D. Martin Davies

Kinetics of the hydrolysis and perhydrolysis of tetraacetylethylenediamine, a peroxide bleach activator

Hydrogen peroxide and water react with tetraacetylethylenediamine (TAED) to form consecutively triacetylethylenediamine and diacetylethylenediamine with the release of two molecules of peracetic acid or acetic acid. The effect of pH, specific buffers and temperature on the rates of hydrolysis and perhydrolysis are compared. Peracetic acid reacts with TAED very slowly. The ratio of the second-order rate constants for the reaction of TAED with hydroperoxide and peracetate anions is exceptionally large after taking into account the difference in pKa values of their conjugate acids. The relative reactivity of various nucleophiles with TAED is discussed in terms of its performance as a bleach activator.

Kinetic data for redox reactions of cytochrome c with Fe(CN)5X complexes and the question of association prior to electron transfer

Use of rigorous equilibration kinetics to evaluate rate constants for the Fe(CN)6 4- reduction of horse-heart cytochrome c in the oxidized form, cyt c (III), has shown that limiting kinetics do not apply with concentrations of Fe(CN)6 4- (the reactant in excess) in the range 2-10 x 10(-4) M, I = 0.10 M (NaCl). The reaction conforms to a first-order rate law in each reactant, and at 25 degrees C, pH 7.2 (Tris), it is concluded that K for association prior to electron transfer is less than 200 M-1. From previous studies at 25 degrees C, ph 7.0 (10(-1) M phosphate), I = 0.242 M (NaCl), a value K = 2.4 x 10(3) M-1 has been reported. Had such a value applied, some or all of the redox inactive complexes Mo(CN)8 4-, Co(CN)6 3-, Cr(CN)6 3-, Zr(C2O4)4 4- present in amounts 5-20 x 10(-4) M would have been expected to associate at the same site and partially block the redox process. No effect on rats was observed. With the reductants Fe(CN)5(4-NH2-py)3- and Fe(CN)5(imid)3-, reactions proceeded to greater than 90% completion and rate laws were again first order in each reactant. Rate constants (M-1 sec-1) at 25 degrees C, pH 7.2 (Tris), I = 0.10 M (NaCl), are Fe(CN)6 4- (3.5 x 10(4)), Fe(CN)5(4-NH2py)3- (6.7 x 10(5), and Fe(CN)5(imid)3- (4.2 x 10(5). Related reactions in which cyt c(II) is oxidized are also first order in each reactant, Fe(CN)6 3- (9.1 x 10(6)), Fe(CN)5(NCS)3- (1.3 x 10(6)), Fe(CN)5(4-NH2py)2- (3.8 x 10(6) at pH 9.4), and Fe(CN)5(NH3)2- (2.75 x 10(6) at ph 8). Redox inactive Co(CN)6 3- (1.0 x 10(-3) M) has no effect on the reaction of Fe(CN)6 3- which suggests that a recent interpretation for the Fe(CN)6 3- oxidation of cyt c(II), I = 0.07 M, may also require reappraisal.

Cooperativity and steric hindrance: important factors in the binding of α-cyclodextrin with para-substituted aryl alkyl sulfides, sulfoxides and sulfones

Binding constants for 22 para-substituted aryl alkyl sulfides, sulfoxides and sulfones have been determined spectrophotometrically. It was found that the presence of sulfur containing substituents generally results in destabilisation of complex formation, and it is postulated that the angle of the sulfur bond in these compounds results in steric hindrance with the 5-H protons at the rear of the cyclodextrin cavity, causing it to be displaced from its optimal position or orientation. Additionally, the observation, for several sulfides, of cooperativity in the binding of a second molecule of cyclodextrin is discussed in terms of binding induced changes in electrical potential within the cyclodextrin cavity.

Kinetic studies on 1:1 electron transfer reactions involving blue copper proteins: 8. Reactions of plastocyanin and azurin with cytochrome c and high potential iron-sulfur protein

Abstract Rate constants determined by the stopped-flow method for four protein-protein reactions at 25°C, pHs in the range 5.8–7.5. I = 0.10 M (NaCI), are as follows: cytochrome c(II) with plastocyanin, PCu(II). 1.5 × 10 6 M −1 sec −1 , pH 7.6; high-potential iron-sulfur protein (Hipip) with PCu(II), 3.7 × 10 5 M −1 sec −1 . pH 5.8; cytochrome c(II) with azurin, ACu(ll). 6.4 × 10 3 M −1 sec −1 , pH 6.1; Hipip with ACu(II), 2.2 × 10 5 M −1 sec −1 , pH 5.8. Activation parameters have been determined for all four reactions; they indicate higher enthalpy requirements and less negative entropy requirements for the PCu(II) as opposed to ACu(II) reactions. Equilibrium constants K for association prior to electron transfer are −1 for the cytochrome c(II) reduction of PCu(II) (estimated charges 8 + and 9-,respectively), and −1 for the other reactions, indicating no favorable interactions. Rate constants have been analyzed in terms of the simple Marcus theory, which has previously given an excellent fit to thirteen protein-protein reactions considered by Wherland and Pecht. No similar correlation exists in the present studies, and calculated rate constants differ by orders of magnitude from experimentally determined values.

Catalysis and inhibition of the iodide reduction of peracids by surfactants: partitioning of reactants, product and transition state between aqueous and micellar pseudophases

A multiple micellar pseudophase model of kinetics in aqueous surfactant solutions is described. The model has been developed using the transition state pseudoequilibrium constant approach. The advantage of this approach is that no assumptions are made about the nature of the micelle pseudophases. The kinetics of the reduction of pernonanoic and 3-chloroperbenzoic acids by iodide in SDS, Brij-35 and Triton X-100 are reported. The results are used to obtain the micellar association constants of the peracids and also the apparent (virtual) micellar association constants of the transition states. These are compared with the micellar association constants of the parent acids, obtained by pH titration. The association constants are discussed in terms of micellar catalysis and inhibition. Ratios of association constants indicate the relationship between initial, transition and final states.

Effect of α-cyclodextrin on the oxidation of aryl alkyl sulfides by peracids

Substituent and leaving group effects on the uncatalysed reaction were in good agreement with literature studies. The effect of α-cyclodextrin on the kinetics of aryl alkyl sulfide oxidation by peracids was investigated by studying the following reaction series: (a) a range of aryl alkyl sulfides with three different perbenzoic acids and (b) a range of alkyl peracids and perbenzoic acids with five different aryl alkyl sulfides. For peracids which bind strongly to α-cyclodextrin, the observed second-order rate constant increases to a maximum with increasing cyclodextrin concentration and thereafter non-productive binding of the sulfide causes a decline in rate. Weakly binding peracids, such as peracetic acid show only a decline in rate constant with increasing cyclodextrin concentration. Linear free energy relationships reveal that transition state stabilisation by one molecule of cyclodextrin shows a far greater dependence on the stability of the peracid-cyclodextrin complex than on the stability of the sulfide-cyclodextrin complex, indicating that the principle pathway for the cyclodextrin mediated reaction is that between the peracidcyclodextrin complex and uncomplexed sulfide. Additionally, a linear free energy relationship comparing transition state stabilisation for the α-cyclodextrin mediated oxidation of iodide and methyl 4-nitrophenyl sulfide by peracids indicates a common mechanism of catalysis for both substrates, although the catalysis of sulfide oxidation is more effective. Several possible mechanisms of catalysis are discussed. Transition state stabilisation by two molecules of α-cyclodextrin was observed for those peracids which demonstrate significant 2:1 complex formation. Here the principal pathway is the reaction of the 2:1 cyclodextrin–peracid complex with the unbound sulfide, although the extent of transition state stabilisation by the second cyclodextrin molecule is only about the same as its stabilisation of peracid in the ground state.

Multiple pathways in the α-cyclodextrin catalysed reaction of iodide and substituted perbenzoic acids

The kinetic rate equation for the title reaction in aqueous acetate buffer has both first-order and second-order terms with respect to cyclodextrin concentration, due to catalysis both by one and by two molecules of cyclodextrin. The stabilisation of the transition state of the iodide–peracid reaction by cyclodextrin is examined using the pseudoequilibrium constant approach of Tee, Carbohydr. Res., 1989, 192, 181. This approach indicates that, depending on the nature of the peracid, the predominant pathway catalysed by one cyclodextrin molecule involves the reaction of either free iodide and a cyclodextrin–peracid complex or free peracid and a cyclodextrin–iodide complex. The latter two pathways are kinetically indistinguishable, but the corresponding terms in the rate equation are separated using the extrakinetic assumption of a Bronsted-type relationship. This assumption is reasonable since the uncatalysed reaction and that catalysed by two molecules of cyclodextrin show Bronsted-type relationships. The mechanism of catalysis is discussed in terms of the effect of cyclodextrin on the nucleophilicity of the iodide and acid catalysis via the protonation of the benzoate leaving group.

Kinetic treatment of the reaction of m-chloroperbenzoic acid and iodide in mixed anionic/non-ionic micelles

Catalysis and inhibition of the title reaction in mixed micelles of Brij-35 and SDS is described. The kinetics are treated using a combined multiple micellar pseudophase model and transition state pseudoequilibrium constant approach that was previously used for the catalysis and inhibition of the reaction of peracids and iodide in non-ionic and anionic micelles, respectively (D. M. Davies, N. D. Gillitt and P. M. Paradis, J. Chem. Soc., Perkin Trans. 2, 1996, 659). In the present mixed micellar system the following factors are taken into account: (i) the mole ratio of the surfactants in the mixed micelle, which is the relevant quantity for the micellar kinetics and is different from the stoichiometric mole ratio of the surfactants; (ii) non-ideal mixing of the surfactants, which also influences the composition of the mixed micelle; (iii) the ideal behaviour of the apparent molar volume of micellized surfactant in mixed micelles; and (iv) deviations from ideality of the partitioning of reactants and transition state between mixed micelles and the bulk aqueous phase. Logarithms of micellar association constants of the reactants and transition state in the two surfactants have been obtained, together with parameters closely related to the excess Gibbs function associated with the deviations from ideality described by (iv). The relationship between the parameters that describe non-ideal mixing of the surfactants in (ii) and the deviations from ideality in (iv) is discussed.

The interaction of α-cyclodextrin with aliphatic, aromatic and inorganic peracids, the corresponding parent acids and their respective anions

Potentiometric or combined potentiometric and spectrophotometric or kinetic techniques have been used to determine stability constants for complexes between α-cyclodextrin and 20 of the title compounds. Linear free energy relationships indicate that 4-substituted benzoic acids, perbenzoic acids and perbenzoates have predominantly the same orientation within the cyclodextrin cavity, with the carboxylic acid, percarboxylic acid and percarboxylate groups located at the narrow (primary hydroxy) end of the cavity. 4-Substituted benzoates orientate in the opposite way with the carboxylate group located at the wide end of the cavity. Alkyl carboxylic acids, percarboxylic acids and their anions show a linear dependence between log stability constant and the number of carbons. They are likely to bind with the functional group at the narrow end of the cavity, although the carboxylate groups will probably be located outside the cavity because of solvation requirements. 2:1 cyclodextrin-guest complexes are observed for several of the compounds studied.

Cyclodextrin complexes of substituted perbenzoic and benzoic acids and their conjugate bases: free energy relationships show the interaction of polar and steric factors

The stability constants of the complexes of α-cyclodextrin and 4-methyl-, 4-nitro-, 4-sulfonato- and 3-chloro-substituted perbenzoic acids, perbenzoates and benzoates, but not benzoic acids, show linear free energy relationships. In contrast to α-cyclodextrin, the stability constants of the β-cyclodextrin complexes of perbenzoic acid and benzoic acids do show the same trend. The stability constants are discussed in terms of the orientation of the guest species in the cyclodextrin cavity.