RESUMEN
Photocatalysis is a versatile and rapidly developing field with applications spanning artificial photosynthesis, photo-biocatalysis, photoredox catalysis in solution or supramolecular structures, utilization of abundant metals and organocatalysts, sustainable synthesis, and plastic degradation. In this Perspective, we summarize conclusions from an interdisciplinary workshop of young principal investigators held at the Lorentz Center in Leiden in March 2023. We explore how diverse fields within photocatalysis can benefit from one another. We delve into the intricate interplay between these subdisciplines, by highlighting the unique challenges and opportunities presented by each field and how a multidisciplinary approach can drive innovation and lead to sustainable solutions for the future. Advanced collaboration and knowledge exchange across these domains can further enhance the potential of photocatalysis. Artificial photosynthesis has become a promising technology for solar fuel generation, for instance, via water splitting or CO2 reduction, while photocatalysis has revolutionized the way we think about assembling molecular building blocks. Merging such powerful disciplines may give rise to efficient and sustainable protocols across different technologies. While photocatalysis has matured and can be applied in industrial processes, a deeper understanding of complex mechanisms is of great importance to improve reaction quantum yields and to sustain continuous development. Photocatalysis is in the perfect position to play an important role in the synthesis, deconstruction, and reuse of molecules and materials impacting a sustainable future. To exploit the full potential of photocatalysis, a fundamental understanding of underlying processes within different subfields is necessary to close the cycle of use and reuse most efficiently. Following the initial interactions at the Lorentz Center Workshop in 2023, we aim to stimulate discussions and interdisciplinary approaches to tackle these challenges in diverse future teams.
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Molecular catalysts based on abundant elements that function in neutral water represent an essential component of sustainable hydrogen production. Artificial hydrogenases based on protein-inorganic hybrids have emerged as an intriguing class of catalysts for this purpose. We have prepared a novel artificial hydrogenase based on cobaloxime bound to a de novo three alpha-helical protein, α3C, via a pyridyl-based unnatural amino acid. The functionalized de novo protein was characterised by UV-visible, CD, and EPR spectroscopy, as well as MALDI spectrometry, which confirmed the presence and ligation of cobaloxime to the protein. The new de novo enzyme produced hydrogen under electrochemical, photochemical and reductive chemical conditions in neutral water solution. A change in hydrogen evolution capability of the de novo enzyme compared with native cobaloxime was observed, with turnover numbers around 80% of that of cobaloxime, and hydrogen evolution rates of 40% of that of cobaloxime. We discuss these findings in the context of existing literature, how our study contributes important information about the functionality of cobaloximes as hydrogen evolving catalysts in protein environments, and the feasibility of using de novo proteins for development into artificial metalloenzymes. Small de novo proteins as enzyme scaffolds have the potential to function as upscalable bioinspired catalysts thanks to their efficient atom economy, and the findings presented here show that these types of novel enzymes are a possible product.
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Hidrógeno , Hidrogenasas , Hidrógeno/química , Hidrógeno/metabolismo , Hidrogenasas/química , Hidrogenasas/metabolismo , Compuestos Organometálicos/química , Compuestos Organometálicos/metabolismo , Compuestos Organometálicos/síntesis química , CatálisisRESUMEN
Concerted electron-proton transfer (CEPT) reactions avoid charged intermediates and may be energetically favorable for redox and radical-transfer reactions in natural and synthetic systems. Tryptophan (W) often partakes in radical-transfer chains in nature but has been proposed to only undergo sequential electron transfer followed by proton transfer when water is the primary proton acceptor. Nevertheless, our group has shown that oxidation of freely solvated tyrosine and W often exhibit weakly pH-dependent proton-coupled electron transfer (PCET) rate constants with moderate kinetic isotope effects (KIE ≈ 2-5), which could be associated with a CEPT mechanism. These results and conclusions have been questioned. Here, we present PCET rate constants for W derivatives with oxidized Ru- and Zn-porphyrin photosensitizers, extracted from laser flash-quench studies. Alternative quenching/photo-oxidation methods were used to avoid complications of previous studies, and both the amine and carboxylic acid groups of W were protected to make the indole the only deprotonable group. With a suitably tuned oxidant strength, we found an ET-limited reaction at pH < 4 and weakly pH-dependent rates at pH > â¼5 that are intrinsic to the PCET of the indole group with water (H2O) as the proton acceptor. The observed rate constants are up to more than 100 times higher than those measured for initial electron transfer, excluding the electron-first mechanism. Instead, the reaction can be attributed to CEPT. These conclusions are important for our view of CEPT in water and of PCET-mediated radical reactions with solvent-exposed tryptophan in natural systems.
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Protones , Triptófano , Transporte de Electrón , Electrones , Concentración de Iones de Hidrógeno , Indoles , Oxidación-Reducción , Triptófano/metabolismo , AguaRESUMEN
Proton-coupled electron transfer (PCET) reactions are fundamental to energy transformation reactions in natural and artificial systems and are increasingly recognized in areas such as catalysis and synthetic chemistry. The interdependence of proton and electron transfer brings a mechanistic richness of reactivity, including various sequential and concerted mechanisms. Delineating between different PCET mechanisms and understanding why a particular mechanism dominates are crucial for the design and optimization of reactions that use PCET. This Perspective provides practical guidelines for how to discern between sequential and concerted mechanisms based on interpretations of thermodynamic data with temperature-, pressure-, and isotope-dependent kinetics. We present new PCET-zone diagrams that show how a mechanism can switch or even be eliminated by varying the thermodynamic (ΔGPT° and ΔGET°) and coupling strengths for a PCET system. We discuss the appropriateness of asynchronous concerted PCET to rationalize observations in organic reactions, and the distinction between hydrogen atom transfer and other concerted PCET reactions. Contemporary issues and future prospects in PCET research are discussed.
RESUMEN
Proton-coupled electron transfer (PCET) from tyrosine produces a neutral tyrosyl radical (Yâ¢) 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 Y32 in the α3Y model protein. The previously solved α3Y solution NMR structure shows that Y32 is sequestered â¼7.7 ± 0.3 Å 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 Y32 oxidation. Y32⢠was generated in a bimolecular reaction with [Ru(bpy)3]3+ formed by flash photolysis. At pH > 8, the rate constant of Y32⢠formation (kPCET) 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 ≈ 2.5, which best fits a concerted mechanism. kPCET 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 α3Y and hydrogen bond to Y32, as well as the possibility of hydrogen-bonding interactions between Y32 and E13, 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.
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Proteínas/química , Protones , Tirosina/química , Transporte de Electrón , Enlace de Hidrógeno , Concentración de Iones de Hidrógeno , Cinética , Simulación de Dinámica Molecular , Oxidación-ReducciónRESUMEN
Isotopic labeling of recombinantly expressed proteins is generally required for investigation by modern nuclear magnetic resonance (NMR) methods. Purification strategies of the labeled proteins often include the use of a polyhistidine affinity tag (His-tag) and immobilized metal ion affinity chromatography (IMAC). Described herein are rapid and inexpensive qualitative and quantitative assays to determine the concentration of paramagnetic Ni2+ in protein samples purified by IMAC. Both qualitative and quantitative colorimetric methods detect the amount of Ni2+ via the color change produced when a [Ni(PAR)n]2+ (PAR=4-(2-pyridylazo)resorcinol, n=1, 2) complex is formed. The qualitative assay provides a rapid visual test for the presence of Ni2+ in the low micromolar range in a sample of interest. The usefulness of the spectroscopic quantitative assay is illustrated by: (i) detecting a 12µM Ni2+ contamination in an NMR sample containing 950µM of the 7.5kDa α3W protein purified by a standard His-tag Ni2+/IMAC approach and (ii) showing that the 15N-HSQC spectrum of the α3W NMR sample, containing 1 paramagnetic Ni2+ ion per 80 protein molecules, displays clear line broadening of both water and protein spectral lines. We also (iii) measured Ni2+ release during the equilibration, wash, and elution steps of three commonly used Ni2+/IMAC resins when following manufacturer's protocols. The concentration of Ni2+ detected in elutes of the three resins ranged from 2µM to nearly 1mM.
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Compuestos Cromogénicos/química , Colorimetría/métodos , Complejos de Coordinación/análisis , Espectroscopía de Resonancia Magnética/métodos , Níquel/análisis , Resorcinoles/química , Cationes Bivalentes , Cromatografía de Afinidad , Complejos de Coordinación/química , Escherichia coli/genética , Escherichia coli/metabolismo , Expresión Génica , Vectores Genéticos/química , Vectores Genéticos/metabolismo , Níquel/química , Proteínas Recombinantes de Fusión/química , Proteínas Recombinantes de Fusión/genética , Proteínas Recombinantes de Fusión/metabolismo , Sensibilidad y Especificidad , Tiorredoxinas/química , Tiorredoxinas/genética , Tiorredoxinas/metabolismoRESUMEN
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 (E°') 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 pKOX of 3.4, a E°'(W32(Nâ¢+/NH)) of 1293 mV, and a E°'(W32(Nâ¢/NH); pH 7.0) of 1095 ± 4 mV versus the normal hydrogen electrode. W32(Nâ¢/NH) is 109 ± 4 mV more oxidizing than Y32(Oâ¢/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 â¼104 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.
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Proteínas/química , Protones , Triptófano/química , Transporte de Electrón , Radicales Libres/química , Cinética , Oxidación-Reducción , TermodinámicaRESUMEN
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), dO···N, giving rise to values ranging from 2.567 to 2.8487 Å, and resulting in calculated proton tunneling distances, r0, 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 dO···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 dO···N decreased, as can be expected from a static proton tunneling model. An exponential attenuation of the PCET rate constant was found: kPCET(d) = k0PCETexp[-ß(d - d0)], with ß â¼ 10 Å-1. The observed kinetic isotope effect (KIE = kH/kD) ranged from 1.2 to 1.4, where the KIE was observed to decrease slightly with increasing dO···N. Both ß 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.
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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.
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The influence of H-bond geometry on the dynamics of excited state intramolecular proton transfer (ESIPT) and photoinduced tautomerization in a series of phenol-quinoline compounds is investigated. Control over the proton donor-acceptor distance (dDA ) and dihedral angle between the proton donor-acceptor subunits is achieved by introducing methylene backbone straps of increasing lengths to link the phenol and quinoline. We demonstrate that a long dDA correlates with a higher barrier for ESIPT, while a large dihedral angle opens highly efficient deactivation channels after ESIPT, preventing the formation of the fully relaxed tautomer photoproduct.
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Transient absorption decay rate constants (kobs) for reactions of electronically excited zinc tetraphenylporphyrin ((3)ZnTPP*) with triruthenium oxo-centered acetate-bridged clusters [Ru3(µ3-O)(µ-CH3CO2)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, kq, for (3)ZnTPP*/1-3 are 3.0 × 10(9), 1.5 × 10 (9), and 1.1 × 10(9) M(-1) s(-1), respectively). Values of kq for reactions of (3)ZnTPP* with 1-3 and Ru3(µ3-O)(µ-CH3CO2)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) Ru3O-cluster products were observed.
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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 (kPCET) is pH dependent: 1.4 × 10(4) M(-1) s(-1) (pH 5.5), 1.8 × 10(5) M(-1) s(-1) (pH 8.5), 5.4 × 10(3) M(-1) s(-1) (pD 5.5), and 4.0 × 10(4) M(-1) s(-1) (pD 8.5). k(H)/k(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, t1/2 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.
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Radicales Libres/química , Procesos Fotoquímicos , Proteínas/química , Protones , Tirosina/química , Secuencia de Aminoácidos , Transporte de Electrón , Concentración de Iones de Hidrógeno , Cinética , Modelos Moleculares , Datos de Secuencia Molecular , Compuestos Organometálicos/química , Oxidantes/química , Estructura Secundaria de ProteínaRESUMEN
Application of a semiclassical three-state model of mixed valency to complexes of the type [Ru(3)(µ(3)-O)(OAc)(6)(CO)(py)-(µ(2)-BL)-Ru(3)(µ(3)-O)(OAc)(6)(CO)(py)](-1), where BL = 1,4-pyrazine or 4,4'-bipyridine and py = 4-dimethylaminopyridine, pyridine, or 4-cyanopyridine is described. The appearance of two intervalence charge transfer (IVCT) bands in the near-infrared (NIR) region of the electronic spectra of these complexes is explained well by the three-state model. An important feature of the three-state model is that the IVCT band evolves into two bands: one that is metal-to-bridging-ligand-charge-transfer (MBCT) in character and another that is metal-to-metal-charge-transfer (MMCT) in character. The three-state model also fully captures the observed spectroscopic behavior in which the MBCT transition increases in energy and the MMCT band decreases in energy with increasing electronic communication in a series of mixed valence ions. The appearance of both the MBCT and MMCT bands is found to persist as coalescence of infrared (IR) vibrational spectra suggest a ground state delocalized on the picosecond time scale. The solvent and temperature dependence of the MBCT and MMCT electronic transitions defines the mixed valence complexes reported here as lying on the borderline of delocalization.
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Rutenio/química , Elementos de Transición/química , Estructura Molecular , Compuestos Orgánicos/química , Compuestos Organometálicos/química , Solventes/química , Espectroscopía Infrarroja CortaRESUMEN
The field of molecular magnetism has grown tremendously since the discovery of single-molecule magnets, but it remains centred around the superexchange mechanism. The possibility of instead using a double-exchange mechanism (based on electron delocalization rather than Heisenberg exchange through a non-magnetic bridge) presents a tantalizing prospect for synthesizing molecules with high-spin ground states that are well isolated in energy. We now demonstrate that magnetic double exchange can be sustained by simple imidazolate bridging ligands, known to be well suited for the construction of coordination clusters and solids. A series of mixed-valence molecules of the type [(PY5Me(2))V(II)(micro-L(br)) V(III)(PY5Me(2))](4+) were synthesized and their electron delocalization probed through cyclic voltammetry and spectroelectrochemistry. Magnetic susceptibility data reveal a well-isolated S = 5/2 ground state arising from double exchange for [(PY5Me(2))(2)V(2)(micro-5,6-dimethylbenzimidazolate)](4+). Combined modelling of the magnetic data and spectral analysis leads to an estimate of the double-exchange parameter of B = 220 cm(-1) when vibronic coupling is taken into account.
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Complejos de Coordinación/química , Electrones , Imidazoles/química , Vanadio/química , Cristalografía por Rayos X , Ligandos , Magnesio/química , Magnetismo , Modelos Moleculares , Conformación Molecular , VibraciónRESUMEN
The dependence of the rates of intramolecular electron transfer (ET) of mixed-valence complexes of the type {[Ru3O(OAc)6(CO)(L)]2-BL}-1, where L is the pyridyl ligand and BL is the pyrazine on solvent type and temperature is described. Complexes were reduced chemically to obtain the mixed-valence anions in acetonitrile (CH3CN) and methylene chloride (CH2Cl2). Rate constants for intramolecular ET were estimated by simulating the observed degree of nu(CO) infrared (IR) bandshape coalescence in the mixed-valence state. In the strongly coupled mixed-valence states of these complexes, the electronic coupling, HAB, approaches lambda/2, where lambda is the total reorganization energy. The activation energy is thus nearly zero, and rate constants are in the 'ultrafast' regime where they depend on the pre-exponential terms within the frequency factor, nuN. The frequency factor contains both external (solvent dynamics) and internal (molecular vibrations) contributions. In general, external solvent motions are slower than internal vibrations, and therefore control ET rates in fluid solution. A profound increase in the degree of nu(CO) IR bandshape coalescence is observed as the temperature approaches the freezing points of the solvents methylene chloride (f.p. -92 degrees C) and acetonitrile (f.p. -44 degrees C). Decoupling the slower solvent motions involved in the frequency factor nuN for ET by freezing the solvent causes a transition from solvent dynamics to internal vibration-limited rates. The solvent phase transition causes a localized-to-delocalized transition in the mixed-valence ions that accelerates the rate of ET.
RESUMEN
We relate the solvent and temperature dependence of the rates of intramolecular electron transfer (ET) of mixed valence complexes of the type {[Ru3O(OAc)6(CO)(L)]2-BL}-1, where L = pyridyl ligand and BL = pyrazine. Complexes were reduced chemically or electrochemically to obtain the mixed valence anions in seven solvents: acetonitrile, methylene chloride, dimethylformamide, tetrahydrofuran, dimethylsulfoxide, chloroform, and hexamethylphosphoramide. Rate constants for intramolecular ET were estimated by simulating the observed degree of nu(CO) IR band shape coalescence in the mixed valence state. Correlations between rate constants for ET and solvent properties including static dielectric constant, optical dielectric constant, the quantity 1/epsilonop - 1/epsilonS, microscopic solvent polarity, viscosity, cardinal rotational moments of inertia, and solvent relaxation times were examined. In the temperature study, the complexes displayed a sharp increase in the ket as the freezing points of the solvents methylene chloride and acetonitrile were approached. The solvent phase transition causes a localized-to-delocalized transition in the mixed valence ions and an acceleration in the rate of ET. This is explained in terms of decoupling the slower solvent motions involved in the frequency factor nuN which increases the value of nuN. The observed solvent and temperature dependence of the ket for these complexes is used in order to formulate a new definition for Robin-Day class II-III mixed valence compounds. Specifically, it is proposed that class II-III compounds are those for which thermodynamic properties of the solvent exert no control over ket, but the dynamic properties of the solvent still influence ket.