Electron-Transfer Reactions of Electronically Excited Zinc Tetraphenylporphyrin with Multinuclear Ruthenium Complexes

Transient absorption decay rate constants (k_(obs)) for reactions of electronically excited zinc tetraphenylporphyrin (^3ZnTPP*) with triruthenium oxo-centered acetate-bridged clusters [Ru_3(μ_3-O)(μ-CH_3CO_2)_6(CO)(L)]_2(μ-pz), where pz = pyrazine and L = 4-cyanopyridine (cpy) (1), pyridine (py) (2), or 4-dimethylaminopyridine (dmap) (3), were obtained from nanosecond flash-quench spectroscopic data (quenching constants, k_q, for ^3ZnTPP*/1–3 are 3.0 × 10^9, 1.5 × 10^9, and 1.1 × 10^9 M^(–1) s^(–1), respectively). Values of k_q for reactions of ^3ZnTPP* with 1–3 and Ru_3(μ_3-O)(μ-CH_3CO_2)_6(CO)(L)_2 [L = cpy (4), py (5), dmap (6)] monomeric analogues suggest that photoinduced electron transfer is the main pathway of excited-state decay; this mechanistic proposal is consistent with results from a photolysis control experiment, where growth of characteristic near-IR absorption bands attributable to reduced (mixed-valence) Ru_3O-cluster products were observed

Proton-Coupled Electron Transfer from Tyrosine in the Interior of a de novo Protein : Mechanisms and Primary Proton Acceptor

Proton-coupled electron transfer (PCET) from tyrosine produces a neutral tyrosyl radical (Y-center dot) that is vital to many catalytic redox reactions. To better understand how the protein environment influences the PCET properties of tyrosine, we have studied the radical formation behavior of Y-32 in the alpha Y-3 model protein. The previously solved alpha Y-3 solution NMR structure shows that Y-32 is sequestered similar to 7.7 +/- 0.3 angstrom below the protein surface without any primary proton acceptors nearby. Here we present transient absorption kinetic data and molecular dynamics (MD) simulations to resolve the PCET mechanism associated with Y-32 oxidation. Y-32(center dot). was generated in a bimolecular reaction with [Ru(bpy)(3)](3+) formed by flash photolysis. At pH > 8, the rate constant of Y-32(center dot). formation (k(P)(CET)) increases by one order of magnitude per pH unit, corresponding to a proton-first mechanism via tyrosinate (PTET). At lower pH < 7.5, the pH dependence is weak and shows a previously measured KIE approximate to 2.5, which best fits a concerted mechanism. k(PC)(ET) is independent of phosphate buffer concentration at pH 6.5. This provides clear evidence that phosphate buffer is not the primary proton acceptor. MD simulations show that one to two water molecules can enter the hydrophobic cavity of alpha Y-3 and hydrogen bond to Y-32, as well as the possibility of hydrogen-bonding interactions between Y-32 and E-13, through structural fluctuations that reorient surrounding side chains. Our results illustrate how protein conformational motions can influence the redox reactivity of a tyrosine residue and how PCET mechanisms can be tuned by changing the pH even when the PCET occurs within the interior of a protein

Pourbaix Diagram, Proton-Coupled Electron Transfer, and Decay Kinetics of a Protein Tryptophan Radical: Comparing the Redox Properties of W₃₂^• and Y₃₂^• Generated Inside the Structurally Characterized α₃W and α₃Y Proteins

Protein-based “hole” hopping typically involves spatially arranged redox-active tryptophan or tyrosine residues. Thermodynamic information is scarce for this type of process. The well-structured α₃W model protein was studied by protein film square wave voltammetry and transient absorption spectroscopy to obtain a comprehensive thermodynamic and kinetic description of a buried tryptophan residue. A Pourbaix diagram, correlating thermodynamic potentials (<i><i>E</i>°</i>′) with pH, is reported for W₃₂ in α₃W and compared to equivalent data recently presented for Y₃₂ in α₃Y (Ravichandran, K. R.; Zong, A. B.; Taguchi, A. T.; Nocera, D. G.; Stubbe, J.; Tommos, C. J. Am. Chem. Soc. 2017, 139, 2994−3004). The α₃W Pourbaix diagram displays a p<i>K</i>_OX of 3.4, a <i><i>E</i>°</i>′(W₃₂(N^•+/NH)) of 1293 mV, and a <i><i>E</i>°</i>′(W₃₂(N^•/NH); pH 7.0) of 1095 ± 4 mV versus the normal hydrogen electrode. W₃₂(N^•/NH) is 109 ± 4 mV more oxidizing than Y₃₂(O^•/OH) at pH 5.4–10. In the voltammetry measurements, W₃₂ oxidation–reduction occurs on a time scale of about 4 ms and is coupled to the release and subsequent uptake of one full proton to and from bulk. Kinetic analysis further shows that W₃₂ oxidation likely involves pre-equilibrium electron transfer followed by proton transfer to a water or small water cluster as the primary acceptor. A well-resolved absorption spectrum of W₃₂^• is presented, and analysis of decay kinetics show that W₃₂^• persists ∼10⁴ times longer than aqueous W^• due to significant stabilization by the protein. The redox characteristics of W₃₂ and Y₃₂ are discussed relative to global and local protein properties

Isolating the Effects of the Proton Tunneling Distance on Proton-Coupled Electron Transfer in a Series of Homologous Tyrosine-Base Model Compounds

The distance dependence of concerted proton-coupled electron transfer (PCET) reactions was probed in a series of three new compounds, where a phenol is covalently bridged by a 5, 6, or 7 membered carbocycle to the quinoline. The carbocycle bridge enforces the change in distance between the phenol oxygen (proton donor) and quinoline nitrogen (proton acceptor), <i>d</i>_O···N, giving rise to values ranging from 2.567 to 2.8487 Å, and resulting in calculated proton tunneling distances, <i>r</i>₀, that span 0.719 to 1.244 Å. Not only does this series significantly extend the range of distances that has been previously accessible for experimental distance dependent PCET studies of synthetic model compounds, but it also greatly improves the isolation of <i>d</i>_O···N as a variable compared to earlier reports. Rates of PCET were determined by time-resolved optical spectroscopy with flash-quench generated [Ru­(bpy)₃]³⁺ and [Ru­(dce)₃]³⁺, where bpy = 2,2′-bipyridyl and dce = 4,4′-dicarboxyethylester-2,2′-bipyridyl. The rates increased as <i>d</i>_O···N decreased, as can be expected from a static proton tunneling model. An exponential attenuation of the PCET rate constant was found: <i>k</i>_PCET(<i>d</i>) = <i>k</i>⁰_PCETexp­[−β­(<i>d</i> – <i>d</i>₀)], with β ∼ 10 Å^–1. The observed kinetic isotope effect (KIE = <i>k</i>_H/<i>k</i>_D) ranged from 1.2 to 1.4, where the KIE was observed to <i>decrease</i> slightly with increasing <i>d</i>_O···N. Both β and KIE values are significantly smaller than what is predicted by a static proton tunneling model. We conclude that vibrational compression of the tunneling distances, as well as higher vibronic transitions, that contribute to concerted proton coupled electron transfer must also be considered

Photochemical Tyrosine Oxidation in the Structurally Well-Defined α₃Y Protein: Proton-Coupled Electron Transfer and a Long-Lived Tyrosine Radical

Tyrosine oxidation–reduction involves proton-coupled electron transfer (PCET) and a reactive radical state. These properties are effectively controlled in enzymes that use tyrosine as a high-potential, one-electron redox cofactor. The α₃Y model protein contains Y32, which can be reversibly oxidized and reduced in voltammetry measurements. Structural and kinetic properties of α₃Y are presented. A solution NMR structural analysis reveals that Y32 is the most deeply buried residue in α₃Y. Time-resolved spectroscopy using a soluble flash-quench generated [Ru­(2,2′-bipyridine)₃]³⁺ oxidant provides high-quality Y32–O• absorption spectra. The rate constant of Y32 oxidation (<i>k</i>_PCET) is pH dependent: 1.4 × 10⁴ M^–1 s^–1 (pH 5.5), 1.8 × 10⁵ M^–1 s^–1 (pH 8.5), 5.4 × 10³ M^–1 s^–1 (pD 5.5), and 4.0 × 10⁴ M^–1 s^–1 (pD 8.5). <i>k</i>^H/<i>k</i>^D of Y32 oxidation is 2.5 ± 0.5 and 4.5 ± 0.9 at pH­(D) 5.5 and 8.5, respectively. These pH and isotope characteristics suggest a concerted or stepwise, proton-first Y32 oxidation mechanism. The photochemical yield of Y32–O• is 28–58% versus the concentration of [Ru­(2,2′-bipyridine)₃]³⁺. Y32–O• decays slowly, <i>t</i>_1/2 in the range of 2–10 s, at both pH 5.5 and 8.5, via radical–radical dimerization as shown by second-order kinetics and fluorescence data. The high stability of Y32–O• is discussed relative to the structural properties of the Y32 site. Finally, the static α₃Y NMR structure cannot explain (i) how the phenolic proton released upon oxidation is removed or (ii) how two Y32–O• come together to form dityrosine. These observations suggest that the dynamic properties of the protein ensemble may play an essential role in controlling the PCET and radical decay characteristics of α₃Y

Electron Dynamics and IR Peak Coalescence in Bridged Mixed Valence Dimers Studied by Ultrafast 2D-IR Spectroscopy

Dynamic IR peak coalescence and simulations based on the optical Bloch equations have been used previously to predict the rates of intramolecular electron transfer in a group of bridged mixed valence dimers of the type [Ru₃(O)­(OAc)₆(CO)­L]-BL-[Ru₃(O) (OAc)₆(CO)­L]. However, limitations of the Bloch equations for the analysis of dynamical coalescence in vibrational spectra have been described. We have used ultrafast 2D-IR spectroscopy to investigate the vibrational dynamics of the CO spectator ligands of several dimers in the group. These experiments reveal that no electron site exchange occurs on the time scale required to explain the observed peak coalescence. The high variability in FTIR peak shapes for these mixed valence systems is suggested to be the result of fluctuations in the charge distributions at each metal cluster within a single-well potential energy surface, rather than the previous model of two-site exchange