Interaction of the substrate radical and the 5'-deoxyadenosine-5'-methyl group in vitamin B(12) coenzyme-dependent ethanolamine deaminase

The distance and relative orientation of the C5' methyl group of 5'-deoxyadenosine and the substrate radical in vitamin B(12) coenzyme-dependent ethanolamine deaminase from Salmonella typhimurium have been characterized by using X-band two-pulse electron spin-echo envelope modulation (ESEEM) spectroscopy in the disordered solid state. The (S)-2-aminopropanol-generated substrate radical catalytic intermediate was prepared by cryotrapping steady-state mixtures of enzyme in which catalytically exchangeable hydrogen sites in the active site had been labeled by previous turnover on (2)H(4)-ethanolamine. Simulation of the time- and frequency-domain ESEEM requires two types of coupled (2)H. The strongly coupled (2)H has an effective dipole distance (r(eff)) of 2.2 A, and isotropic coupling constant (A(iso)) of -0.35 MHz. The weakly coupled (2)H has r(eff) = 3.8 A and A(iso) = 0 MHz. The best (2)H ESEEM time- and frequency-domain simulations are achieved with a model in which the hyperfine couplings arise from one strongly coupled hydrogen site and two equivalent weakly coupled hydrogen sites located on the C5' methyl group of 5'-deoxyadenosine. This model indicates that the unpaired electron on C1 of the substrate radical and C5' are separated by 3.2 A and are thus at closest contact. The close proximity of C1 and C5' indicates that C5' of the 5'-deoxyadenosyl moiety directly mediates radical migration between cobalt in cobalamin and the substrate/product site over a distance of 5-7 A in the active site of ethanolamine deaminase.     

Mechanism of cobalt(II) porphyrin-catalyzed C-H amination with organic azides: radical nature and H-atom abstraction ability of the key cobalt(III)-nitrene intermediates

The mechanism of cobalt(II) porphyrin-catalyzed benzylic C-H bond amination of ethylbenzene, toluene, and 1,2,3,4-tetrahydronaphthalene (tetralin) using a series of different organic azides [N(3)C(O)OMe, N(3)SO(2)Ph, N(3)C(O)Ph, and N(3)P(O)(OMe)(2)] as nitrene sources was studied by means of density functional theory (DFT) calculations and electron paramagnetic resonance (EPR) spectroscopy. The DFT computational study revealed a stepwise radical process involving coordination of the azide to the metal center followed by elimination of dinitrogen to produce unusual "nitrene radical" intermediates (por)Co(III)-N(?)Y (4) [Y = -C(O)OMe, -SO(2)Ph, -C(O)Ph, -P(O)(OMe)(2)]. Formation of these nitrene radical ligand complexes is exothermic, predicting that the nitrene radical ligand complexes should be detectable species in the absence of other reacting substrates. In good agreement with the DFT calculations, isotropic solution EPR signals with g values characteristic of ligand-based radicals were detected experimentally from (por)Co complexes in the presence of excess organic azide in benzene. They are best described as nitrene radical anion ligand complexes (por)Co(III)-N(?)Y, which have their unpaired spin density located almost entirely on the nitrogen atom of the nitrene moiety. These key cobalt(III)-nitrene radical intermediates readily abstract a hydrogen atom from a benzylic position of the organic substrate to form the intermediate species 5, which are close-contact pairs of the thus-formed organic radicals R'(?) and the cobalt(III)-amido complexes (por)Co(III)-NHY ({R'(?)···(por)Co(III)-NHY}). These close-contact pairs readily collapse in a virtually barrierless fashion (via transition state TS3) to produce the cobalt(II)-amine complexes (por)Co(II)-NHYR', which dissociate to afford the desired amine products NHYR' (6) with regeneration of the (por)Co catalyst. Alternatively, the close-contact pairs {R'(?)···(por)Co(III)-NHY} 5 may undergo β-hydrogen-atom abstraction from the benzylic radical R'(?) by (por)Co(III)-NHY (via TS4) to form the corresponding olefin and (por)Co(III)-NH(2)Y, which dissociates to give Y-NH(2). This process for the formation of olefin and Y-NH(2) byproducts is also essentially barrierless and should compete with the collapse of 5 via TS3 to form the desired amine product. Alternative processes leading to the formation of side products and the influence of different porphyrin ligands with varying electronic properties on the catalytic activity of the cobalt(II) complexes have also been investigated.     

Electron spin-echo envelope modulation and pulse electron nuclear double resonance studies of Cu2+...beta-carotene interactions in Cu-MCM-41 molecular sieves

Perdeuterated all-trans beta-carotene imbedded in activated Cu-MCM-41 was examined by electron paramagnetic resonance (EPR) and electron spin-echo envelope modulation (ESEEM) spectroscopies. The EPR study showed that complexation and electron transfer between Cu2+ and deuterated beta-carotene occurs. The interaction was confirmed by detecting the spin-echo modulation of deuterium in the ESEEM spectra of Cu2+. Ratio analysis of ESEEM was used to determine the number of deuterons which interact with Cu2+ and the distance between deuteron(s) and Cu2+. The bonding site of beta- carotene determined by ESEEM and pulse electron nuclear double resonance is the C15=C15' double bond.     

Electronic structure description of a [Co(III)3Co(IV)O4] cluster: a model for the paramagnetic intermediate in cobalt-catalyzed water oxidation

Multifrequency electron paramagnetic resonace (EPR) spectroscopy and electronic structure calculations were performed on [Co(4)O(4)(C(5)H(5)N)(4)(CH(3)CO(2))(4)](+) (1(+)), a cobalt tetramer with total electron spin S = 1/2 and formal cobalt oxidation states III, III, III, and IV. The cuboidal arrangement of its cobalt and oxygen atoms is similar to that of proposed structures for the molecular cobaltate clusters of the cobalt-phosphate (Co-Pi) water-oxidizing catalyst. The Davies electron-nuclear double resonance (ENDOR) spectrum is well-modeled using a single class of hyperfine-coupled (59)Co nuclei with a modestly strong interaction (principal elements of the hyperfine tensor are equal to [-20(±2), 77(±1), -5(±15)] MHz). Mims (1)H ENDOR spectra of 1(+) with selectively deuterated pyridine ligands confirm that the amount of unpaired spin on the cobalt-bonding partner is significantly reduced from unity. Multifrequency (14)N ESEEM spectra (acquired at 9.5 and 34.0 GHz) indicate that four nearly equivalent nitrogen nuclei are coupled to the electron spin. Cumulatively, our EPR spectroscopic findings indicate that the unpaired spin is delocalized almost equally across the eight core atoms, a finding corroborated by results from DFT calculations. Each octahedrally coordinated cobalt ion is forced into a low-spin electron configuration by the anionic oxo and carboxylato ligands, and a fractional electron hole is localized on each metal center in a Co 3d(xz,yz)-based molecular orbital for this essentially [Co(+3.125)(4)O(4)] system. Comparing the EPR spectrum of 1(+) with that of the catalyst film allows us to draw conclusions about the electronic structure of this water-oxidation catalyst.     

Suicide Inactivation of Dioldehydrase by 2‐Chloroacetaldehyde: Formation of the ‘cis‐Ethanesemidione’ Radical,and the Role of a Monovalent Cation

Dioldehydrase is an adenosylcobalamin‐dependent enzyme that catalyzes the dehydration of (R)‐ or (S)‐propane‐1,2‐diol to propanal. The reaction proceeds by a radical mechanism initiated by the homolytic scission of the covalent Co? C(5′) bond in the coenzyme to form cob(II)alamin and the 5‐deoxyadenosyl radical as transient intermediates. Dioldehydrase is subject to ‘suicide inactivation’ by substrate/product analogs. Inactivation by 2‐chloroacetaldehyde converts the inactivator into the ‘cis‐ethanesemidione’ radical. A mechanism for this process includes reaction of chloroacetaldehyde in the reverse of the normal catalytic process to a rearranged radical that eliminates HCl. K⁺ and other monovalent cations of similar size, including Tl⁺, are required for dioldehydrase activity and for suicide inactivation by glycolaldehyde or 2‐chloroacetaldehyde. A K⁺ ion is bound to propane‐1,2‐diol in dioldehydrase. Both EPR and pulsed‐EPR experiments show that the magnetic nuclei of thallous ions (²⁰³Tl⁺, ²⁰⁵Tl⁺) do not interact with the unpaired electron in the cis‐ethanesemidione radical at the active site of dioldehydrase. Pulsed‐EPR experiments implicate a ¹⁴NH group, possibly of His¹⁴³, interacting with the radical at the active site.     

Isolation and characterization of four isomers of a C(60) bisadduct with a TTF derivative. Study of their radical ions.

A family of triads composed of C(60) attached by a rigid spacer to two identical TTF moieties has been synthesized, and some of the isomers have been isolated and characterized by UV-vis spectroscopy, LDI-TOF-MS, and HMBC NMR spectroscopy. AM1 semiempirical calculations of the dipolar moments and the heats of formation of the different isomers have been carried out in order to verify their assignments. Oxidation and reduction of the triads affords the derived radical ion systems, TTF(+*)-C(60)-TTF(+*) and TTF-C(60)(-*)-TTF, which were studied by EPR spectroscopy. Spin density distributions of these radical cations and radical anions show that the unpaired electron is located mainly on the TTF and fullerene moieties, respectively. However, while the EPR signals obtained from the radical cations are very similar for all the isomers, the structured signals observed for the radical anions arising from the coupling of the unpaired electron with the hydrogen atoms of the methylene bridges in the spacer show that there is a strong influence of the isomerism on the spin distribution. Importantly, the theoretical calculations of the spin density distributions of the radical anions fit well with the experimental EPR results.     

Direct determination of product radical structure reveals the radical rearrangement pathway in a coenzyme B12-dependent enzyme

A carbinolamine (1-aminoethan-1-ol-2-yl) structure for the product radical in the CoII product radical pair catalytic intermediate state in coenzyme B12 (adenosylcobalamin)-dependent ethanolamine deaminase from Salmonella typhimurium has been determined by using isotope labeling and techniques of electron paramagnetic resonance (EPR) spectroscopy. The presence of nitrogen is detected from the difference in the EPR line shapes of the product radicals that are cryotrapped during steady-state turnover on either 14N- or 15N-labeled aminoethanol substrate. Three-pulse electron spin-echo envelope modulation (ESEEM) spectroscopy of the product radical labeled with 2H reveals two types of beta-2H hyperfine couplings. A structural model is proposed in which the two beta-2H couplings arise from two C1-C2 product radical rotamer states. The sum of the dihedral angles between the C2 p-orbital axis and C1-Hbeta bonds is 120 degrees , which indicates sp3-hybridization at C1. This confirms the C1 carbinolamine structure. The identification of the carbinolamine product radical indicates that the radical rearrangement in ethanolamine deaminase deviates from the solution elimination reaction pathway and proceeds by migration of the amine from C2 of the substrate radical to C1 of the product radical.     

Corroborative models of the cobalt(II) inhibited Fe/Mn superoxide dismutases

Attempting to model superoxide dismutase (SOD) enzymes, we designed two new N3O-donor ligands to provide the same set of donor atoms observed in the active site of these enzymes: K(i)Pr2TCMA (potassium 1,4-diisopropyl-1,4,7-triazacyclononane-N-acetate) and KBPZG (potassium N,N-bis(3,5-dimethylpyrazolylmethyl) glycinate). Five new Co(II) complexes (1-5) were obtained and characterized by X-ray crystallography, mass spectrometry, electrochemistry, magnetochemistry, UV-vis, and electron paramagnetic resonance (EPR) spectroscopies. The crystal structures of 1 and 3-5 revealed five-coordinate complexes, whereas complex 2 is six-coordinate. The EPR data of complexes 3 and 4 agree with those of the Co(II)-substituted SOD, which strongly support the proposition that the active site of the enzyme structurally resembles these models. The redox behavior of complexes 1-5 clearly demonstrates the stabilization of the Co(II) state in the ligand field provided by these ligands. The irreversibility displayed by all of the complexes is probably related to an electron-transfer process followed by a rearrangement of the geometry around the metal center for complexes 1 and 3-5 that probably changes from a trigonal bipyramidal (high spin, d7) to octahedral (low spin, d6) as Co(II) is oxidized to Co(III), which is also expected to be accompanied by a spin-state conversion. As the redox potentials to convert the Co(II) to Co(III) are high, it can be inferred that the redox potential of the Co(II)-substituted SOD may be outside the range required to convert the superoxide radical (O2*-) to hydrogen peroxide, and this is sufficient to explain the inactivity of the enzyme. Finally, the complexes reported here are the first corroborative structural models of the Co(II)-substituted SOD.     

Identification of primary free radicals in trehalose dihydrate single crystals X-irradiated at 10 K

Primary free radical formation in trehalose dihydrate single crystals X-irradiated at 10 K was investigated at the same temperature using X-band Electron Paramagnetic Resonance (EPR), Electron Nuclear Double Resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques. The ENDOR results allowed the unambiguous determination of six proton hyperfine coupling (HFC) tensors. Using the EIE technique, these HF interactions were assigned to three different radicals, labeled R1, R2 and R3. The anisotropy of the EPR and EIE spectra indicated that R1 and R2 are alkyl radicals (i.e. carbon-centered) and R3 is an alkoxy radical (i.e. oxygen-centered). The EPR data also revealed the presence of an additional alkoxy radical species, labeled R4. Molecular modeling using periodic Density Functional Theory (DFT) calculations for simulating experimental data suggests that R1 and R2 are the hydrogen-abstracted alkyl species centered at C5' and C5, respectively, while the alkoxy radicals R3 and R4 have the unpaired electron localized mainly at O2 and O4'. Interestingly, the DFT study on R4 demonstrates that the trapping of a transferred proton can significantly influence the conformation of a deprotonated cation. Comparison of these results with those obtained from sucrose single crystals X-irradiated at 10 K indicates that the carbon situated next to the ring oxygen and connected to the CH(2)OH hydroxymethyl group is a better radical trapping site than other positions.     

The ESR study of reactivity of perfluoroacetyldiisopropylmethyl radical

The kinetics of hydrogen abstraction from hydrocarbons by the air-stable perfluoroacetyldiisopropylmethyl radical was studied by ESR, and a reaction mechanism was proposed. The degree of delocalization of the unpaired electron in the model C(5)F₃−C(1)·[C(2)(O(4))C(3)F₃]C(6)F₃ radical was calculated by the MNDO/PM3 method in the UHF approximation. For the conformation in which the CO group lies in the plane passing through the C(1), C(2), and C(5) atoms, the electron density on the O atom is 0.22. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1954–1957, November, 1997.     

Radiation products at 77 K in trehalose single crystals: EMR and DFT analysis

The radicals obtained in trehalose dihydrate single crystals after 77 K X-irradiation have been investigated at the same temperature using X-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EIE) techniques. Five proton hyperfine coupling tensors were unambiguously determined from the ENDOR measurements and assigned to three carbon-centered radical species (T1, T1*, and T2) based on the EIE spectra. EPR angular variations revealed the presence of four additional alkoxy radical species (T3 to T6) and allowed determination of their g tensors. Using periodic density functional theory (DFT) calculations, T1/T1*, T2, and T3 were identified as H-loss species centered at C4, C1', and O2', respectively. The T4 radical is proposed to have the unpaired electron at O4, but considerable discrepancies between experimental and calculated HFC values indicate it is not simply the (net) H-loss species. No suitable models were found for T5 and T6. These exhibit a markedly larger g anisotropy than T3 and T4, which were not reproduced by any of our DFT calculations.     

Hydrogen abstraction from deoxyribose by a neighbouring uracil-5-yl radical

Hydrogen abstraction from the C1' and C2' positions of deoxyadenosine by a neighbouring uracil-5-yl radical in the 5'-AU*-3' DNA sequence is explored using DFT. This hydrogen abstraction is the first step in a sequence leading to single or double strand break in DNA. The uracil-5-yl radical can be the result of photolysis or low-energy electron (LEE) attachment. If the radical is produced by photolysis the neighbouring adenine will become a cation radical and if it is produced by LEE the adenine will remain neutral. The hydrogen abstraction reactions for both cases were investigated. It is concluded that it is possible for the uracil-5-yl to abstract hydrogen from C1' and C2'. When adenine is neutral there is a preference for the C1' site and when the adenine is a radical cation the C2' site is the preferred. If adenine is positively charged, the rate-limiting step when abstracting hydrogen from C1' is the formation of an intermediate crosslink between uracil and adenine. This crosslink might be avoided in dsDNA, making C1' the preferred site for abstraction.     

Synthesis of isotopically labeled arachidonic acids to probe the reaction mechanism of prostaglandin H synthase

Prostaglandin H synthase (PGHS) catalyzes the conversion of arachidonic acid to prostaglandin G(2) in the cyclooxygenase reaction. The first step of the mechanism has been proposed to involve abstraction of the pro-S hydrogen atom from C13 to generate a pentadienyl radical spanning C11-C15. We report here the synthesis of six site-specifically deuterated arachidonic acids to investigate the structure of the radical intermediate. The preparation of these compounds was achieved using a divergent scheme that involved one advanced intermediate for all targets. The synthetic design introduced the label late in the routes and allowed the utilization of common synthetic intermediates in the preparation of various targets. Both 13(R)- and 13(S)-deuterium-labeled arachidonic acids were synthesized in high enantiomeric purity as deduced from soybean lipoxygenase assays and mass spectrometric analysis of the resulting enzymatic products. Each synthetic compound was reacted under anaerobic conditions with the wide singlet tyrosyl radical of PGHS-2 to generate a radical intermediate that was analyzed by EPR. Deuterium substitution at positions 11, 13(S), and 15 resulted in the loss of one hyperfine interaction, indicating that the protons at these positions interact with the unpaired electron. Simulation of the spectra was achieved with one set of parameters that are consistent with the assignment of a pentadienyl radical. Use of 16-[(2)H(2)]-arachidonic acid indicated that only one of the protons at C16 gives rise to a strong hyperfine interaction. The findings are discussed in the context of two proposed mechanisms for the cyclooxygenase reaction.     

An EPR study of γ-irradiated single crystals of cholesta-4, 6-diene-3-one

Cholesta-4, 6-diene-3-one single crystals irradiated with γ-rays at room temperature have been investigated by electron spin resonance. EPR spectra at room temperature exhibit a characteristics triplet which splits into two doublets. The main triplet has been interpreted as being caused by the addition of a hydrogen atom to the 7-position of the molecule, leaving an unpaired electron in the 2p₂ orbital of the carbon atoms in position 6 and 4. The hyperfine spectrum is generated by interaction of the unpaired electron with two equivalent -protons in position 4 and 6 and with tow non-equivalent β-protons in position 7. The principal values of the hydrogen hyperfine tensors are determined together with the g tensor of this radical.     

An EPR analysis of β-dimerization in α-blocked pyrroles in oxidant conditions

Electron paramagnetic resonance (EPR) analysis of neutral and acidic solutions of 2,5-dimethyl-1-phenylpyrrol (1) and meta-, para-, and ortho-bis(2,5-dimethylpyrrol-1-yl)benzenes (4-6) in the presence of Tl(III) trifluoroacetate as oxidant reveals the poor stability of their generated monomeric radical cations which dimerize through C(β)-C(β) bond formation. EPR spectra of the monomeric radical cations 4(?+) , 5(?+) , and 6(?+) coincide with that of 1(?+) , suggesting that the unpaired electron in these charged species is confined in one of the pyrrolic rings. The very twisted angles between pyrrolic and phenyl planes due to steric hindrance in the X-ray analysis of the molecular structure of 4 confirm the absence of extended conjugation in the π-system.     

ESR investigation of a stable trapped hydrogen atom in X-ray-irradiated beta-tricalcium phosphate at room temperature

A stable trapped hydrogen atom in X-ray-irradiated beta-tricalcium phosphate (beta-Ca3(PO4)2, beta-TCP) was successfully detected at room temperature. This hydrogen atom is stable at ambient temperature for several months. Hyperfine structure of the hydrogen atom and superhyperfine structures of the two phosphorus atoms were observed by means of electron spin resonance (ESR) spectroscopy. Electron spin-echo (ESE) of the hydrogen atom was observed in X-ray-irradiated beta-TCP. At room temperature, relaxation times of the hydrogen atom in X-ray-irradiated beta-TCP were very long (phase memory time TM = 19.4 mus, spin-lattice relaxation time T1 = 75.8 mus) compared with those of usual paramagnetic species. The most important facts are the detections of ESE and electron spin-echo envelope modulation (ESEEM) at room temperature. At room temperature, the observations of ESE and ESEEM and the estimations for the relaxation times (TM, T1) of the hydrogen atom were carried out for the first time until now. TM was able to be measured from room temperature to 9 K. The short relaxation time TM below 20 K might be explained by the quantum tunneling effect of the hydrogen atom. Fourier transformation of the electron spin-echo envelope modulation (FT-ESEEM) at room temperature suggests the overlapping of the wave functions between the hydrogen atom and the two phosphorus atoms. The site of the hydrogen atom in the X-ray-irradiated beta-TCP was discussed on the basis of the continuous wave ESR (CW-ESR) and pulse-ESR analyses.     

Reactivity of substituted charged phenyl radicals toward components of nucleic acids

Reactions of differently substituted phenyl radicals with components of nucleic acids have been investigated in the gas phase. A positively charged group located meta with respect to the radical site was employed to allow manipulation of the radicals in a Fourier-transform ion cyclotron resonance mass spectrometer. All of these electrophilic radicals react with sugars via exclusive hydrogen atom abstraction, with adenine and uracil almost exclusively via addition (likely at the C8 and C5 carbons, respectively), and with the nucleoside thymidine by hydrogen atom abstraction and addition at C5 in the base moiety (followed by elimination of (*)CH(3)). These findings parallel the reactivity of the phenyl radical with components of nucleic acids in solution, except that the selectivity for addition is different. Like HO(*), the electrophilic charged phenyl radicals appear to favor addition to the C5-end of the C5-C6 double bond of thymine and thymidine, whereas the phenyl radical preferentially adds to C6. The charged phenyl radicals do not predominantly add to thymine, as the neutral phenyl radical and HO(*), but mainly react by hydrogen atom abstraction from the methyl group (some addition to C5 in the base followed by loss of (*)CH(3) also occurs). Adenine appears to be the preferred target among the nucleobases, while uracil is the least favored. A systematic increase in the electrophilicity of the radicals by modification of the radicals' structures was found to facilitate all reactions, but the addition even more than hydrogen atom abstraction. Therefore, the least reactive radicals are most selective toward hydrogen atom abstraction, while the most reactive radicals also efficiently add to the base. Traditional enthalpy arguments do not rationalize the rate variations. Instead, the rates reflect the radicals' electron affinities used as a measure for their ability to polarize the transition state of each reaction.     

Crystal environments probed by EPR spectroscopy. Variations in the EPR spectra of Co(II)(octaethylporphyrin) doped in crystalline diamagnetic hosts and a reassessment of the electronic structure of four-coordinate cobalt(II)

The powder and single-crystal EPR spectra of Co(II)(OEP) (OEP is the dianion of octaethylporphyrin) doped into a range of diamagnetic crystals including simple four-coordinate hosts, H(2)(OEP), the triclinic B form of Ni(II)(OEP), the tetragonal form of Ni(II)(OEP) and Zn(II)(OEP); five-coordinate hosts, micro-dioxane)[Zn(II)(OEP)](2) and (py)Zn(II)(OEP); six-coordinate hosts, (py)(2)Zn(II)(OEP) and (py)(2)Mg(II)(OEP); and hosts containing fullerenes, C(60).2Zn(II)(OEP).CHCl(3), C(70).Ni(II)(OEP).C(6)H(6).CHCl(3), and C(60).Ni(II)(OEP).2C(6)H(6) have been obtained and analyzed. Spectra were simulated using a program that employed the exact diagonalization of the 16 x 16 complex spin Hamiltonian matrix. The EPR spectra of these doped samples are very sensitive to the environment within each crystal with the crystallographic site symmetry determining whether axial or rhombic resonance patterns are observed. For Co(II)(OEP) doped into tetragonal Ni(II)(OEP) (which displays a very large g( perpendicular ) of 3.405 and a very small g( parallel ) of 1.544) and several other crystals containing four-coordinate metal sites, the g components could not be fit using existing theory with the assumption of the usual z(2) ground state. However, reasonable agreement of the observed EPR parameters could be obtained by assuming that the unpaired electron resides in an xy orbital in the four-coordinate complexes.     

Suicide inactivation of dioldehydratase by glycolaldehyde and chloroacetaldehyde: an examination of the reaction mechanism

High-level ab initio calculations have been used to study the mechanism for the inactivation of diol dehydratase (DDH) by glycolaldehyde or 2-chloroacetaldehyde. As in the case of the catalytic substrates of DDH, e.g., ethane-1,2-diol, the 5'-deoxyadenosyl radical (Ado*) is able to abstract a hydrogen atom from both substrate analogues in the initial step on the reaction pathway, as evidenced by comparable energy barriers. However, in subsequent step(s), each substrate analogue produces the highly stable glycolaldehyde radical. The barrier for hydrogen atom reabstraction by the glycolaldehyde radical is calculated to be too high ( approximately 110 kJ mol-1) to allow Ado* to be regenerated and recombine with the cob(II)alamin radical, the latter therefore remaining tightly bound to DDH. As a consequence, the catalytic pathway is disrupted, and DDH becomes an impotent enzyme. Interconversion of equivalent structures of the glycolaldehyde radical via the symmetrical cis-ethanesemidione radical is calculated to require 38 kJ mol-1. EPR indications of a symmetrical cis-ethanesemidione structure are likely to be the result of formation of an equilibrium mixture of glycolaldehyde radical structures, this equilibration being facilitated by partial deprotonation of the glycolaldehyde radical by the carboxylate of an amino acid residue within the active site of DDH.     

An Isolable Diphosphene Radical Cation Stabilized by Three‐Center Three‐Electron π‐Bonding with Chromium: End‐On versus Side‐On Coordination

Two salts ( 2 and 4 ) containing the radical cations of complexed diphosphenes have been isolated and characterized by electron paramagnetic resonance (EPR) spectroscopy, IR spectroscopy, and single‐crystal X‐ray diffraction. The P?P bond is coordinated to the Cr center either in an end‐on (in 2 ) or a side‐on (in 4 ) fashion. The spin density of the radical is delocalized over the Cr atom and the two P atoms in 2 whereas the unpaired electron is mainly localized on the Cr atom in 4 . This work provides the first example of a complexed diphosphene radical ( 2 ) featuring novel three‐center three‐electron (3c‐3e) π‐bonding in the Cr‐P‐P unit, and the first example of a 17 e Cr radical with a side‐on π‐bonded ligand ( 4 ).