(2).

Decomposition kinetics of Ni(III)-peptide complexes with histidine and histamine as the third residue.

The decomposition kinetics of the Ni(III) complexes of Gly(2)HisGly and Gly(2)Ha are studied from p[H(+)] 3.5 to 10, where His is l-histidine and Ha is histamine. In these redox reactions, at least two Ni(III) complexes are reduced to Ni(II) while oxidizing a single peptide ligand. The rate of Ni(III) loss is first order at low pH, mixed order from pH 7.0 to 8.5, and second order at higher pH. The transition from first- to second-order kinetics is attributed to the formation of an oxo-bridged Ni(III)-peptide dimer. The rates of decay of the Ni(III) complexes are general-base assisted with Brønsted beta values of 0.62 and 0.59 for Ni(III)Gly(2)HisGly and Ni(III)Gly(2)Ha, respectively. The coordination of Gly(2)HisGly and Gly(2)Ha to Ni(II) are examined by UV-vis and CD spectroscopy. The square planar Ni(II)(H(-2)Gly(2)HisGly)(-) and Ni(II)(H(-2)Gly(2)Ha) complexes lose an additional proton from an imidazole nitrogen at high pH with pK(a) values of 11.74 and 11.54, respectively. The corresponding Ni(III) complexes have axially coordinated water molecules with pK(a) values of 9.37 and 9.44. At higher pH an additional proton is lost from the imidazole nitrogen with a pK(a) value of 10.50 to give Ni(III)(H(-3)Gly(2)Ha)(H(2)O)(OH)(2-).