抄録
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.
| 元の言語 | 英語 |
|---|---|
| ページ（範囲） | 2083-2089 |
| ページ数 | 7 |
| ジャーナル | Biochemistry |
| 巻 | 12 |
| 発行部数 | 11 |
| DOI | |
| 出版物ステータス | 出版済み - 5 1 1973 |
| 外部発表 | Yes |
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All Science Journal Classification (ASJC) codes
- Biochemistry
これを引用
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. / Bruice, Thomas C.; Hevesi, Laszlo; Shinkai, Seiji.
：: Biochemistry, 巻 12, 番号 11, 01.05.1973, p. 2083-2089.研究成果: ジャーナルへの寄稿 › 記事
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TY - JOUR
T1 - 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
AU - Bruice, Thomas C.
AU - Hevesi, Laszlo
AU - Shinkai, Seiji
PY - 1973/5/1
Y1 - 1973/5/1
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=0015932727&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=0015932727&partnerID=8YFLogxK
U2 - 10.1021/bi00735a010
DO - 10.1021/bi00735a010
M3 - Article
C2 - 4705987
AN - SCOPUS:0015932727
VL - 12
SP - 2083
EP - 2089
JO - Biochemistry
JF - Biochemistry
SN - 0006-2960
IS - 11
ER -