Proton-coupled electron transfer from tryptophan: A concerted mechanism with water as proton acceptor

Ming Tian Zhang, Leif Hammarström

Research output: Contribution to journalArticle

54 Citations (Scopus)

Abstract

The mechanism of proton-coupled electron transfer (PCET) from tyrosine in enzymes and synthetic model complexes is under intense discussion, in particular the pH dependence of the PCET rate with water as proton acceptor. Here we report on the intramolecular oxidation kinetics of tryptophan derivatives linked to [Ru(bpy)3]2+ units with water as proton acceptor, using laser flash-quench methods. It is shown that tryptophan oxidation can proceed not only via a stepwise electron-proton transfer (ETPT) mechanism that naturally shows a pH-independent rate, but also via another mechanism with a pH-dependent rate and higher kinetic isotope effect that is assigned to concerted electron-proton transfer (CEP). This is in contrast to current theoretical models, which predict that CEP from tryptophan with water as proton acceptor can never compete with ETPT because of the energetically unfavorable PT part (pKa(TrpH+) = 4.7 ≫ pK a(H3O+) ≈ -1.5). The moderate pH dependence we observe for CEP cannot be explained by first-order reactions with OH - or the buffers and is similar to what has been demonstrated for intramolecular PCET in [Ru(bpy)3]3+-tyrosine complexes (Sjödin, M.; et al. J. Am. Chem. Soc.2000, 122, 3932. Irebo, T.; et al. J. Am. Chem. Soc.2007, 129, 15462). Our results suggest that CEP with water as the proton acceptor proves a general feature of amino acid oxidation, and provide further experimental support for understanding of the PCET process in detail.

Original languageEnglish
Pages (from-to)8806-8809
Number of pages4
JournalJournal of the American Chemical Society
Volume133
Issue number23
DOIs
Publication statusPublished - Jun 15 2011

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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