Free-radical scavenging by tryptophan and its metabolites through electron transfer based processes

  • Adriana Pérez-González
  • Juan Raúl Alvarez-Idaboy
  • Annia Galano
Original Paper


Free-radical scavenging by tryptophan and eight of its metabolites through electron transfer was investigated in aqueous solution at physiological pH, using density functional theory and the Marcus theory. A test set of 30 free radicals was employed. Thermochemical and kinetic data on the corresponding reactions are provided here for the first time. Two different pathways were found to be the most important: sequential proton loss electron transfer (SPLET) and sequential double proton loss electron transfer (SdPLET). Based on kinetic analyses, it is predicted that the tryptophan metabolites kynurenic acid and xanthurenic acid are the best free-radical scavengers among the tested compounds; they were estimated to be at least 24 and 12 times more efficient than Trolox for scavenging OOH. These findings are in line with previous reports suggesting that the antioxidant activity that has been attributed to tryptophan is actually due to its metabolites, and they demonstrate the particular importance of phenolic metabolites to such activity.

Graphical Abstract

Kynurenic acid (KNA) and xanthurenic acid (XNA) are the major contributors to the free-radical scavenging activity of tryptophan


Free-radical scavenger Rate constants Single electron transfer 



We gratefully acknowledge the Laboratorio de Visualización y Cómputo Paralelo at Universidad Autónoma Metropolitana-Iztapalapa and the Dirección General de Servicios de Cómputo Académico (DGSCA) at Universidad Nacional Autónoma de México. This work was partially supported by a grant from the DGAPA UNAM (PAPIIT-IN209812), and projects SEP-CONACyT 167491 and 167430.

Supplementary material

894_2015_2758_MOESM1_ESM.pdf (665 kb)
ESM 1 (PDF 664 kb)


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Copyright information

© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  • Adriana Pérez-González
    • 1
  • Juan Raúl Alvarez-Idaboy
    • 1
  • Annia Galano
    • 2
  1. 1.Departamento de Física y Química Teórica, Facultad de QuímicaUniversidad Nacional Autónoma de MéxicoMéxicoMexico
  2. 2.Departamento de QuímicaUniversidad Autónoma Metropolitana-IztapalapaMéxicoMexico

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