## 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 pK_{a5}) the pH and buffer independent equilibrium constant (K_{e} = [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 K_{x} = [4a]/[I] and K_{y} = [5^{–}]/[I] upon pH and total sulfite buffer concentration ([S]_{T}) it was determined that K_{x} was proportional to [HSO_{3} ^{–}]^{1.0} and K_{y} proportional to [SO_{3} ^{2–}]^{1.0}. These dependencies of K_{x} and K_{y} upon concentrations of buffer species establish that the forward reactions from I to 4a and 5^{–} have in their rate expressions the terms [HSO_{3} ^{–}] and [SO_{3} ^{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] ≫ [I_{0}], The pseudo-first-order rate constants (k_{obSd}) were found to be dependent upon three terms (eq 14); the first contained the product [HSO_{3} ^{–}][SO_{3} ^{2–}], the second [SO_{3} ^{2–}], and the third was independent of buffer species but dependent upon the mole fraction of a reactant of pK_{app} (pK_{a5}) assignable to dissociation of 5H → 5^{–}. With the knowledge of the dependence of the equilibrium ratios [4a]/[I] and [5^{–}]/[I] upon [HSO_{3} ^{–}] and [SO_{3} ^{2–}], the rate terms were assignable (Scheme I) to: general acid (by HSO_{3} ^{–}) catalysis of nucleophilic attack of SO_{3} ^{2–} upon I to yield 4a and by microscopic reversibility general base (by SO_{3} ^{2–}) catalysis of conversion of 4a → I and unassisted nucleophilic attack of SO_{3} ^{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 HSO_{3} ^{–}) catalyzed attack of SO_{3} ^{2–} and to 5^{–} by unassisted attack of SO_{3} ^{2–}, 4a arising from rearrangement of 5H. Arguments are presented which favor the mechanism of Scheme I.

本文言語 | 英語 |
---|---|

ページ（範囲） | 2083-2089 |

ページ数 | 7 |

ジャーナル | Biochemistry |

巻 | 12 |

号 | 11 |

DOI | |

出版ステータス | 出版済み - 5 1 1973 |

外部発表 | はい |

## All Science Journal Classification (ASJC) codes

- Biochemistry