Deactivation of Triplet-Excited Riboflavin by Purine Derivatives: Important Role of Uric Acid in Light-Induced Oxidation of Milk Sensitized by Riboflavin
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- 作者：Daniel R. Cardoso ; Paula Homem-de-Mello ; Karsten Olsen ; Albé ; rico B. F. da Silva ; Douglas W. Franco ; and Leif H. Skibsted
- 刊名：Journal of Agricultural and Food Chemistry
- 出版年：2005
- 出版时间：May 4, 2005
- 年：2005
- 卷：53
- 期：9
- 页码：3679 - 3684
- 全文大小：129K
- 年卷期：v.53,no.9(May 4, 2005)
- ISSN：1520-5118
文摘
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 flashphotolysis), 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 
f">C, in thepresence of uric acid, xanthine, and hypoxanthine strongly suggests a direct electron transfer fromthe purine to the triplet-excited riboflavin with k = 2.9 × 109 M-1 s-1 (
fchars/Delta.gif" BORDER=0 >H
f"> = 14.7 kJ mol-1, fchars/Delta.gif" BORDER=0 >Sf"> =-15.6 J mol-1 K-1), 1.2 × 109 M-1 s-1 (fchars/Delta.gif" BORDER=0 >Hf"> = 34.3 kJ mol-1, fchars/Delta.gif" BORDER=0 >Sf"> = +45.3 J mol-1 K-1), and 1.7 ×108 M-1 s-1 (fchars/Delta.gif" BORDER=0 >Hf"> = 122 kJ mol-1, fchars/Delta.gif" BORDER=0 >Sf"> = +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 vsnormal hydrogen electrode, NHE), xanthine (E = +1.106 vs NHE), and hypoxanthine (E = +1.654vs NHE), the overall free energy changes for electron transfer from the quencher to the triplet-excitedriboflavin are as follows: uric acid (fchars/Delta.gif" BORDER=0 >Gf"> = -114 kJ mol-1), xanthine (fchars/Delta.gif" BORDER=0 >Gf"> = -73.5 kJ mol-1),hypoxanthine (fchars/Delta.gif" BORDER=0 >Gf"> = -20.6 kJ mol-1), and purine (fchars/Delta.gif" BORDER=0 >Gf"> > 0). The inertness observed for purinetoward triplet-excited riboflavin corroborates with its electrochemical inactivity in the potential rangefrom 0 up to 2 V vs NHE. These data are in agreement with the DFT results, which show that theenergy of the purine highest occupied molecular orbital (HOMO) (-0.2685 arbitrary unit) is lowerthan the energy of the semioccupied molecular orbital (SOMO) (-0.2557 a.u.) of triplet-excitedriboflavin, indicating an endergonic process for the electron-transfer process. The rate-determiningstep for deactivation by purine derivatives can be assigned to an electron transfer from the purinederivative to the SOMO orbital of the triplet-excited riboflavin. The results show that uric acid maycompete with oxygen and other antioxidants to deactivate triplet-excited riboflavin in milk serum andother biological fluids leading to a free radical process.Keywords: Riboflavin; uric acid; light oxidation; milk; kinetics; antioxidant
f">C, in thepresence of uric acid, xanthine, and hypoxanthine strongly suggests a direct electron transfer fromthe purine to the triplet-excited riboflavin with k = 2.9 × 109 M-1 s-1 (fchars/Delta.gif" BORDER=0 >Hf"> = 14.7 kJ mol-1, fchars/Delta.gif" BORDER=0 >Sf"> =-15.6 J mol-1 K-1), 1.2 × 109 M-1 s-1 (fchars/Delta.gif" BORDER=0 >Hf"> = 34.3 kJ mol-1, fchars/Delta.gif" BORDER=0 >Sf"> = +45.3 J mol-1 K-1), and 1.7 ×108 M-1 s-1 (fchars/Delta.gif" BORDER=0 >Hf"> = 122 kJ mol-1, fchars/Delta.gif" BORDER=0 >Sf"> = +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 vsnormal hydrogen electrode, NHE), xanthine (E = +1.106 vs NHE), and hypoxanthine (E = +1.654vs NHE), the overall free energy changes for electron transfer from the quencher to the triplet-excitedriboflavin are as follows: uric acid (fchars/Delta.gif" BORDER=0 >Gf"> = -114 kJ mol-1), xanthine (fchars/Delta.gif" BORDER=0 >Gf"> = -73.5 kJ mol-1),hypoxanthine (fchars/Delta.gif" BORDER=0 >Gf"> = -20.6 kJ mol-1), and purine (fchars/Delta.gif" BORDER=0 >Gf"> > 0). The inertness observed for purinetoward triplet-excited riboflavin corroborates with its electrochemical inactivity in the potential rangefrom 0 up to 2 V vs NHE. These data are in agreement with the DFT results, which show that theenergy of the purine highest occupied molecular orbital (HOMO) (-0.2685 arbitrary unit) is lowerthan the energy of the semioccupied molecular orbital (SOMO) (-0.2557 a.u.) of triplet-excitedriboflavin, indicating an endergonic process for the electron-transfer process. The rate-determiningstep for deactivation by purine derivatives can be assigned to an electron transfer from the purinederivative to the SOMO orbital of the triplet-excited riboflavin. The results show that uric acid maycompete with oxygen and other antioxidants to deactivate triplet-excited riboflavin in milk serum andother biological fluids leading to a free radical process.Keywords: Riboflavin; uric acid; light oxidation; milk; kinetics; antioxidant