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Irebo, Tania
Publications (3 of 3) Show all publications
Johannissen, L. O., Irebo, T., Sjödin, M., Johansson, O. & Hammarström, L. (2009). The Kinetic Effect of Internal Hydrogen Bonds on Proton-Coupled Electron Transfer from Phenols: A Theoretical Analysis with Modeling of Experimental Data. Journal of Physical Chemistry B, 113(50), 16214-16225
Open this publication in new window or tab >>The Kinetic Effect of Internal Hydrogen Bonds on Proton-Coupled Electron Transfer from Phenols: A Theoretical Analysis with Modeling of Experimental Data
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2009 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 113, no 50, p. 16214-16225Article in journal (Refereed) Published
Abstract [en]

Proton-coupled electron transfer (PCET) was studied in two biomimetic covalently linked Ru(bpy)3−tyrosine complexes with the phenolic proton hydrogen-bonded to an internal carboxylate group. The phenolic group is either a salicylic acid (o-hydroxybenzoic acid, SA) or an o-hydroxyphenyl-acetic acid (PA), where the former gives a resonance-assisted hydrogen bond. Transient absorption data allowed direct determination of the rate constant for these intramolecular, bidirectional, and concerted PCET (CEP) reactions, as a function of temperature and H/D isotope. We found, unexpectedly, that the hydrogen bond in SA is in fact weaker than the hydrogen bond in the complex with PA, which forced us to reassess an earlier hypothesis that the proton coupling term for CEP with SA is increased by a stronger hydrogen bond. Consequently, the kinetic data was modeled numerically using a quantum mechanical rate expression. Sufficient experimentally determined observables were available to give robust and well-determined parameter values. This analysis, coupled with DFT/B3LYP and MP2 calculations and MD simulations, gave a detailed insight into the parameters that control the CEP reactions, and the effect of internal hydrogen bonds. We observed that a model with a static proton-tunneling distance is unable to describe the reaction correctly, requiring unrealistic values for the equilibrium proton-tunneling distances. Instead, when promoting vibrations that modulate the proton donor−acceptor distance were included, satisfactory fits to the experimental data were obtained, with parameter values that agree with DFT calculations and MD simulations. According to these results, it is in fact the weaker hydrogen bond of SA which increases the proton coupling. The inner reorganization energy of the phenolic groups is a significant factor contributing to the CEP barriers, but this is reduced by the hydrogen bonds to 0.35 and 0.50 eV for the two complexes. The promoting vibrations increase the rate of CEP by over 2 orders of magnitude, and dramatically reduce the kinetic isotope effect from ca. 40 for the static case to a modest value of 2−3.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-111831 (URN)10.1021/jp9048633 (DOI)000272560100015 ()
Available from: 2009-12-22 Created: 2009-12-22 Last updated: 2017-12-12Bibliographically approved
Irebo, T., Reece, S. Y., Sjödin, M., Nocera, D. G. & Hammarström, L. (2007). Proton-coupled electron transfer of tyrosine oxidation: buffer dependence and parallel mechanisms. Journal of the American Chemical Society, 129(50), 15462-15464
Open this publication in new window or tab >>Proton-coupled electron transfer of tyrosine oxidation: buffer dependence and parallel mechanisms
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2007 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 129, no 50, p. 15462-15464Article in journal (Refereed) Published
Abstract [en]

The proton-coupled electron transfer (PCET) from tyrosine covalently linked to a metal complex has been studied. The reaction was induced by laser flash excitation of the metal complex, and PCET was bidirectional, with electron transfer to the excited or flash-quenched oxidized metal complex and proton transfer to water or added buffers in the solution. We found a competition between three different PCET mechanisms: (1) A concerted PCET with water as the proton acceptor, which indeed shows a pH-dependence as earlier reported (Sjödin, M.; Styring, S.; Åkermark, B.; Sun, L.; Hammarström, L. J. Am. Chem. Soc. 2000, 122, 3932); (2) a stepwise electron transfer-proton transfer (ETPT) that is pH-independent; (3) a buffer-assisted concerted PCET. The relative importance of reaction 2 increases with oxidant strength, while that of reaction 1 increases with pH. At higher buffer concentrations reaction 3 becomes important, and the rate follows the expected first-order dependence on the concentration of the buffer base. Most importantly, the pH-dependence of reaction 1, with a slope of 0.4-0.5 in a plot of log k vs pH, is independent of buffer and cannot be explained by reaction schemes with simple first-order dependencies on [OH-], [H3O+], or buffer species.

Keywords
Chemistry, Multidisciplinary
National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-12424 (URN)10.1021/ja073012u (DOI)000251581900026 ()18027937 (PubMedID)
Available from: 2007-12-18 Created: 2007-12-18 Last updated: 2017-12-11Bibliographically approved
Sjödin, M., Irebo, T., Utas, J. E., Lind, J., Merenyi, G., Åkermark, B. & Hammarström, L. (2006). Kinetic effects of hydrogen bonds on proton-coupled electron transfer from phenols. Journal of the American Chemical Society, 128(40), 13076-13083
Open this publication in new window or tab >>Kinetic effects of hydrogen bonds on proton-coupled electron transfer from phenols
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2006 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 128, no 40, p. 13076-13083Article in journal (Refereed) Published
Abstract [en]

The kinetics and mechanism of proton-coupled electron transfer (PCET) from a series of phenols to a laser flash generated [Ru(bpy)(3)](3+) oxidant in aqueous solution was investigated. The reaction followed a concerted electron-proton transfer mechanism (CEP), both for the substituted phenols with an intramolecular hydrogen bond to a carboxylate group and for those where the proton was directly transferred to water. Without internal hydrogen bonds the concerted mechanism gave a characteristic pH-dependent rate for the phenol form that followed a Marcus free energy dependence, first reported for an intramolecular PCET in Sjodin, M. et al. J. Am. Chem. Soc. 2000, 122, 3932-3962 and now demonstrated also for a bimolecular oxidation of unsubstituted phenol. With internal hydrogen bonds instead, the rate was no longer pH-dependent, because the proton was transferred to the carboxylate base. The results suggest that while a concerted reaction has a relatively high reorganization energy (lambda), this may be significantly reduced by the hydrogen bonds, allowing for a lower barrier reaction path. It is further suggested that this is a general mechanism by which proton-coupled electron transfer in radical enzymes and model complexes may be promoted by hydrogen bonding. This is different from, and possibly in addition to, the generally suggested effect of hydrogen bonds on PCET in enhancing the proton vibrational wave function overlap between the reactant and donor states. In addition we demonstrate how the mechanism for phenol oxidation changes from a stepwise electron transfer-proton transfer with a stronger oxidant to a CEP with a weaker oxidant, for the same series of phenols. The hydrogen bonded CEP reaction may thus allow for a low energy barrier path that can operate efficiently at low driving forces, which is ideal for PCET reactions in biological systems.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-83646 (URN)10.1021/ja063264f (DOI)000241030500024 ()17017787 (PubMedID)
Available from: 2006-11-07 Created: 2006-11-07 Last updated: 2017-12-14Bibliographically approved
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