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Author: Hui Zheng
Publisher:
ISBN:
Category :
Languages : en
Pages : 440
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Author: Hui Zheng
Publisher:
ISBN:
Category :
Languages : en
Pages : 440
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Author: Shaun Di Hang Wong
Publisher:
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Category :
Languages : en
Pages :
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Mononuclear non-heme iron (NHFe) enzymes catalyze a wide variety of biologically-important reactions such as hydroxylation, halogenation, desaturation, ring closure, and electrophilic aromatic substitution. The key intermediate in the catalytic cycle is the S = 2 Fe(IV)=O species, capable of abstracting an H-atom from inert C--H bonds as strong as 106 kcal/mol. The Fe(IV)=O intermediate in enzymes is transient and difficult to trap; as such, stable synthetic analogs have proven invaluable for spectroscopic elucidation of the geometric/electronic structure of the Fe(IV)=O unit and how it is activated for reactivity. Such biomimetic Fe(IV)=O model complexes can be either intermediate-spin (S = 1) or high-spin (S = 2) in contrast to the S = 2 ground state of enzyme intermediates. For an S = 1 Fe(IV)=O species, the Fe--oxo [beta] [pi]*-frontier molecular orbital (FMO) [from the combination of Fe d(xz/yz) and oxo p(x/y)] is involved in H-atom abstraction, and this FMO requires a side-on approach ([pi]-attack) to achieve maximum overlap with the substrate C--H bond. Through magnetic circular dichroism (MCD) and nuclear vibrational resonance spectroscopy (NRVS) studies, the reactivity of the S = 1 Fe(IV)=O unit has been shown to be affected by the oxo contribution in the [pi]*-FMO, where a larger oxo contribution results in greater orbital overlap (with the substrate C--H) and higher reactivity; also, the [pi]-attack pathway results in steric clashes between substrate and ligand, giving a significant steric contribution to the energy of the reaction barrier. For an S = 2 Fe(IV)=O species, the Fe--oxo [alpha] [sigma]*-FMO [Fe d(z2) and oxo p(z)] is spin-polarized (exchange-stabilized) to an energy level comparable with its [pi]*-FMO, making it accessible as a second pathway ([sigma]-attack) for reactivity. In the S = 2 Fe(IV)=O model complex ligated by TMG3tren, this [sigma]*-FMO is active but is axially hindered by the ligand, again giving a large steric contribution to the reaction barrier; however, the intrinsic electronic reaction barriers of the S = 2 [sigma]*-FMO and the S = 1 [pi]*-FMO are comparable, suggesting they are similarly active in H-atom abstraction. Furthermore, MCD excited-state spectroscopy in combination with multiconfigurational calculations on the S = 2 model reveal two different [pi]-pathways for reactivity involving Fe(III)--oxyl[p(x), [pi]] character, in addition to the [sigma]-pathway involving Fe(III)--oxyl[p(z), [sigma]] character, showing that the S = 2 Fe(IV)=O unit is activated for both [pi] and [sigma] H-atom abstraction reactivities. Finally, the S = 2 enzyme intermediate for the halogenase SyrB2 was trapped and structurally characterized by NRVS, revealing two possible 5-coordinate trigonal bipyramidal candidates with the Fe--oxo vector oriented either perpendicular or parallel to the substrate C--H bond. Importantly, this difference in orientation leads to Fe(III)--OH products oriented efficiently for different rebound reactivities -- native halogenation in the case of perpendicular orientation and non-native hydroxylation in the case of parallel orientation.
Author: Lei Liu
Publisher:
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Category :
Languages : en
Pages :
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Mononuclear non-heme iron enzymes catalyze wide varieties of important biological reactions with industrial, medical, and environmental applications. These enzymes can be classified into two classes, O2 activating FeII enzymes and substrate activating FeIII enzymes. This thesis focuses on understanding the geometric and electronic structures of the peroxo level intermediates and their reactivities in two O2 activating FeII enzymes, bleomycin and Rieske dioxygenases related model complexes, by using a combination of spectroscopic and computational methods. Bleomycin is a glycopeptide anticancer drug capable of effecting single- and double-strand DNA cleavage. The last detectable intermediate prior to DNA cleavage is a low spin S = 1/2 FeIII--OOH species, termed activated bleomycin (ABLM). The DNA strand scission is initiated through the abstraction of the C-4' hydrogen atom of the deoxyribose sugar unit. Nuclear resonance vibrational spectroscopy (NRVS) aided by extended X-ray absorption fine structure (EXAFS) spectroscopy and density functional theory (DFT) calculations are applied to define the natures of FeIIIBLM and ABLM as (BLM)FeIII--OH and (BLM)FeIII([eta]1--OOH) species, respectively. The NRVS spectra of FeIIIBLM and ABLM are strikingly different because in ABLM the Fe--O--O bending mode mixes with, and energetically splits, the doubly degenerate, intense O--Fe--Nax trans-axial bends. DFT calculations of the reaction of ABLM with DNA, based on the species defined by the NRVS data, show that the direct H-atom abstraction by ABLM is thermodynamically favored over other proposed reaction pathways. Previously, the rate of ABLM decay had been found, based on indirect methods, to be independent of the presence of DNA. In this thesis, we use a circular dichroism (CD) feature unique to ABLM to directly monitor the kinetics of ABLM reaction with a DNA oligonucleotide. Our results show that the ABLM + DNA reaction is appreciably faster, has a different kinetic isotope effect, and has a lower Arrhenius activation energy than does ABLM decay. In the ABLM reaction with DNA, the small normal kH/kD ratio is attributed to a secondary solvent effect through DFT vibrational analysis of reactant and transition state (TS) frequencies, and the lower Ea is attributed to the weaker bond involved in the abstraction reaction (C--H for DNA and N--H for the decay in the absence of DNA). The DNA dependence of the ABLM reaction indicates that DNA is involved in the TS for ABLM decay and thus reacts directly with (BLM)FeIII([eta]1--OOH) instead of its decay product. Oxygen-containing mononuclear iron species, FeIII--peroxo, FeIII--hydroperoxo and FeIV--oxo, are key intermediates in the catalytic activation of dioxygen by iron-containing metalloenzymes. It has been difficult to generate synthetic analogues of these three active iron--oxygen species in identical host complexes, which is necessary to elucidate changes to the structure of the iron center during catalysis and the factors that control their chemical reactivities with substrates. Here we report the high-resolution crystal structure of a mononuclear non-haem side-on FeIII--peroxo complex, [Fe(III)(TMC)(OO)]+. We also report a series of chemical reactions in which this iron(III)--peroxo complex is cleanly converted to the FeIII--hydroperoxo complex, [Fe(III)(TMC)(OOH)]2+, via a short-lived intermediate on protonation. This iron(III)--hydroperoxo complex then cleanly converts to the ferryl complex, [Fe(IV)(TMC)(O)]2+, via homolytic O--O bond cleavage of the iron(III)--hydroperoxo species. All three of these iron species--the three most biologically relevant iron--oxygen intermediates--have been spectroscopically characterized; we note that they have been obtained using a simple macrocyclic ligand. We have performed relative reactivity studies on these three iron species which reveal that the iron(III)--hydroperoxo complex is the most reactive of the three in the deformylation of aldehydes and that it has a similar reactivity to the iron(IV)--oxo complex in C--H bond activation of alkylaromatics. These reactivity results demonstrate that iron(III)--hydroperoxo species are viable oxidants in both nucleophilic and electrophilic reactions by iron-containing enzymes. The geometric and electronic structure and reactivity of an S = 5/2 (HS) mononuclear non-heme (TMC)FeIII-OOH complex was studied by spectroscopy, calculations, and kinetics for comparison to our past study of an S = 1/2 (LS) FeIII-OOH complex to understand their mechanisms of O-O bond homolysis and electrophilic H-atom abstraction. The homolysis reaction of the HS [(TMC)FeIII-OOH]2+ complex is found to involve axial ligand coordination and a crossing to the LS surface for O-O bond homolysis. Both HS and LS FeIII-OOH complexes are found to perform direct H-atom abstraction reactions but with very different reaction coordinates. For the LS FeIII-OOH, the transition state is late in O-O and early in C-H coordinates. However, for the HS FeIII-OOH, the transition state is early in O-O and further along in the C-H coordinate. In addition, there is a significant amount of electron transfer from substrate to HS FeIII-OOH at transition state, but does not occur in the LS transition state. Thus in contrast to the behavior of LS FeIII-OOH, the H-atom abstraction reactivity of HS FeIII-OOH is found to be highly dependent on both the ionization potential and C-H bond strength of substrate. LS FeIII-OOH is found to be more effective in H-atom abstraction for strong C-H bonds, while the higher reduction potential of HS FeIII-OOH allows it be active in electrophilic reactions without the requirement of O-O cleavage. This is relevant to the Rieske dioxygenases, which are proposed to use a HS FeIII-OOH to catalyze cis-dihydroxylation of a wide range of aromatic compounds. S K-edge XAS is a direct experimental probe of metal ion electronic structure as the pre-edge energy reflects its oxidation state, and the energy splitting pattern of the pre-edge transitions reflects its spin state. The combination of sulfur K-edge XAS and DFT calculations indicates that the electronic structures of {FeNO}7 (S = 3/2) (SMe2N4(tren)Fe(NO), complex I) and {FeNO}7 (S = 1/2) ((bme-daco)Fe(NO), complex II) are FeIII(S=5/2)--NO-- (S = 1) and FeIII(S=3/2)--NO-- (S = 1), respectively. When an axial ligand is computationally added to complex II, the electronic structure becomes FeII(S = 0)--NO[*] (S = 1/2). These studies demonstrate how the ligand field of the Fe center defines its spin state and thus changes the electron exchange, an important factor in determining the electron distribution over {FeNO}7 and {FeO2}8 sites.
Author: Sam P. De Visser
Publisher: Royal Society of Chemistry
ISBN: 1849731810
Category : Science
Languages : en
Pages : 463
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Book Description
Mononuclear iron containing enzymes are important intermediates in bioprocesses and have potential in the industrial biosynthesis of specific products. This book features topical review chapters by leaders in this field and its various sub-disciplines.
Author: Andrea Decker
Publisher:
ISBN:
Category :
Languages : en
Pages : 606
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Author: Shyam Rajan Iyer
Publisher:
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Category :
Languages : en
Pages :
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Author: Konstantin Bryliakov
Publisher: CRC Press
ISBN: 1466588578
Category : Science
Languages : en
Pages : 166
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Catalysis plays a vital role in chemical, petroleum, agriculture, polymer, electronics, pharmaceutical, and other industries. Over 90 percent of chemicals originate from catalytic processes. Toughening economic and environmental constraints have proven to be a challenge for meeting the demand of novel efficient and sustainable regio- and stereoselective catalyst systems. Environmentally Sustainable Catalytic Asymmetric Oxidations provides a comprehensive overview of existing ecologically friendly catalyst systems for various asymmetric oxidation processes. Topics include: A survey of existing transition metal-based catalyst systems for asymmetric epoxidations (AEs) with O2 and H2O2 Asymmetric sulfoxidations with H2O2 on chiral metal complexes An overview of various transition metal-catalyzed oxidative transformations with H2O2 or O2 used as the terminal oxidant Organocatalytic asymmetric oxidations Catalytic processes of stereospecific oxidations of C-H functional groups The role that oxoiron(V) intermediates play in chemo- and stereoselective oxidations catalyzed by non-heme iron complexes The book concludes with a discussion of the opportunities and problems associated with the industrial application of stereoselective processes of catalytic oxidation with H2O2 and O2. It also provides examples of processes with industrial potential. Some of the catalysts presented in this book may serve as promising alternatives for existing catalysts—progressively replacing them in manufacturing processes and ultimately making the chemical industry greener and cleaner.
Author: Agustí Lledós
Publisher: Springer Nature
ISBN: 3030569969
Category : Science
Languages : en
Pages : 276
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This book focuses on the computational modeling of organometallic reactivity. In recent years, computational methods, particularly those based on Density Functional Theory (DFT) have been fully incorporated into the toolbox of organometallic chemists’ methods. Nowadays, energy profiles of multistep processes are routinely calculated, and detailed mechanistic pictures of the reactions arise from these calculations. This type of analysis is increasingly performed even by experimentalists themselves. The volume aims to connect established computational organometallics with the more recent theoretical and methodological developments applied to this field. This would allow broadening of the simulation scope toward emergent organometallic areas (as ligand design or photoactivated processes), to narrow the gap between calculations and experiments (microkinetic models) and even to discover new reactions (automated methods). Given the broad interest and extensive application that computational methods have reached within the organometallic community, this new volume will attract the interest of both experimental and computational organometallic chemists.
Author: Marcel Swart
Publisher: John Wiley & Sons
ISBN: 1118898303
Category : Science
Languages : en
Pages : 472
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Book Description
It has long been recognized that metal spin states play a central role in the reactivity of important biomolecules, in industrial catalysis and in spin crossover compounds. As the fields of inorganic chemistry and catalysis move towards the use of cheap, non-toxic first row transition metals, it is essential to understand the important role of spin states in influencing molecular structure, bonding and reactivity. Spin States in Biochemistry and Inorganic Chemistry provides a complete picture on the importance of spin states for reactivity in biochemistry and inorganic chemistry, presenting both theoretical and experimental perspectives. The successes and pitfalls of theoretical methods such as DFT, ligand-field theory and coupled cluster theory are discussed, and these methods are applied in studies throughout the book. Important spectroscopic techniques to determine spin states in transition metal complexes and proteins are explained, and the use of NMR for the analysis of spin densities is described. Topics covered include: DFT and ab initio wavefunction approaches to spin states Experimental techniques for determining spin states Molecular discovery in spin crossover Multiple spin state scenarios in organometallic reactivity and gas phase reactions Transition-metal complexes involving redox non-innocent ligands Polynuclear iron sulfur clusters Molecular magnetism NMR analysis of spin densities This book is a valuable reference for researchers working in bioinorganic and inorganic chemistry, computational chemistry, organometallic chemistry, catalysis, spin-crossover materials, materials science, biophysics and pharmaceutical chemistry.
Author:
Publisher: Academic Press
ISBN: 0128128356
Category : Science
Languages : en
Pages : 346
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Book Description
Inorganic Reaction Mechanisms, Volume 70 is the latest volume in the Advances in Inorganic Chemistry series that presents timely summaries of current progress in inorganic chemistry, ranging from bio-inorganic to solid state studies. Topics covered in this updated volume include The Kinetics and Mechanism of Complex Redox Reactions in Aqueous Solution: The Tools of the Trade, O-O Bond Activation in Cu and Fe-Based Coordination Complexes: Breaking it Makes the Difference, μ-Nitrido Diiron Phthalocyanine and Porphyrin Complexes: Unusual Structures With Interesting Catalytic Properties, and The Role of Nonheme Transition Metal-Oxo, -Peroxo and -Superoxo Intermediates in Enzyme Catalysis and Reactions of Bioinspired Complexes. This acclaimed serial features reviews written by experts in the field, serving as an indispensable reference to advanced researchers. Each volume contains an index and chapters are fully referenced. Features comprehensive reviews on the latest developments in inorganic reaction mechanisms, a subfield of inorganic chemistry Includes contributions from leading experts in the field of inorganic reaction mechanisms Serves as an indispensable reference to advanced researchers in inorganic reaction mechanisms
