Cardoso Daniel R, Homem-de-Mello Paula, Olsen Karsten, da Silva Albérico B F, Franco Douglas W, Skibsted Leif H
Departamento de Química e Física Molecular, Instituto de Química de São Carlos, Universidade de São Paulo, Avenue Trabalhador São Carlense 400, CP 780, CEP 13560-970, São Carlos SP, Brazil.
J Agric Food Chem. 2005 May 4;53(9):3679-84. doi: 10.1021/jf048347z.
The reactivity of purine derivatives (uric acid, xanthine, hypoxanthine, and purine) toward triplet-excited riboflavin in aqueous solution at pH 6.4 is described on the basis of kinetic (laser flash photolysis), electrochemical (square-wave voltammetry), and theoretical data (density functional theory, DFT). Direct deactivation of triplet-excited riboflavin in aqueous solution, pH 6.4 at 24 degrees C, in the presence of uric acid, xanthine, and hypoxanthine strongly suggests a direct electron transfer from the purine to the triplet-excited riboflavin with k = 2.9 x 10(9) M(-1) s(-1) (DeltaH(++) = 14.7 kJ mol(-1), DeltaS(++) = -15.6 J mol(-1) K(-1)), 1.2 x 10(9) M(-1) s(-1) (DeltaH(++) = 34.3 kJ mol(-1), DeltaS(++) = +45.3 J mol(-1) K(-1)), and 1.7 x10(8) M(-1) s(-1) (DeltaH(++) = 122 kJ mol(-1), DeltaS(++) = +319 J mol(-1) K(-1)), respectively. From the respective one-electron oxidation potentials collected in aqueous solution at pH 6.4 for uric acid (E = +0.686 vs normal hydrogen electrode, NHE), xanthine (E = +1.106 vs NHE), and hypoxanthine (E = +1.654 vs NHE), the overall free energy changes for electron transfer from the quencher to the triplet-excited riboflavin are as follows: uric acid (DeltaG(o) = -114 kJ mol(-1)), xanthine (DeltaG(o) = -73.5 kJ mol(-1)), hypoxanthine (DeltaG(o) = -20.6 kJ mol(-1)), and purine (DeltaG(o) > 0). The inertness observed for purine toward triplet-excited riboflavin corroborates with its electrochemical inactivity in the potential range from 0 up to 2 V vs NHE. These data are in agreement with the DFT results, which show that the energy of the purine highest occupied molecular orbital (HOMO) (-0.2685 arbitrary unit) is lower than the energy of the semioccupied molecular orbital (SOMO) (-0.2557 a.u.) of triplet-excited riboflavin, indicating an endergonic process for the electron-transfer process. The rate-determining step for deactivation by purine derivatives can be assigned to an electron transfer from the purine derivative to the SOMO orbital of the triplet-excited riboflavin. The results show that uric acid may compete with oxygen and other antioxidants to deactivate triplet-excited riboflavin in milk serum and other biological fluids leading to a free radical process.
基于动力学（激光闪光光解）、电化学（方波伏安法）和理论数据（密度泛函理论，DFT），描述了嘌呤衍生物（尿酸、黄嘌呤、次黄嘌呤和嘌呤）在pH 6.4的水溶液中对三重态激发核黄素的反应活性。在24℃、pH 6.4的水溶液中，尿酸、黄嘌呤和次黄嘌呤存在时，三重态激发核黄素的直接失活强烈表明从嘌呤到三重态激发核黄素存在直接电子转移，速率常数k分别为2.9×10⁹ M⁻¹ s⁻¹（ΔH⁺⁺ = 14.7 kJ mol⁻¹，ΔS⁺⁺ = -15.6 J mol⁻¹ K⁻¹）、1.2×10⁹ M⁻¹ s⁻¹（ΔH⁺⁺ = 34.3 kJ mol⁻¹，ΔS⁺⁺ = +45.3 J mol⁻¹ K⁻¹）和1.7×10⁸ M⁻¹ s⁻¹（ΔH⁺⁺ = 122 kJ mol⁻¹，ΔS⁺⁺ = +319 J mol⁻¹ K⁻¹）。根据在pH 6.4的水溶液中收集的尿酸（E = +0.686 vs标准氢电极，NHE）、黄嘌呤（E = +1.106 vs NHE）和次黄嘌呤（E = +1.654 vs NHE）各自的单电子氧化电位，从猝灭剂到三重态激发核黄素的电子转移的总自由能变化如下：尿酸（ΔG⁰ = -114 kJ mol⁻¹）、黄嘌呤（ΔG⁰ = -73.5 kJ mol⁻¹）、次黄嘌呤（ΔG⁰ = -20.6 kJ mol⁻¹）和嘌呤（ΔG⁰ > 0）。观察到嘌呤对三重态激发核黄素的惰性与其在0至2 V vs NHE电位范围内的电化学惰性相符。这些数据与DFT结果一致，DFT结果表明嘌呤最高占据分子轨道（HOMO）的能量（-0.2685任意单位）低于三重态激发核黄素的半占据分子轨道（SOMO）的能量（-0.2557 a.u.），表明电子转移过程是一个吸能过程。嘌呤衍生物失活的速率决定步骤可归因于从嘌呤衍生物到三重态激发核黄素的SOMO轨道的电子转移。结果表明，尿酸可能与氧气和其他抗氧化剂竞争，使乳清和其他生物流体中的三重态激发核黄素失活，从而引发自由基过程。
Zotero 插件