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Proton-Coupled Electron Transfer from Hydrogen-Bonded Phenols
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Photochemistry and Molecular Science, Chemical Physics.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Proton-coupled electron transfer (PCET) is one of the elementary reactions occurring in many chemical and biological systems, such as photosystem II where the oxidation of tyrosine (TyrZ) is coupled to deprotonation of the phenolic proton. This reaction is here modelled by the oxidation of a phenol covalently linked to a Ru(bpy)32+-moitey, which is photo-oxidized by a laser flash-quench method. This model system is unusual as mechanism of PCET is studied in a unimolecular system in water solution. Here we address the question how the nature of the proton accepting base and its hydrogen bond to phenol influence the PCET reaction.

In the first part we investigate the effect of an internal hydrogen bond PCET from. Two similar phenols are compared. For both these the proton accepting base is a carboxylate group linked to the phenol on the ortho-position directly or via a methylene group. On the basis of kinetic and thermodynamic arguments it is suggested that the PCET from these occurs via a concerted electron proton transfer (CEP). Moreover, numerical modelling of the kinetic data provides an in-depth analysis of this CEP reaction, including promoting  vibrations  along the O–H–O coordinate that are required to explain the data.

The second part describes the study on oxidation of phenol where either water or an external base the proton acceptor. The pH-dependence of the kinetics reveals four mechanistic regions for PCET within the same molecule when water is the base. It is shown that the competition between the mechanisms can be tuned by the strength of the oxidant. Moreover, these studies reveal the conditions that may favour a buffer-assisted PCET over that with deprotonation to water solution.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2010. , p. 69
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 706
Keywords [en]
Proton-coupled electron transfer, phenol oxidation, hydrogen bonds, artificial photosynthesis, promoting vibrations, proton transfer, laser flash-quench, transient absorption.
National Category
Physical Chemistry
Research subject
Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-112060ISBN: 978-91-554-7699-1 (print)OAI: oai:DiVA.org:uu-112060DiVA, id: diva2:284621
Public defence
2010-02-19, Häggsalen, Ångströmslaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2010-01-28 Created: 2010-01-08 Last updated: 2010-01-28Bibliographically approved
List of papers
1. The rate ladder of proton-coupled tyrosine oxidation in water: A systematic dependence on hydrogen bonds and protonation state
Open this publication in new window or tab >>The rate ladder of proton-coupled tyrosine oxidation in water: A systematic dependence on hydrogen bonds and protonation state
2008 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 130, no 29, p. 9194-+Article in journal (Refereed) Published
Abstract [en]

Proton coupled electron transfer (PCET) from tyrosine covalently linked to Ru(bPY)(3)(2+) has been studied with laser flash-quench techniques. Two new complexes with internal hydrogen bonding bases to the phenolic proton have been synthesized. Depending on the hydrogen bonding and protonation situation the rate constant of PCET spanned over 5 orders of magnitude and revealed a systematic dependence on pH. This resulted in a previously predicted "rate ladder" scheme: (i) pH dependent concerted electron-proton transfer (CEP) with deprotonation to bulk water, giving low PCET rates, (ii) pH independent CEP with deprotonation to the internal base, giving intermediate PCET rates, and (iii) pure electron transfer from tyrosinate, giving high rates. This behavior is reminiscent of Y-z oxidation in Mn-depleted and native photosystem II. The study also revealed important differences in rates between phenols with strong and weak hydrogen bonds, and for the latter a hydrogen bond-gated PCET was observed.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-109962 (URN)10.1021/ja802076v (DOI)000257796500008 ()
Available from: 2009-11-02 Created: 2009-11-02 Last updated: 2017-12-12Bibliographically approved
2. Proton-coupled electron transfer of tyrosine oxidation: buffer dependence and parallel mechanisms
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
3. Spanning Four Mechanistic Regions of Intramolecular Proton-Coupled Electron Transfer in a Ru(bpy)32+-Tyrosine Complex
Open this publication in new window or tab >>Spanning Four Mechanistic Regions of Intramolecular Proton-Coupled Electron Transfer in a Ru(bpy)32+-Tyrosine Complex
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2012 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 134, no 39, p. 16247-16254Article in journal (Refereed) Published
Abstract [en]

Proton-coupled electron transfer (PCET) from tyrosine (TyrOH) to a covalently linked [Ru(bpy)(3)](2+) photosensitizer in aqueous media has been systematically reinvestigated by laser flash-quench kinetics as a model system for PCET in radical enzymes and in photochemical energy conversion. Previous kinetic studies on Ru-TyrOH molecules (Sjodin et al. J. Am. Chem. Soc. 2000, 122, 3932; Irebo et al. J. Am. Chem. Soc. 2007, 129, 15462) have established two mechanisms. Concerted electron-proton (CEP) transfer has been observed when pH < pK(a)(TyrOH), which is pH-dependent but not first-order in [OH-] and not dependent on the buffer concentration when it is sufficiently low (less than ca. 5 mM). In addition, the pH-independent rate constant for electron transfer from tyrosine phenolate (TyrO(-)) was reported at pH >10. Here we compare the PCET rates and kinetic isotope effects (k(H)/k(D)) of four Ru-TyrOH molecules with varying Ru-III/II oxidant strengths over a pH range of 1-12.5. On the basis of these data, two additional mechanistic regimes were observed and identified through analysis of kinetic competition and kinetic isotope effects (KIE): (i) a mechanism dominating at low pH assigned to a stepwise electron-first PCET and (ii) a stepwise proton-first PCET with OH- as proton acceptor that dominates around pH = 10. The effect of solution pH and electrochemical potential of the Ru-III/II oxidant on the competition between the different mechanisms is discussed. The systems investigated may serve as models for the mechanistic diversity of PCET reactions in general with water (H2O, OH-) as primary proton acceptor.

Keywords
proton-coupled electron transfer, phenol oxidation, artificial photosynthesis
National Category
Chemical Sciences
Research subject
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-112056 (URN)10.1021/ja3053859 (DOI)000309335000029 ()
Note

De 2 första författarna delar förstaförfattarskapet.

Available from: 2010-01-07 Created: 2010-01-07 Last updated: 2017-12-12Bibliographically approved
4. The Kinetic Effect of Internal Hydrogen Bonds on Proton-Coupled Electron Transfer from Phenols: A Theoretical Analysis with Modeling of Experimental Data
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
5. Kinetic effects of hydrogen bonds on proton-coupled electron transfer from phenols
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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