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Reaction Kinetics of Cyanide Binding to a Cobalt Schiff-Base Macrocycle Relevant to Its Mechanism of Antidotal Action.
Chem Res Toxicol 2019; 32(8):1630-1637CR

Abstract

The Co(II/III)-containing macrocycle, cobalt 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(17)2,11,13,15-pentaenyl cation, or CoN4[11.3.1], is a potential cyanide-scavenging agent. The rate of reduction of Co(III)N4[11.3.1] by ascorbate is reasonably facile under pseudo-first-order conditions; a second-order rate constant of 11.7(±0.4) M-1 s-1 was determined at 25 °C and pH 7.4, along with the activation parameters for the reaction (ΔH⧧ = 53.9(±0.8) kJ mol-1; ΔS -79(±3) J mol-1 K-1). It follows that any cyanide-decorporating capability of the cobalt complex should depend more on the cyanide-binding characteristics of Co(II)N4[11.3.1] than the oxidized form. The kinetics of the reaction of cyanide with Co(II)N4[11.3.1] under anaerobic pseudo-first-order conditions is rapid and resulted in a linear dependence on the cyanide concentration, kHCN = 8 × 104 M-1 s-1, with a nonlinear intercept of 420 s-1 at 10 °C, pH 7.6. The observed reaction rate increases significantly with increasing pH. A rate law is suggested, kobs = k'[X] + (kHCN + kCNKa/[H+])[HCN], where kCN is estimated to be ∼2 × 106 M-1 s-1. Activation parameters for the reaction with HCN (ΔH⧧ = 10.7(±0.4) kJ mol-1; ΔS⧧ = -153(±1) J mol-1 K-1) suggest an associative mechanism. In the presence of excess oxygen, i.e., at higher levels than free oxygen in vivo, the reaction rate was too fast to be measured, and the final product was the oxidized complex, Co(III)N4[11.3.1], where any cyanide ligands had been lost. This is much more rapid than the oxidation of the parent compound by oxygen, for which a second-order rate constant of 0.5(±0.02) M-1 s-1 at 25 °C was obtained. The study has gone some way toward enhancing our understanding of the reaction of Co(II)N4[11.3.1] with cyanide. The fast reaction rate implies a high efficacy of the cyanide-scavenging capability of the complex and further supports the suggestion stemming from our previous work that Co(II)N4[11.3.1] could prove to be a better and more cost-effective cyanide antidote than the FDA-approved hydroxocobalamin.

Authors+Show Affiliations

Department of Environmental and Occupational Health, Graduate School of Public Health , The University of Pittsburgh , Pittsburgh , Pennsylvania 15261 , United States.Department of Environmental and Occupational Health, Graduate School of Public Health , The University of Pittsburgh , Pittsburgh , Pennsylvania 15261 , United States.Department of Environmental and Occupational Health, Graduate School of Public Health , The University of Pittsburgh , Pittsburgh , Pennsylvania 15261 , United States.

Pub Type(s)

Journal Article

Language

eng

PubMed ID

31241903

Citation

Praekunatham, Hirunwut, et al. "Reaction Kinetics of Cyanide Binding to a Cobalt Schiff-Base Macrocycle Relevant to Its Mechanism of Antidotal Action." Chemical Research in Toxicology, vol. 32, no. 8, 2019, pp. 1630-1637.
Praekunatham H, Pearce LL, Peterson J. Reaction Kinetics of Cyanide Binding to a Cobalt Schiff-Base Macrocycle Relevant to Its Mechanism of Antidotal Action. Chem Res Toxicol. 2019;32(8):1630-1637.
Praekunatham, H., Pearce, L. L., & Peterson, J. (2019). Reaction Kinetics of Cyanide Binding to a Cobalt Schiff-Base Macrocycle Relevant to Its Mechanism of Antidotal Action. Chemical Research in Toxicology, 32(8), pp. 1630-1637. doi:10.1021/acs.chemrestox.9b00170.
Praekunatham H, Pearce LL, Peterson J. Reaction Kinetics of Cyanide Binding to a Cobalt Schiff-Base Macrocycle Relevant to Its Mechanism of Antidotal Action. Chem Res Toxicol. 2019 Aug 19;32(8):1630-1637. PubMed PMID: 31241903.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Reaction Kinetics of Cyanide Binding to a Cobalt Schiff-Base Macrocycle Relevant to Its Mechanism of Antidotal Action. AU - Praekunatham,Hirunwut, AU - Pearce,Linda L, AU - Peterson,Jim, Y1 - 2019/07/12/ PY - 2019/6/27/pubmed PY - 2019/6/27/medline PY - 2019/6/27/entrez SP - 1630 EP - 1637 JF - Chemical research in toxicology JO - Chem. Res. Toxicol. VL - 32 IS - 8 N2 - The Co(II/III)-containing macrocycle, cobalt 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(17)2,11,13,15-pentaenyl cation, or CoN4[11.3.1], is a potential cyanide-scavenging agent. The rate of reduction of Co(III)N4[11.3.1] by ascorbate is reasonably facile under pseudo-first-order conditions; a second-order rate constant of 11.7(±0.4) M-1 s-1 was determined at 25 °C and pH 7.4, along with the activation parameters for the reaction (ΔH⧧ = 53.9(±0.8) kJ mol-1; ΔS -79(±3) J mol-1 K-1). It follows that any cyanide-decorporating capability of the cobalt complex should depend more on the cyanide-binding characteristics of Co(II)N4[11.3.1] than the oxidized form. The kinetics of the reaction of cyanide with Co(II)N4[11.3.1] under anaerobic pseudo-first-order conditions is rapid and resulted in a linear dependence on the cyanide concentration, kHCN = 8 × 104 M-1 s-1, with a nonlinear intercept of 420 s-1 at 10 °C, pH 7.6. The observed reaction rate increases significantly with increasing pH. A rate law is suggested, kobs = k'[X] + (kHCN + kCNKa/[H+])[HCN], where kCN is estimated to be ∼2 × 106 M-1 s-1. Activation parameters for the reaction with HCN (ΔH⧧ = 10.7(±0.4) kJ mol-1; ΔS⧧ = -153(±1) J mol-1 K-1) suggest an associative mechanism. In the presence of excess oxygen, i.e., at higher levels than free oxygen in vivo, the reaction rate was too fast to be measured, and the final product was the oxidized complex, Co(III)N4[11.3.1], where any cyanide ligands had been lost. This is much more rapid than the oxidation of the parent compound by oxygen, for which a second-order rate constant of 0.5(±0.02) M-1 s-1 at 25 °C was obtained. The study has gone some way toward enhancing our understanding of the reaction of Co(II)N4[11.3.1] with cyanide. The fast reaction rate implies a high efficacy of the cyanide-scavenging capability of the complex and further supports the suggestion stemming from our previous work that Co(II)N4[11.3.1] could prove to be a better and more cost-effective cyanide antidote than the FDA-approved hydroxocobalamin. SN - 1520-5010 UR - https://www.unboundmedicine.com/medline/citation/31241903/Reaction_Kinetics_of_Cyanide_Binding_to_a_Cobalt_Schiff-Base_Macrocycle_Relevant_to_Its_Mechanism_of_Antidotal_Action L2 - https://dx.doi.org/10.1021/acs.chemrestox.9b00170 DB - PRIME DP - Unbound Medicine ER -