Article

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

Department of Photochemistry and Molecular Science, Uppsala University, Box 523, SE-751 20 Uppsala, Sweden.
Journal of the American Chemical Society (Impact Factor: 10.68). 06/2011; 133(23):8806-9. DOI: 10.1021/ja201536b
Source: PubMed

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 (pK(a)(Trp(•)H(+)) = 4.7 ≫ pK(a)(H(3)O(+)) ≈ -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.

0 Bookmarks
 · 
106 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: Proton-coupled electron transfers (PCET) are ubiquitous in natural and synthetic processes. This review focuses on reactions where the two events are concerted. Semiclassical models of such reactions allow their kinetic characterization through activation versus driving force relationships, estimates of reorganization energies, effects of the nature of the proton acceptor, and H/D kinetic isotope effect as well as their discrimination from stepwise pathways. Several homogeneous reactions (through stopped-flow and laser flash-quench techniques) and electrochemical processes are discussed in this framework. Once the way has been rid of the improper notion of pH-dependent driving force, water appears as a remarkable proton acceptor in terms of reorganization energy and pre-exponential factor, thanks to its H-bonded and H-bonding properties, similarly to purposely synthesized "H-bond train" molecules. The most recent developments are in modeling and description of emblematic concerted proton-electron transfer (CPET) reactions associated with the breaking of a heavy-atom bond in an all-concerted process.
    Annual review of analytical chemistry (Palo Alto, Calif.). 06/2014; 7(1):537-60.
  • [Show abstract] [Hide abstract]
    ABSTRACT: QM cluster and QM/MM protein models have been employed to understand aspects of the reaction mechanism of plant allene oxide synthase (pAOS). In this study we have investigated two reaction mechanisms for pAOS. The standard pAOS mechanism was contrasted with an alternative involving an additional active site molecule which has been shown to facilitate proton coupled electron transfer (PCET) in related systems. Firstly, we found that the results from QM/MM protein model are comparable with those from the QM cluster model, presumably due to the large active site used. Furthermore, the results from the QM cluster model show that the FeIII and FeIV pathways for the standard mechanism have similar energetic and structural properties, indicating that the reaction mechanism may well proceed via both pathways. However, while the PCET process is facilitated by an additional active site bound water in other related families, in pAOS it is not, suggesting this type of process is not general to all closely related family members.
    Journal of Molecular Graphics and Modelling 07/2014; · 2.33 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A new D-π-A type 4-(dicyanomethylene)-2-methyl-6-((1,4-diethyl-1,2,3,4-tetrahydroquinoxalin-6-yl)vinyl)-4H-pyran (DCM-Q) dye has been prepared by condensation. Under neutral or alkaline conditions, DCM-Q had a long absorption wavelength, because of intramolecular charge transfer (ICT), but weak fluorescence, because of photoinduced electron transfer (PET) in the excited state. In the presence of protons, the absorption wavelength of DCM-Q was blue shifted, owing to weak ICT, whereas the fluorescence intensity increased, because of limited PET. DCM-Q has potential as an efficient fluorescence “OFF–ON” switch for proton determination.
    Research on Chemical Intermediates · 1.54 Impact Factor

Full-text

Download
44 Downloads
Available from
May 20, 2014