Mechanistic Studies on the Reaction between Aquacobalamin and the HNO Donor Piloty's Acid over a Wide pH Range in Aqueous Solution

Justyna Polaczek, Harishchandra Subedi, Łukasz Orzeł, Lynn S. Lisboa, Ruth B. Cink, Grażyna Stochel, Nicola E. Brasch, Rudi Van Eldik

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5 Citations (Scopus)

Abstract

Detailed kinetic and mechanistic studies have been carried out on the reaction between aquacobalamin/hydroxocobalamin (CblOH2+/CblOH) and nitroxyl (HNO) generated by Piloty's acid (PA, N-hydroxybenzenesulfonamide) over a wide pH range (3.5-13). The resulting data showed that in a basic solution HNO can react with hydroxocobalamin to form nitrosylcobalamin despite the inert nature of CblOH. It was shown that at low PA concentrations the rate-determining step is the decomposition of PhSO2NHO- to release HNO, whereas the reaction between CblOH and HNO becomes the rate-determining step at high PA concentrations. Data from kinetic studies on the reaction of CblOH with an excess of HNO enabled us to experimentally determine the pKa(HNO) value from initial rate data as a function of pH, giving pKa(HNO) = 11.47 ± 0.04. An especially interesting observation was made in the neutral pH range, where PA is stable and does not produce HNO. Under such conditions, rapid formation of CblNO was observed in the studied system. The obtained data suggest that CblOH2+ reacts directly with PA to form a Piloty's acid-bound cobalamin intermediate, which deprotonates rapidly at neutral pH followed by rate-determining S-N bond cleavage to give CblNO and release PhSO2-.

Original languageEnglish
Pages (from-to)2964-2975
Number of pages12
JournalInorganic Chemistry
Volume60
Issue number5
Early online date29 Jan 2021
DOIs
Publication statusPublished - 1 Mar 2021
Externally publishedYes

Keywords

  • Absorption
  • Kinetics
  • Organic reactions
  • Solution chemistry
  • pH

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