Biomedical subjects

Dale W Margerum

Publications and source records attributed to Dale W Margerum.

9 recordsLinked to original sources

PubMed2003

Kinetics and mechanisms of the reactions of hypochlorous acid, chlorine, and chlorine monoxide with bromite ion.

The reaction between BrO2(-) and excess HOCl (p[H+] 6-7, 25.0 degrees C) proceeds through several pathways. The primary path is a multistep oxidation of HOCl by BrO(2)(-) to form ClO(3)(-) and HOBr (85% of the initial 0.15 mM BrO(2)(-)). Another pathway produces ClO(2) and HOBr (8%), and a third pathway produces BrO(3)(-) and Cl(-) (7%). With excess HOCl concentrations, Cl(2)O also is a reactive species. In the proposed mechanism, HOCl and Cl(2)O react with BrO(2)(-) to form steady-state species, HOClOBrO(-) and ClOClOBrO(-). Acid facilitates the conversion of HOClOBrO(-) and ClOClOBrO(-) to HOBrOClO(-). These reactions require a chainlike connectivity of the intermediates with alternating halogen-oxygen bonding (i.e. HOBrOClO(-)) as opposed to Y-shaped intermediates with a direct halogen-halogen bond (i.e. HOBrCl(O)O(-)). The HOBrOClO(-) species dissociates into HOBr and ClO(2)(-) or reacts with general acids to form BrOClO. The distribution of products suggests that BrOClO exists as a BrOClO.HOCl adduct in the presence of excess HOCl. The primary products, ClO(3)(-) and HOBr, are formed from the hydrolysis of BrOClO.HOCl. A minor hydrolysis path for BrOClO.HOCl gives BrO(3)(-) and Cl(-). An induction period in the formation of ClO(2) is observed due to the buildup of ClO(2)(-), which reacts with BrOClO.HOCl to give 2 ClO(2) and Br(-). Second-order rate constants for the reactions of HOCl and Cl(2)O with BrO(2)(-) are k(1)(HOCl) = 1.6 x 10(2) M(-1) s(-1) and k(1)(Cl)()2(O) = 1.8 x 10(5) M(-)(1) s(-)(1). When Cl(-) is added in large excess, a Cl(2) pathway exists in competition with the HOCl and Cl(2)O pathways for the loss of BrO(2)(-). The proposed Cl(2) pathway proceeds by Cl(+) transfer to form a steady-state ClOBrO species with a rate constant of k(1)(Cl2) = 8.7 x 10(5) M(-1) s(-1).

Journal Article↗

PubMed2003

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.

Alkenes↗

PubMed2003

Kinetics and mechanisms of S(IV) reductions of bromite and chlorite ions.

The reaction of bromite with aqueous S(IV) is first order in both reactants and is general-acid catalyzed. The reaction half-lives vary from 5 ms (p[H+] 5.9) to 210 s (p[H+] 13.1) for 0.7 mM excess S(IV) at 25 degrees C. The proposed mechanism includes a rapid reaction (k(1) = 3.0 x 10(7) M(-1) s(-1)) between BrO(2)(-) and SO(3)(2-) to form a steady-state intermediate, (O(2)BrSO(3))(3-). General acids assist the removal of an oxide ion from (O(2)BrSO(3))(3-) to form OBrSO(3)(-), which hydrolyzes rapidly to give OBr(-) and SO(4)(2-). Subsequent fast reactions between HOBr/OBr(-) and SO(3)(2-) give Br(-) and SO(4)(2-) as final products. In contrast, the chlorite reactions with S(IV) are 5-6 orders of magnitude slower. These reactions are specific-acid, not general-acid, catalyzed. In the proposed mechanism, ClO(2)(-) and SO(3)H(-)/SO(2) react to form (OClOSO(3)H)(2)(-) and (OClOSO(2))(-) intermediates which decompose to form OCl(-) and SO(4)(2-). Subsequent fast reactions between HOCl/OCl(-) and S(IV) give Cl- and SO(4)(2-) as final products. SO(2) is 6 orders of magnitude more reactive than SO(3)H-, where k(5)(SO(2)/ClO(2)(-)) = 6.26 x 10(6) M(-1) s(-1) and k(6)(SO(3)H(-)/ClO(2)(-)) = 5.5 M(-1) s(-1). Direct reaction between ClO(2)(-) and SO(3)(2-) is not observed. The presence or absence of general-acid catalysis leads to the proposal of different connectivities for the initial reactive intermediates, where a Br-S bond forms with BrO(2)(-) and SO(3)(2-), while an O-S bond forms with ClO(2)(-) and SO(3)H-.

Journal Article↗

PubMed2002

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).

Journal Article↗

PubMed2002

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.

Journal Article↗

PubMed2002

Kinetics and mechanisms of the ozone/bromite and ozone/chlorite reactions.

Ozone reactions with XO(2)(-) (X = Cl or Br) are studied by stopped-flow spectroscopy under pseudo-first-order conditions with excess XO(2)(-). The O(3)/XO(2)(-) reactions are first-order in [O(3)] and [XO(2)(-)], with rate constants k(1)(Cl) = 8.2(4) x 10(6) M(-1) s(-1) and k(1)(Br) = 8.9(3) x 10(4) M(-1) s(-1) at 25.0 degrees C and mu = 1.0 M. The proposed rate-determining step is an electron transfer from XO(2)(-) to O(3) to form XO(2) and O(3)(-). Subsequent rapid reactions of O(3)(-) with general acids produce O(2) and OH. The OH radical reacts rapidly with XO(2)(-) to form a second XO(2) and OH(-). In the O(3)/ClO(2)(-) reaction, ClO(2) and ClO(3)(-) are the final products due to competition between the OH/ClO(2)(-) reaction to form ClO(2) and the OH/ClO(2) reaction to form ClO(3)(-). Unlike ClO(2), BrO(2) is not a stable product due to its rapid disproportionation to form BrO(2)(-) and BrO(3)(-). However, kinetic spectra show that small but observable concentrations of BrO(2) form within the dead time of the stopped-flow instrument. Bromine dioxide is a transitory intermediate, and its observed rate of decay is equal to half the rate of the O(3)/BrO(2)(-) reaction. Ion chromatographic analysis shows that O(3) and BrO(2)(-) react in a 1/1 ratio to form BrO(3)(-) as the final product. Variation of k(1)(X) values with temperature gives Delta H(++)(Cl) = 29(2) kJ mol(-1), DeltaS(++)(Cl) = -14.6(7) J mol(-1) K(-1), Delta H(++)(Br) = 54.9(8) kJ mol(-1), and Delta S(++)(Br) = 34(3) J mol(-1) K(-1). The positive Delta S(++)(Br) value is attributed to the loss of coordinated H(2)O from BrO(2)(-) upon formation of an [O(3)BrO(2)(-)](++) activated complex.

Journal Article↗

PubMed2002

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).

Journal Article↗

PubMed2002

Bromite ion catalysis of the disproportionation of chlorine dioxide with nucleophile assistance of electron-transfer reactions between ClO(2) and BrO(2) in basic solution.

The rate of ClO(2) conversion to ClO(2)(-) and ClO(3)(-) is accelerated by BrO(2)(-), repressed by ClO(2)(-), and greatly assisted by many nucleophiles (Br(-) > PO(4)(3-) > HPO(4)(2-) > CO(3)(2-) > Cl(-) approximately OH(-) > CH(3)COO(-) approximately SO(4)(2-) approximately C(5)H(5)N >> H(2)O). The kinetics (at p[H(+)] = 9.3-12.9) show that the first step of the mechanism is an electron transfer between ClO(2) and BrO(2)(-) (k(1) = 36 M(-1) s(-1)) to give ClO(2)(-) and BrO(2). This highly reversible reaction (k(1)/k(-1) = 1 x 10(-6)) accounts for the observed inhibition by ClO(2)(-). The second step is an electron transfer between ClO(2) and BrO(2) to regenerate BrO(2)(-) and form ClO(3)(-). A novel aspect of the second step is the large kinetic contribution from nucleophiles (k(Nu)) that assist the electron transfer between ClO(2) and BrO(2). The k(Nu) (M(-2) s(-1)) values at 25.0 degrees C vary from 2.89 x 10(8) for Br(-) to 2.0 x 10(4) for H(2)O.

Journal Article↗

PubMed2002

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.

Bromides↗