Yisong Guo | Carnegie Mellon University (original) (raw)
Papers by Yisong Guo
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](Inorganic chemistry, Jan 3, 2017
Tetramethylcyclam (TMC, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) exhibits two face... more Tetramethylcyclam (TMC, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) exhibits two faces in supporting an oxoiron(IV) moiety, as exemplified by the prototypical [(TMC)Fe(IV)(Oanti)(NCCH3)](OTf)2, where anti indicates that the O atom is located on the face opposite all four methyl groups, and the recently reported syn isomer [(TMC)Fe(IV)(Osyn)(OTf)](OTf). The ability to access two isomers of [(TMC)Fe(IV)(Oanti/syn)] raises the fundamental question of how ligand topology can affect the properties of the metal center. Previously, we have reported the formation of [(CH3CN)(TMC)Fe(III)-Oanti-Cr(III)(OTf)4(NCCH3)] (1) by inner-sphere electron transfer between Cr(OTf)2 and [(TMC)Fe(IV)(Oanti)(NCCH3)](OTf)2. Herein we demonstrate that a new species 2 is generated from the reaction between Cr(OTf)2 and [(TMC)Fe(IV)(Osyn)(NCCH3)](OTf)2, which is formulated as [(TMC)Fe(III)-Osyn-Cr(III)(OTf)4(NCCH3)] based on its characterization by UV-vis, resonance Raman, Mössbauer, and X-ray absor...
Angewandte Chemie International Edition, 2016
The biological activation of N2 occurs at the FeMo-cofactor, a 7Fe-9S-Mo-C-homocitrate cluster. F... more The biological activation of N2 occurs at the FeMo-cofactor, a 7Fe-9S-Mo-C-homocitrate cluster. FeMo-cofactor formation involves assembly of a Fe6-8 -SX -C core precursor, NifB-co, which occurs on the NifB protein. Characterization of NifB-co in NifB is complicated by the dynamic nature of the assembly process and the presence of a permanent [4Fe-4S] cluster associated with the radical SAM chemistry for generating the central carbide. We have used the physiological carrier protein, NifX, which has been proposed to bind NifB-co and deliver it to the NifEN protein, upon which FeMo-cofactor assembly is ultimately completed. Preparation of NifX in a fully NifB-co-loaded form provided an opportunity for Mössbauer analysis of NifB-co. The results indicate that NifB-co is a diamagnetic (S=0) 8-Fe cluster, containing two spectroscopically distinct Fe sites that appear in a 3:1 ratio. DFT analysis of the (57) Fe electric hyperfine interactions deduced from the Mössbauer analysis suggests that NifB-co is either a 4Fe(2+) -4Fe(3+) or 6Fe(2+) -2Fe(3+) cluster having valence-delocalized states.
Journal of the American Chemical Society, Jan 20, 2016
High valent Fe-OH species are often invoked as key intermediates but have only been observed in C... more High valent Fe-OH species are often invoked as key intermediates but have only been observed in Compound II of cytochrome P450s. To further address the properties of non-heme Fe(IV)-OH complexes we demonstrate the reversible protonation of a synthetic Fe(IV)-oxo species containing a tris-urea tripodal ligand. The same protonated Fe(IV)-oxo species can be prepared via oxidation, suggesting a putative Fe(V)-oxo species was initially generated. Computational, Mössbauer, XAS, and NRVS studies indicate that protonation of the Fe(IV)-oxo complex most likely occur on the tripodal ligand. We suggest that similar species for protonated high valent Fe-oxo species may occur in the active sites of proteins. This finding further argues for caution when assigning unverified high valent Fe-OH species to mechanisms.
Journal of the American Chemical Society, 2016
The ultimate step in chloramphenicol (CAM) biosynthesis is a six-electron oxidation of an aryl-am... more The ultimate step in chloramphenicol (CAM) biosynthesis is a six-electron oxidation of an aryl-amine precursor (NH2-CAM) to the aryl-nitro group of CAM catalyzed by the non-heme diiron cluster-containing oxygenase CmlI. Upon exposure of the diferrous cluster to O2, CmlI forms a long-lived peroxo intermediate, P, which reacts with NH2-CAM to form CAM. Since P is capable of at most a two-electron oxidation, the overall reaction must occur in several steps. It is unknown whether P is the oxidant in each step or whether another oxidizing species participates in the reaction. Mass spectrometry product analysis of reactions under (18)O2 show that both oxygen atoms in the nitro function of CAM derive from O2. However, when the single-turnover reaction between (18)O2-P and NH2-CAM is carried out in an (16)O2 atmosphere, CAM nitro groups contain both (18)O and (16)O, suggesting that P can be reformed during the reaction sequence. Such reformation would require reduction by a pathway intermediate, shown here to be NH(OH)-CAM. Accordingly, the aerobic reaction of NH(OH)-CAM with diferric CmlI yields P and then CAM without an external reductant. A catalytic cycle is proposed in which NH2-CAM reacts with P to form NH(OH)-CAM and diferric CmlI. Then the NH(OH)-CAM rereduces the enzyme diiron cluster, allowing P to reform upon O2 binding, while itself being oxidized to NO-CAM. Finally, the reformed P oxidizes NO-CAM to CAM with incorporation of a second O2-derived oxygen atom. The complete six-electron oxidation requires only two exogenous electrons and could occur in one active site.
Journal of the American Chemical Society, 2016
Mechanisms have been proposed for α-KG dependent non-heme iron enzyme catalyzed oxygen atom inser... more Mechanisms have been proposed for α-KG dependent non-heme iron enzyme catalyzed oxygen atom insertion into an olefinic moiety in various natural products, but not examined in detail. Using a combination of methods including transient kinetics, Mössbauer spectroscopy and mass spectrometry, we demonstrate that AsqJ catalyzed (-)-4'methoxy-cyclopenin formation uses a high-spin Fe(IV)-oxo intermediate to carry out epoxidation. Furthermore, product analysis on 16O/18O isotope incorporation from the reactions using the native substrate, 4'-methoxy-dehydrocyclopeptin, and a mechanistic probe, dehydrocyclopeptin, reveals evidence supporting oxo-hydroxo tautomerism of the Fe(IV)-oxo species in the non-heme iron enzyme catalysis.
Journal of the American Chemical Society, 2016
The enzyme isopenicillin N synthase (IPNS) installs the β-lactam and thiazolidine rings of the pe... more The enzyme isopenicillin N synthase (IPNS) installs the β-lactam and thiazolidine rings of the penicillin core into the linear tripeptide l-δ-aminoadipoyl-l-Cys-d-Val (ACV) on the pathways to a number of important antibacterial drugs. A classic set of enzymological and crystallographic studies by Baldwin and co-workers established that this overall four-electron oxidation occurs by a sequence of two oxidative cyclizations, with the β-lactam ring being installed first and the thiazolidine ring second. Each phase requires cleavage of an aliphatic C-H bond of the substrate: the pro-S-CCys,β-H bond for closure of the β-lactam ring, and the CVal,β-H bond for installation of the thiazolidine ring. IPNS uses a mononuclear non-heme-iron(II) cofactor and dioxygen as cosubstrate to cleave these C-H bonds and direct the ring closures. Despite the intense scrutiny to which the enzyme has been subjected, the identities of the oxidized iron intermediates that cleave the C-H bonds have been addressed only computationally; no experimental insight into their geometric or electronic structures has been reported. In this work, we have employed a combination of transient-state-kinetic and spectroscopic methods, together with the specifically deuterium-labeled substrates, A[d2-C]V and AC[d8-V], to identify both C-H-cleaving intermediates. The results show that they are high-spin Fe(III)-superoxo and high-spin Fe(IV)-oxo complexes, respectively, in agreement with published mechanistic proposals derived computationally from Baldwin's founding work.
Journal of the American Chemical Society, Jan 2, 2016
The non-heme iron halogenases CytC3 and SyrB2 catalyze C-H bond halogenation in the biosynthesis ... more The non-heme iron halogenases CytC3 and SyrB2 catalyze C-H bond halogenation in the biosynthesis of some natural products via S = 2 oxoiron(IV)-halide intermediates. These oxidants abstract a hydrogen atom from a substrate C-H bond to generate an alkyl radical that reacts with the bound halide to form a C-X bond chemoselectively. The origin of this selectivity has been explored in biological systems but has not yet been investigated with synthetic models. Here we report the characterization of S = 2 [Fe(IV)(O)(TQA)(Cl/Br)](+) (TQA = tris(quinolyl-2-methyl)amine) complexes that can preferentially halogenate cyclohexane. These are the first synthetic oxoiron(IV)-halide complexes that serve as spectroscopic and functional models for the halogenase intermediates. Interestingly, the nascent substrate radicals generated by these synthetic complexes are not as short-lived as those obtained from heme-based oxidants and can be intercepted by O2 to prevent halogenation, supporting an emerging...
Nature, Jan 2, 2015
Many peroxy-containing secondary metabolites have been isolated and shown to provide beneficial e... more Many peroxy-containing secondary metabolites have been isolated and shown to provide beneficial effects to human health. Yet, the mechanisms of most endoperoxide biosyntheses are not well understood. Although endoperoxides have been suggested as key reaction intermediates in several cases, the only well-characterized endoperoxide biosynthetic enzyme is prostaglandin H synthase, a haem-containing enzyme. Fumitremorgin B endoperoxidase (FtmOx1) from Aspergillus fumigatus is the first reported α-ketoglutarate-dependent mononuclear non-haem iron enzyme that can catalyse an endoperoxide formation reaction. To elucidate the mechanistic details for this unique chemical transformation, we report the X-ray crystal structures of FtmOx1 and the binary complexes it forms with either the co-substrate (α-ketoglutarate) or the substrate (fumitremorgin B). Uniquely, after α-ketoglutarate has bound to the mononuclear iron centre in a bidentate fashion, the remaining open site for oxygen binding and ...
Inorganic chemistry, Jan 17, 2015
We report herein the first example of an oxoiron(IV) complex of an ethylene-bridged dialkylcyclam... more We report herein the first example of an oxoiron(IV) complex of an ethylene-bridged dialkylcyclam ligand, [Fe(IV)(O)(Me2EBC)(NCMe)](2+) (2; Me2EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). Complex 2 has been characterized by UV-vis, (1)H NMR, resonance Raman, Mössbauer, and X-ray absorption spectroscopy as well as electrospray ionization mass spectrometry, and its properties have been compared with those of the closely related [Fe(IV)(O)(TMC)(NCMe)](2+) (3; TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), the intensively studied prototypical oxoiron(IV) complex of the macrocyclic tetramethylcyclam ligand. Me2EBC has an N4 donor set nearly identical with that of TMC but possesses an ethylene bridge in place of the 1- and 8-methyl groups of TMC. As a consequence, Me2EBC is forced to deviate from the trans-I configuration typically found for Fe(IV)(O)(TMC) complexes and instead adopts a folded cis-V stereochemistry that requires the MeCN ligand to coordi...
Journal of Inorganic Biochemistry, 2015
We have used femtosecond pump-probe spectroscopy (FPPS) to study the FeMo-cofactor within the nit... more We have used femtosecond pump-probe spectroscopy (FPPS) to study the FeMo-cofactor within the nitrogenase (N2ase) MoFe protein from Azotobacter vinelandii. A sub-20-fs visible laser pulse was used to pump the sample to an excited electronic state, and a second sub-10-fs pulse was used to probe changes in transmission as a function of probe wavelength and delay time. The excited protein relaxes to the ground state with a ~1.2ps time constant. With the short laser pulse we coherently excited the vibrational modes associated with the FeMo-cofactor active site, which are then observed in the time domain. Superimposed on the relaxation dynamics, we distinguished a variety of oscillation frequencies with the strongest band peaks at ~84, 116, 189, and 226cm(-1). Comparison with data from nuclear resonance vibrational spectroscopy (NRVS) shows that the latter pair of signals comes predominantly from the FeMo-cofactor. The frequencies obtained from the FPPS experiment were interpreted with normal mode calculations using both an empirical force field (EFF) and density functional theory (DFT). The FPPS data were also compared with the first reported resonance Raman (RR) spectrum of the N2ase MoFe protein. This approach allows us to outline and assign vibrational modes having relevance to the catalytic activity of N2ase. In particular, the 226cm(-1) band is assigned as a potential 'promoting vibration' in the H-atom transfer (or proton-coupled electron transfer) processes that are an essential feature of N2ase catalysis. The results demonstrate that high-quality room-temperature solution data can be obtained on the MoFe protein by the FPPS technique and that these data provide added insight to the motions and possible operation of this protein and its catalytic prosthetic group.
ChemInform, 2014
Highly active and selective aerobic cross-dehydrogenative coupling of terminal alkynes with thiol... more Highly active and selective aerobic cross-dehydrogenative coupling of terminal alkynes with thiols to construct alkynyl sulfides catalyzed by Cu(I) using molecular oxygen as the oxidant has been demonstrated under mild reaction conditions. The process is applicable to a wide range of alkynes and various thiols and is compatible with a variety of functional groups on both alkyne and thiol coupling partners. † Electronic supplementary information (ESI) available: Detailed experimental procedures, and spectral data for all compounds, including scanned images of 1 H and 13 C NMR spectra. See
European Journal of Inorganic Chemistry, 2015
ABSTRACT The electronic properties of iron tetraamido macrocyclic ligand (TAML) activators of per... more ABSTRACT The electronic properties of iron tetraamido macrocyclic ligand (TAML) activators of peroxides can be finely and coarsely tuned by varying the substituents on the “head” and “tail” macrocyclic components, respectively. By examining the reactivity of the TAML oxidoiron(V) complex with head-NO2 and tail-F substituents, one is able to compare the impact of significantly reduced macrocyclic tetraamide donor capacity on fundamental processes such as hydrogen-atom abstraction, oxygen-atom transfer, and electron transfer by using prior studies with more electron-rich TAML systems. Herein, we demonstrate that the oxidoiron(V) form 3c can be generated by treatment of [Fe{4-NO2C6H3-1,2-(N2COCMe2N3CO)2CF2(Fe–N2)(Fe–N3)}(OH2)]– (1c) with m-chloroperoxybenzoic acid (mCPBA) in MeCN at –40 °C. The oxidation proceeds through the intermediacy of the μ-oxo[iron(IV)]2 dimer. The overall rate of the FeIIIFeV conversion by mCPBA is slightly faster for 1c than that of its less electron-rich precursor [Fe{C6H4-1,2-(N1COCMe2N2CO)2CMe2(Fe–N1)(Fe–N2)}(OH2)]– (1a). Nevertheless, the oxidative reactivity of 3c toward thioanisole and the hydrocarbons ethylbenzene and cyclohexane exceeds that of 3a by 4.3, 2.1, and 2.6 times, respectively. The reactivity of 3c is significantly greater towards ethylbenzene than that of the oxidoiron(V) species 3b derived from [Fe{C6H4-1,2-(N1COCMe2N2CO)2NMe(Fe–N1)(Fe–N2)}(OH2)]– (1b).
Mössbauer Spectroscopy, 2013
Chemical science (Royal Society of Chemistry : 2010), 2012
The pentadentate ligand (n)Bu-P2DA (2(b), (n)Bu-P2DA = N-(1',1'-bis(2-pyridyl)pentyl)imin... more The pentadentate ligand (n)Bu-P2DA (2(b), (n)Bu-P2DA = N-(1',1'-bis(2-pyridyl)pentyl)iminodiacetate) was designed to bind an iron center in a carboxylate-rich environment similar to that found in the active sites of TauD and other α-ketoglutarate-dependent mononuclear non-heme iron enzymes. The iron(II) complex (n)Bu(4)N[Fe(II)(Cl)((n)Bu-P2DA)] (3(b)-Cl) was synthesized and crystallographically characterized to have a 2-pyridine-2-carboxylate donor set in the plane perpendicular to the Fe-Cl bond. Reaction of 3(b)-Cl with N-heterocyclic amines such as pyridine or imidazole yielded the N-heterocyclic amine adducts [Fe(II)(N)((n)Bu-P2DA)]. These adducts in turn reacted with oxo-transfer reagents at -95 °C to afford a short-lived oxoiron(IV) complex [Fe(IV)(O)((n)Bu-P2DA)] (5(b)) in yields as high as 90% depending on the heterocycle used. Complex 5(b) exhibits near-IR absorption features (λ(max) = 770 nm) and Mossbauer parameters (δ = 0.04 mm/s; ΔE(Q) = 1.13 mm/s; D = 27±2 cm(-...
Journal of the American Chemical Society, 2014
The properties of CO-inhibited Azotobacter vinelandii (Av) Mo-nitrogenase (N2ase) have been exami... more The properties of CO-inhibited Azotobacter vinelandii (Av) Mo-nitrogenase (N2ase) have been examined by the combined application of nuclear resonance vibrational spectroscopy (NRVS), extended X-ray absorption fine structure (EXAFS), and density functional theory (DFT). Dramatic changes in the NRVS are seen under high-CO conditions, especially in a 188 cm(-1) mode associated with symmetric breathing of the central cage of the FeMo-cofactor. Similar changes are reproduced with the α-H195Q N2ase variant. In the frequency region above 450 cm(-1), additional features are seen that are assigned to Fe-CO bending and stretching modes (confirmed by (13)CO isotope shifts). The EXAFS for wild-type N2ase shows evidence for a significant cluster distortion under high-CO conditions, most dramatically in the splitting of the interaction between Mo and the shell of Fe atoms originally at 5.08 Å in the resting enzyme. A DFT model with both a terminal -CO and a partially reduced -CHO ligand bound to adjacent Fe sites is consistent with both earlier FT-IR experiments, and the present EXAFS and NRVS observations for the wild-type enzyme. Another DFT model with two terminal CO ligands on the adjacent Fe atoms yields Fe-CO bands consistent with the α-H195Q variant NRVS. The calculations also shed light on the vibrational "shake" modes of the interstitial atom inside the central cage, and their interaction with the Fe-CO modes. Implications for the CO and N2 reactivity of N2ase are discussed.
Science, 2013
The iron-dependent epoxidase HppE converts (S)-2-hydroxypropyl-1-phosphonate (S-HPP) to the antib... more The iron-dependent epoxidase HppE converts (S)-2-hydroxypropyl-1-phosphonate (S-HPP) to the antibiotic fosfomycin [(1R,2S)-epoxypropylphosphonate] in an unusual 1,3-dehydrogenation of a secondary alcohol to an epoxide. HppE has been classified as an oxidase, with proposed mechanisms differing primarily in the identity of the O2-derived iron complex that abstracts hydrogen (H•) from C1 of S-HPP to initiate epoxide ring closure. We show here that the preferred cosubstrate is actually H2O2 and that HppE therefore almost certainly uses an iron(IV)-oxo complex as the H• abstractor. Reaction with H2O2 is accelerated by bound substrate and produces fosfomycin catalytically with a stoichiometry of unity. The ability of catalase to suppress the HppE activity previously attributed to its direct utilization of O2 implies that reduction of O2 and utilization of the resultant H2O2 were actually operant.
Journal of the American Chemical Society, 2011
The trigonal bipyramidal high-spin (S = 2) oxoiron(IV) complex [Fe IV (O)(TMG 2 dien) (CH 3 CN)] ... more The trigonal bipyramidal high-spin (S = 2) oxoiron(IV) complex [Fe IV (O)(TMG 2 dien) (CH 3 CN)] 2+ was synthesized and spectroscopically characterized. Substitution of the CH 3 CN ligand by anions, demonstrated here for X = N 3 − and Cl − , yielded further S = 2 oxoiron(IV) complexes of general formulation [Fe IV (O)(TMG 2 dien)(X)] + (7-X). The reduced steric bulk of 7 relative to the published S = 2 complex [Fe IV (O)(TMG 3 tren)] 2+ (2) was reflected by enhanced rates of intermolecular substrate oxidation. Non-heme monoiron oxygen activating enzymes perform a remarkably diverse array of highly selective oxidative transformations, 1 Most have iron centers with a 2-His-1carboxylate facial triad structural motif, and their catalytic cycles often involve oxoiron(IV) intermediates as oxidants. Within the past several years, such oxoiron(IV) species have been trapped and spectroscopically characterized in several enzymes and found in all cases to be high spin (S = 2). 2 In contrast, the overwhelming majority of existing synthetic oxoiron(IV) complexes have S = 1 ground states. 3 To date the only published examples of S = 2 oxoiron(IV) complexes are [Fe IV (O)(H 2 O) 5 ] 2+ (1), 4 [Fe IV (O)(TMG 3 tren)] 2+ (2, TMG 3 tren = 1,1,1-tris{2-[N 2 -(1,1,3,3-tetramethylguanidino)]-ethyl}amine)) 5 and [Fe IV (O)(H 3 buea)] − (3, H 3 buea = tris[(N′-tert-butylureaylato)-N-ethylene]amine trianion). 6
Inorganic Chemistry, 2011
Currently, there are only a handful of synthetic S = 2 oxoiron(IV) complexes. These serve as mode... more Currently, there are only a handful of synthetic S = 2 oxoiron(IV) complexes. These serve as models for the high-spin (S = 2) oxoiron(IV) species that have been postulated, and confirmed in several cases, as key intermediates in the catalytic cycles of a variety of non-heme oxygen activating enzymes. The trigonal bipyramidal complex [Fe IV (O)(TMG 3 tren)] 2+ (1) was both the first S = 2 oxoiron(IV) model complex to be generated in high yield and the first to be crystallographically characterized. In this study, we demonstrate that the TMG 3 tren ligand is also capable of supporting a tricationic cyanoiron(IV) unit, [Fe IV (CN)(TMG 3 tren)] 3+ (4). This complex was generated by electrolytic oxidation of the high-spin (S = 2) iron(II) complex [Fe II (CN) (TMG 3 tren)] + (2), via the S = 5/2 complex [Fe III (CN)(TMG 3 tren)] 2+ (3), the progress of which was conveniently monitored by using UV-Vis spectroscopy to follow the growth of bathochromically shifting LMCT bands. A combination of XAS, Mössbauer and NMR spectroscopies was used to establish that 4 has a S = 0 iron(IV) center. Consistent with its diamagnetic iron(IV) ground state, EXAFS analysis of 4 indicated a significant contraction of the iron-donor atom bond lengths, relative to those of the crystallographically characterized complexes 2 and 3. Notably, 4 has an Fe IV/III reduction potential of ~1.4 V vs Fc +/o , the highest value yet observed for a monoiron complex. The relatively high stability of 4 (t 1/2 in CD 3 CN solution containing 0.1 M KPF 6 at 25 °C ≈ 15 min), as reflected by its high-yield accumulation via slow bulk electrolysis and amenability to 13 C NMR at −40 °C, highlights the ability of the sterically protecting, highly basic peralkylguanidyl donors of the TMG 3 tren ligand to support highly charged high-valent complexes.
Inorganic chemistry, Jan 3, 2017
Tetramethylcyclam (TMC, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) exhibits two face... more Tetramethylcyclam (TMC, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) exhibits two faces in supporting an oxoiron(IV) moiety, as exemplified by the prototypical [(TMC)Fe(IV)(Oanti)(NCCH3)](OTf)2, where anti indicates that the O atom is located on the face opposite all four methyl groups, and the recently reported syn isomer [(TMC)Fe(IV)(Osyn)(OTf)](OTf). The ability to access two isomers of [(TMC)Fe(IV)(Oanti/syn)] raises the fundamental question of how ligand topology can affect the properties of the metal center. Previously, we have reported the formation of [(CH3CN)(TMC)Fe(III)-Oanti-Cr(III)(OTf)4(NCCH3)] (1) by inner-sphere electron transfer between Cr(OTf)2 and [(TMC)Fe(IV)(Oanti)(NCCH3)](OTf)2. Herein we demonstrate that a new species 2 is generated from the reaction between Cr(OTf)2 and [(TMC)Fe(IV)(Osyn)(NCCH3)](OTf)2, which is formulated as [(TMC)Fe(III)-Osyn-Cr(III)(OTf)4(NCCH3)] based on its characterization by UV-vis, resonance Raman, Mössbauer, and X-ray absor...
Angewandte Chemie International Edition, 2016
The biological activation of N2 occurs at the FeMo-cofactor, a 7Fe-9S-Mo-C-homocitrate cluster. F... more The biological activation of N2 occurs at the FeMo-cofactor, a 7Fe-9S-Mo-C-homocitrate cluster. FeMo-cofactor formation involves assembly of a Fe6-8 -SX -C core precursor, NifB-co, which occurs on the NifB protein. Characterization of NifB-co in NifB is complicated by the dynamic nature of the assembly process and the presence of a permanent [4Fe-4S] cluster associated with the radical SAM chemistry for generating the central carbide. We have used the physiological carrier protein, NifX, which has been proposed to bind NifB-co and deliver it to the NifEN protein, upon which FeMo-cofactor assembly is ultimately completed. Preparation of NifX in a fully NifB-co-loaded form provided an opportunity for Mössbauer analysis of NifB-co. The results indicate that NifB-co is a diamagnetic (S=0) 8-Fe cluster, containing two spectroscopically distinct Fe sites that appear in a 3:1 ratio. DFT analysis of the (57) Fe electric hyperfine interactions deduced from the Mössbauer analysis suggests that NifB-co is either a 4Fe(2+) -4Fe(3+) or 6Fe(2+) -2Fe(3+) cluster having valence-delocalized states.
Journal of the American Chemical Society, Jan 20, 2016
High valent Fe-OH species are often invoked as key intermediates but have only been observed in C... more High valent Fe-OH species are often invoked as key intermediates but have only been observed in Compound II of cytochrome P450s. To further address the properties of non-heme Fe(IV)-OH complexes we demonstrate the reversible protonation of a synthetic Fe(IV)-oxo species containing a tris-urea tripodal ligand. The same protonated Fe(IV)-oxo species can be prepared via oxidation, suggesting a putative Fe(V)-oxo species was initially generated. Computational, Mössbauer, XAS, and NRVS studies indicate that protonation of the Fe(IV)-oxo complex most likely occur on the tripodal ligand. We suggest that similar species for protonated high valent Fe-oxo species may occur in the active sites of proteins. This finding further argues for caution when assigning unverified high valent Fe-OH species to mechanisms.
Journal of the American Chemical Society, 2016
The ultimate step in chloramphenicol (CAM) biosynthesis is a six-electron oxidation of an aryl-am... more The ultimate step in chloramphenicol (CAM) biosynthesis is a six-electron oxidation of an aryl-amine precursor (NH2-CAM) to the aryl-nitro group of CAM catalyzed by the non-heme diiron cluster-containing oxygenase CmlI. Upon exposure of the diferrous cluster to O2, CmlI forms a long-lived peroxo intermediate, P, which reacts with NH2-CAM to form CAM. Since P is capable of at most a two-electron oxidation, the overall reaction must occur in several steps. It is unknown whether P is the oxidant in each step or whether another oxidizing species participates in the reaction. Mass spectrometry product analysis of reactions under (18)O2 show that both oxygen atoms in the nitro function of CAM derive from O2. However, when the single-turnover reaction between (18)O2-P and NH2-CAM is carried out in an (16)O2 atmosphere, CAM nitro groups contain both (18)O and (16)O, suggesting that P can be reformed during the reaction sequence. Such reformation would require reduction by a pathway intermediate, shown here to be NH(OH)-CAM. Accordingly, the aerobic reaction of NH(OH)-CAM with diferric CmlI yields P and then CAM without an external reductant. A catalytic cycle is proposed in which NH2-CAM reacts with P to form NH(OH)-CAM and diferric CmlI. Then the NH(OH)-CAM rereduces the enzyme diiron cluster, allowing P to reform upon O2 binding, while itself being oxidized to NO-CAM. Finally, the reformed P oxidizes NO-CAM to CAM with incorporation of a second O2-derived oxygen atom. The complete six-electron oxidation requires only two exogenous electrons and could occur in one active site.
Journal of the American Chemical Society, 2016
Mechanisms have been proposed for α-KG dependent non-heme iron enzyme catalyzed oxygen atom inser... more Mechanisms have been proposed for α-KG dependent non-heme iron enzyme catalyzed oxygen atom insertion into an olefinic moiety in various natural products, but not examined in detail. Using a combination of methods including transient kinetics, Mössbauer spectroscopy and mass spectrometry, we demonstrate that AsqJ catalyzed (-)-4'methoxy-cyclopenin formation uses a high-spin Fe(IV)-oxo intermediate to carry out epoxidation. Furthermore, product analysis on 16O/18O isotope incorporation from the reactions using the native substrate, 4'-methoxy-dehydrocyclopeptin, and a mechanistic probe, dehydrocyclopeptin, reveals evidence supporting oxo-hydroxo tautomerism of the Fe(IV)-oxo species in the non-heme iron enzyme catalysis.
Journal of the American Chemical Society, 2016
The enzyme isopenicillin N synthase (IPNS) installs the β-lactam and thiazolidine rings of the pe... more The enzyme isopenicillin N synthase (IPNS) installs the β-lactam and thiazolidine rings of the penicillin core into the linear tripeptide l-δ-aminoadipoyl-l-Cys-d-Val (ACV) on the pathways to a number of important antibacterial drugs. A classic set of enzymological and crystallographic studies by Baldwin and co-workers established that this overall four-electron oxidation occurs by a sequence of two oxidative cyclizations, with the β-lactam ring being installed first and the thiazolidine ring second. Each phase requires cleavage of an aliphatic C-H bond of the substrate: the pro-S-CCys,β-H bond for closure of the β-lactam ring, and the CVal,β-H bond for installation of the thiazolidine ring. IPNS uses a mononuclear non-heme-iron(II) cofactor and dioxygen as cosubstrate to cleave these C-H bonds and direct the ring closures. Despite the intense scrutiny to which the enzyme has been subjected, the identities of the oxidized iron intermediates that cleave the C-H bonds have been addressed only computationally; no experimental insight into their geometric or electronic structures has been reported. In this work, we have employed a combination of transient-state-kinetic and spectroscopic methods, together with the specifically deuterium-labeled substrates, A[d2-C]V and AC[d8-V], to identify both C-H-cleaving intermediates. The results show that they are high-spin Fe(III)-superoxo and high-spin Fe(IV)-oxo complexes, respectively, in agreement with published mechanistic proposals derived computationally from Baldwin's founding work.
Journal of the American Chemical Society, Jan 2, 2016
The non-heme iron halogenases CytC3 and SyrB2 catalyze C-H bond halogenation in the biosynthesis ... more The non-heme iron halogenases CytC3 and SyrB2 catalyze C-H bond halogenation in the biosynthesis of some natural products via S = 2 oxoiron(IV)-halide intermediates. These oxidants abstract a hydrogen atom from a substrate C-H bond to generate an alkyl radical that reacts with the bound halide to form a C-X bond chemoselectively. The origin of this selectivity has been explored in biological systems but has not yet been investigated with synthetic models. Here we report the characterization of S = 2 [Fe(IV)(O)(TQA)(Cl/Br)](+) (TQA = tris(quinolyl-2-methyl)amine) complexes that can preferentially halogenate cyclohexane. These are the first synthetic oxoiron(IV)-halide complexes that serve as spectroscopic and functional models for the halogenase intermediates. Interestingly, the nascent substrate radicals generated by these synthetic complexes are not as short-lived as those obtained from heme-based oxidants and can be intercepted by O2 to prevent halogenation, supporting an emerging...
Nature, Jan 2, 2015
Many peroxy-containing secondary metabolites have been isolated and shown to provide beneficial e... more Many peroxy-containing secondary metabolites have been isolated and shown to provide beneficial effects to human health. Yet, the mechanisms of most endoperoxide biosyntheses are not well understood. Although endoperoxides have been suggested as key reaction intermediates in several cases, the only well-characterized endoperoxide biosynthetic enzyme is prostaglandin H synthase, a haem-containing enzyme. Fumitremorgin B endoperoxidase (FtmOx1) from Aspergillus fumigatus is the first reported α-ketoglutarate-dependent mononuclear non-haem iron enzyme that can catalyse an endoperoxide formation reaction. To elucidate the mechanistic details for this unique chemical transformation, we report the X-ray crystal structures of FtmOx1 and the binary complexes it forms with either the co-substrate (α-ketoglutarate) or the substrate (fumitremorgin B). Uniquely, after α-ketoglutarate has bound to the mononuclear iron centre in a bidentate fashion, the remaining open site for oxygen binding and ...
Inorganic chemistry, Jan 17, 2015
We report herein the first example of an oxoiron(IV) complex of an ethylene-bridged dialkylcyclam... more We report herein the first example of an oxoiron(IV) complex of an ethylene-bridged dialkylcyclam ligand, [Fe(IV)(O)(Me2EBC)(NCMe)](2+) (2; Me2EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). Complex 2 has been characterized by UV-vis, (1)H NMR, resonance Raman, Mössbauer, and X-ray absorption spectroscopy as well as electrospray ionization mass spectrometry, and its properties have been compared with those of the closely related [Fe(IV)(O)(TMC)(NCMe)](2+) (3; TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), the intensively studied prototypical oxoiron(IV) complex of the macrocyclic tetramethylcyclam ligand. Me2EBC has an N4 donor set nearly identical with that of TMC but possesses an ethylene bridge in place of the 1- and 8-methyl groups of TMC. As a consequence, Me2EBC is forced to deviate from the trans-I configuration typically found for Fe(IV)(O)(TMC) complexes and instead adopts a folded cis-V stereochemistry that requires the MeCN ligand to coordi...
Journal of Inorganic Biochemistry, 2015
We have used femtosecond pump-probe spectroscopy (FPPS) to study the FeMo-cofactor within the nit... more We have used femtosecond pump-probe spectroscopy (FPPS) to study the FeMo-cofactor within the nitrogenase (N2ase) MoFe protein from Azotobacter vinelandii. A sub-20-fs visible laser pulse was used to pump the sample to an excited electronic state, and a second sub-10-fs pulse was used to probe changes in transmission as a function of probe wavelength and delay time. The excited protein relaxes to the ground state with a ~1.2ps time constant. With the short laser pulse we coherently excited the vibrational modes associated with the FeMo-cofactor active site, which are then observed in the time domain. Superimposed on the relaxation dynamics, we distinguished a variety of oscillation frequencies with the strongest band peaks at ~84, 116, 189, and 226cm(-1). Comparison with data from nuclear resonance vibrational spectroscopy (NRVS) shows that the latter pair of signals comes predominantly from the FeMo-cofactor. The frequencies obtained from the FPPS experiment were interpreted with normal mode calculations using both an empirical force field (EFF) and density functional theory (DFT). The FPPS data were also compared with the first reported resonance Raman (RR) spectrum of the N2ase MoFe protein. This approach allows us to outline and assign vibrational modes having relevance to the catalytic activity of N2ase. In particular, the 226cm(-1) band is assigned as a potential 'promoting vibration' in the H-atom transfer (or proton-coupled electron transfer) processes that are an essential feature of N2ase catalysis. The results demonstrate that high-quality room-temperature solution data can be obtained on the MoFe protein by the FPPS technique and that these data provide added insight to the motions and possible operation of this protein and its catalytic prosthetic group.
ChemInform, 2014
Highly active and selective aerobic cross-dehydrogenative coupling of terminal alkynes with thiol... more Highly active and selective aerobic cross-dehydrogenative coupling of terminal alkynes with thiols to construct alkynyl sulfides catalyzed by Cu(I) using molecular oxygen as the oxidant has been demonstrated under mild reaction conditions. The process is applicable to a wide range of alkynes and various thiols and is compatible with a variety of functional groups on both alkyne and thiol coupling partners. † Electronic supplementary information (ESI) available: Detailed experimental procedures, and spectral data for all compounds, including scanned images of 1 H and 13 C NMR spectra. See
European Journal of Inorganic Chemistry, 2015
ABSTRACT The electronic properties of iron tetraamido macrocyclic ligand (TAML) activators of per... more ABSTRACT The electronic properties of iron tetraamido macrocyclic ligand (TAML) activators of peroxides can be finely and coarsely tuned by varying the substituents on the “head” and “tail” macrocyclic components, respectively. By examining the reactivity of the TAML oxidoiron(V) complex with head-NO2 and tail-F substituents, one is able to compare the impact of significantly reduced macrocyclic tetraamide donor capacity on fundamental processes such as hydrogen-atom abstraction, oxygen-atom transfer, and electron transfer by using prior studies with more electron-rich TAML systems. Herein, we demonstrate that the oxidoiron(V) form 3c can be generated by treatment of [Fe{4-NO2C6H3-1,2-(N2COCMe2N3CO)2CF2(Fe–N2)(Fe–N3)}(OH2)]– (1c) with m-chloroperoxybenzoic acid (mCPBA) in MeCN at –40 °C. The oxidation proceeds through the intermediacy of the μ-oxo[iron(IV)]2 dimer. The overall rate of the FeIIIFeV conversion by mCPBA is slightly faster for 1c than that of its less electron-rich precursor [Fe{C6H4-1,2-(N1COCMe2N2CO)2CMe2(Fe–N1)(Fe–N2)}(OH2)]– (1a). Nevertheless, the oxidative reactivity of 3c toward thioanisole and the hydrocarbons ethylbenzene and cyclohexane exceeds that of 3a by 4.3, 2.1, and 2.6 times, respectively. The reactivity of 3c is significantly greater towards ethylbenzene than that of the oxidoiron(V) species 3b derived from [Fe{C6H4-1,2-(N1COCMe2N2CO)2NMe(Fe–N1)(Fe–N2)}(OH2)]– (1b).
Mössbauer Spectroscopy, 2013
Chemical science (Royal Society of Chemistry : 2010), 2012
The pentadentate ligand (n)Bu-P2DA (2(b), (n)Bu-P2DA = N-(1',1'-bis(2-pyridyl)pentyl)imin... more The pentadentate ligand (n)Bu-P2DA (2(b), (n)Bu-P2DA = N-(1',1'-bis(2-pyridyl)pentyl)iminodiacetate) was designed to bind an iron center in a carboxylate-rich environment similar to that found in the active sites of TauD and other α-ketoglutarate-dependent mononuclear non-heme iron enzymes. The iron(II) complex (n)Bu(4)N[Fe(II)(Cl)((n)Bu-P2DA)] (3(b)-Cl) was synthesized and crystallographically characterized to have a 2-pyridine-2-carboxylate donor set in the plane perpendicular to the Fe-Cl bond. Reaction of 3(b)-Cl with N-heterocyclic amines such as pyridine or imidazole yielded the N-heterocyclic amine adducts [Fe(II)(N)((n)Bu-P2DA)]. These adducts in turn reacted with oxo-transfer reagents at -95 °C to afford a short-lived oxoiron(IV) complex [Fe(IV)(O)((n)Bu-P2DA)] (5(b)) in yields as high as 90% depending on the heterocycle used. Complex 5(b) exhibits near-IR absorption features (λ(max) = 770 nm) and Mossbauer parameters (δ = 0.04 mm/s; ΔE(Q) = 1.13 mm/s; D = 27±2 cm(-...
Journal of the American Chemical Society, 2014
The properties of CO-inhibited Azotobacter vinelandii (Av) Mo-nitrogenase (N2ase) have been exami... more The properties of CO-inhibited Azotobacter vinelandii (Av) Mo-nitrogenase (N2ase) have been examined by the combined application of nuclear resonance vibrational spectroscopy (NRVS), extended X-ray absorption fine structure (EXAFS), and density functional theory (DFT). Dramatic changes in the NRVS are seen under high-CO conditions, especially in a 188 cm(-1) mode associated with symmetric breathing of the central cage of the FeMo-cofactor. Similar changes are reproduced with the α-H195Q N2ase variant. In the frequency region above 450 cm(-1), additional features are seen that are assigned to Fe-CO bending and stretching modes (confirmed by (13)CO isotope shifts). The EXAFS for wild-type N2ase shows evidence for a significant cluster distortion under high-CO conditions, most dramatically in the splitting of the interaction between Mo and the shell of Fe atoms originally at 5.08 Å in the resting enzyme. A DFT model with both a terminal -CO and a partially reduced -CHO ligand bound to adjacent Fe sites is consistent with both earlier FT-IR experiments, and the present EXAFS and NRVS observations for the wild-type enzyme. Another DFT model with two terminal CO ligands on the adjacent Fe atoms yields Fe-CO bands consistent with the α-H195Q variant NRVS. The calculations also shed light on the vibrational "shake" modes of the interstitial atom inside the central cage, and their interaction with the Fe-CO modes. Implications for the CO and N2 reactivity of N2ase are discussed.
Science, 2013
The iron-dependent epoxidase HppE converts (S)-2-hydroxypropyl-1-phosphonate (S-HPP) to the antib... more The iron-dependent epoxidase HppE converts (S)-2-hydroxypropyl-1-phosphonate (S-HPP) to the antibiotic fosfomycin [(1R,2S)-epoxypropylphosphonate] in an unusual 1,3-dehydrogenation of a secondary alcohol to an epoxide. HppE has been classified as an oxidase, with proposed mechanisms differing primarily in the identity of the O2-derived iron complex that abstracts hydrogen (H•) from C1 of S-HPP to initiate epoxide ring closure. We show here that the preferred cosubstrate is actually H2O2 and that HppE therefore almost certainly uses an iron(IV)-oxo complex as the H• abstractor. Reaction with H2O2 is accelerated by bound substrate and produces fosfomycin catalytically with a stoichiometry of unity. The ability of catalase to suppress the HppE activity previously attributed to its direct utilization of O2 implies that reduction of O2 and utilization of the resultant H2O2 were actually operant.
Journal of the American Chemical Society, 2011
The trigonal bipyramidal high-spin (S = 2) oxoiron(IV) complex [Fe IV (O)(TMG 2 dien) (CH 3 CN)] ... more The trigonal bipyramidal high-spin (S = 2) oxoiron(IV) complex [Fe IV (O)(TMG 2 dien) (CH 3 CN)] 2+ was synthesized and spectroscopically characterized. Substitution of the CH 3 CN ligand by anions, demonstrated here for X = N 3 − and Cl − , yielded further S = 2 oxoiron(IV) complexes of general formulation [Fe IV (O)(TMG 2 dien)(X)] + (7-X). The reduced steric bulk of 7 relative to the published S = 2 complex [Fe IV (O)(TMG 3 tren)] 2+ (2) was reflected by enhanced rates of intermolecular substrate oxidation. Non-heme monoiron oxygen activating enzymes perform a remarkably diverse array of highly selective oxidative transformations, 1 Most have iron centers with a 2-His-1carboxylate facial triad structural motif, and their catalytic cycles often involve oxoiron(IV) intermediates as oxidants. Within the past several years, such oxoiron(IV) species have been trapped and spectroscopically characterized in several enzymes and found in all cases to be high spin (S = 2). 2 In contrast, the overwhelming majority of existing synthetic oxoiron(IV) complexes have S = 1 ground states. 3 To date the only published examples of S = 2 oxoiron(IV) complexes are [Fe IV (O)(H 2 O) 5 ] 2+ (1), 4 [Fe IV (O)(TMG 3 tren)] 2+ (2, TMG 3 tren = 1,1,1-tris{2-[N 2 -(1,1,3,3-tetramethylguanidino)]-ethyl}amine)) 5 and [Fe IV (O)(H 3 buea)] − (3, H 3 buea = tris[(N′-tert-butylureaylato)-N-ethylene]amine trianion). 6
Inorganic Chemistry, 2011
Currently, there are only a handful of synthetic S = 2 oxoiron(IV) complexes. These serve as mode... more Currently, there are only a handful of synthetic S = 2 oxoiron(IV) complexes. These serve as models for the high-spin (S = 2) oxoiron(IV) species that have been postulated, and confirmed in several cases, as key intermediates in the catalytic cycles of a variety of non-heme oxygen activating enzymes. The trigonal bipyramidal complex [Fe IV (O)(TMG 3 tren)] 2+ (1) was both the first S = 2 oxoiron(IV) model complex to be generated in high yield and the first to be crystallographically characterized. In this study, we demonstrate that the TMG 3 tren ligand is also capable of supporting a tricationic cyanoiron(IV) unit, [Fe IV (CN)(TMG 3 tren)] 3+ (4). This complex was generated by electrolytic oxidation of the high-spin (S = 2) iron(II) complex [Fe II (CN) (TMG 3 tren)] + (2), via the S = 5/2 complex [Fe III (CN)(TMG 3 tren)] 2+ (3), the progress of which was conveniently monitored by using UV-Vis spectroscopy to follow the growth of bathochromically shifting LMCT bands. A combination of XAS, Mössbauer and NMR spectroscopies was used to establish that 4 has a S = 0 iron(IV) center. Consistent with its diamagnetic iron(IV) ground state, EXAFS analysis of 4 indicated a significant contraction of the iron-donor atom bond lengths, relative to those of the crystallographically characterized complexes 2 and 3. Notably, 4 has an Fe IV/III reduction potential of ~1.4 V vs Fc +/o , the highest value yet observed for a monoiron complex. The relatively high stability of 4 (t 1/2 in CD 3 CN solution containing 0.1 M KPF 6 at 25 °C ≈ 15 min), as reflected by its high-yield accumulation via slow bulk electrolysis and amenability to 13 C NMR at −40 °C, highlights the ability of the sterically protecting, highly basic peralkylguanidyl donors of the TMG 3 tren ligand to support highly charged high-valent complexes.