Mechanisms of Formation and Equilibria of 4a and 5 Adducts of an Isoalloxazine. Reaction of 10-(2′,6′-Dimethylphenyl)-3-methylisoalloxazine-6,8-disulfonate with Sulfite in Aqueous Media

Thomas C. Bruice, Laszlo Hevesi, Seiji Shinkai

Research output: Contribution to journalArticlepeer-review

22 Citations (Scopus)

Abstract

The reaction of 10-(2′,6′-dimethylphenyl)-3-methylisoalloxazine-6,8-disulfonate (I) in aqueous sulfite-bisulfite buffers (30°, μ = 2.0) yields an equilibrium mixture of I plus 4a- and 5-sulfite adducts (4a, 5, and 5H of eq 1). From the pH dependence of the relative concentration of 4a and the kinetically apparent acid dissociation constant of 5H (i.e., 5H → 5 with pKa5) the pH and buffer independent equilibrium constant (Ke = [5H]/[4a]) has been calculated to be 4.12 × 10–2. Therefore, at 30° in aqueous solution, the 4a adduct is thermodynamically favored over the neutral 5 adduct. From studies of the dependence of the equilibrium constants Kx = [4a]/[I] and Ky = [5]/[I] upon pH and total sulfite buffer concentration ([S]T) it was determined that Kx was proportional to [HSO3 ]1.0 and Ky proportional to [SO3 2–]1.0. These dependencies of Kx and Ky upon concentrations of buffer species establish that the forward reactions from I to 4a and 5 have in their rate expressions the terms [HSO3 ] and [SO3 2–], respectively, in excess over the retrograde reactions of 4a → I and S → I. The kinetics for approach to equilibrium in the conversion of I to products (4a, 5, and 5H) were studied under the pseudo-first-order conditions of [buffer] ≫ [I0], The pseudo-first-order rate constants (kobSd) were found to be dependent upon three terms (eq 14); the first contained the product [HSO3 ][SO3 2–], the second [SO3 2–], and the third was independent of buffer species but dependent upon the mole fraction of a reactant of pKapp (pKa5) assignable to dissociation of 5H → 5. With the knowledge of the dependence of the equilibrium ratios [4a]/[I] and [5]/[I] upon [HSO3 ] and [SO3 2–], the rate terms were assignable (Scheme I) to: general acid (by HSO3 ) catalysis of nucleophilic attack of SO3 2– upon I to yield 4a and by microscopic reversibility general base (by SO3 2–) catalysis of conversion of 4a → I and unassisted nucleophilic attack of SO3 2– upon I to yield 5 with spontaneous conversion of 5 → I. An alternate scheme that would satisfy both the thermodynamic and kinetic findings would be that of Scheme II. Here I is converted to 5H via general acid (by HSO3 ) catalyzed attack of SO3 2– and to 5 by unassisted attack of SO3 2–, 4a arising from rearrangement of 5H. Arguments are presented which favor the mechanism of Scheme I.

Original languageEnglish
Pages (from-to)2083-2089
Number of pages7
JournalBiochemistry
Volume12
Issue number11
DOIs
Publication statusPublished - May 1 1973
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Biochemistry

Fingerprint Dive into the research topics of 'Mechanisms of Formation and Equilibria of 4a and 5 Adducts of an Isoalloxazine. Reaction of 10-(2′,6′-Dimethylphenyl)-3-methylisoalloxazine-6,8-disulfonate with Sulfite in Aqueous Media'. Together they form a unique fingerprint.

Cite this