Carbinolamine Intermediate Research Articles

Iron-catalyzed reductive strecker reaction

Strecker reaction is widely applied for the synthesis of amino acids from aldehydes, amines and cyanides. Herein, we report the FeI2-catalyzed reductive Strecker type reaction of formamides instead of aldehydes to produce amino acetonitriles. The challenging capture of carbinolamine intermediates by CN− was achieved via Fe catalysis. This approach afforded better yields than the use of Ir- or Rh-catalysts. The application ability of this methodology is demonstrated by 1) one-pot construction of (13C labeled) complex molecules from CO2via amino acetonitrile intermediates and 2) convenient production of homologated carboxylic acids from aldehydes.

The role of carboxylic acid impurity in the mechanism of the formation of aldimines in aprotic solvents

Aldimines can be readily formed in aprotic solvents from the reaction of amines with aldehydes. However, the true reaction mechanism has not be fully elucidated yet. In this work, the mechanism of methylamine addition to acetaldehyde catalyzed by a second amine, water molecules and the carbinolamine intermediate was investigated by reliable theoretical methods. Although all of these species were able to catalyze this reaction via proton shuttle mechanism, none of them have led to ΔG‡ compatible with experimental data. An additional species presents in the reaction mixture as impurity, acetic acid, which is formed by the contact of acetaldehyde with the oxygen of the air, was also investigated as a catalyst. The calculation of the free energy profile and a microkinetic analysis indicated that a small concentration of acetic acid present in solution is able to promote a very effective catalyst, leading to reaction kinetics compatible with experimental data. Thus, the present study suggest that trace amount of carboxylic acids present in aldehydes catalyze this kind of reaction.

Coupling and decarboxylation mechanism of oxaloacetic acid and ethylenediamine: A theoretical investigation

AbstractThe decarboxylation mechanism of deprotonated oxaloacetate at pH = 8.0 in aid of protonated ethylenediamine was investigated systematically by full optimization at M06‐2X/6‐311++G(d,p) level combined with the CPCM solvation model to consider the effect of bulk water, where the roles of the carbinolamine and imine intermediates were elucidated. In the minimum energy path, the NH3+ group binds to the β‐carboxyl group of oxaloacetate via a hydrogen bond, and the amino group as both a nucleophile and an electrophile connects to the CO group by a hydrogen transfer process with a free‐energy barrier of 131.9 kJ/mol. Then the carbinolamine intermediate is dehydrated to form an imine with a total barrier of 164.2 kJ/mol, which is the rate‐limiting step in this energetically most favorable channel. After a proton transfer process, the β‐decarboxylation barrier is only 49.6 kJ/mol.

ChemInform Abstract: Aza‐Conjugate Addition Methodology for the Synthesis of N‐Hydroxy‐isoindolin‐1‐ones.

AbstractAryl aldehydes containing an ortho‐propiolate or propiolamide moiety are efficiently cyclized to N‐hydroxyisoindoline‐1‐ones, which are subsequently converted into the parent isoindoline‐1‐ones.

Aza-Conjugate Addition Methodology for the Synthesis of N-Hydroxy-isoindolin-1-ones.

Aryl-aldehydes containing ortho-substituted propiolate fragments react with hydroxylamine to afford carbinolamine intermediates that undergo intramolecular aza-conjugate addition reactions to afford N-hydroxy-2.3-dihydro-isoindolin-1-ones that can be reduced to their corresponding isoindolin-1-ones and isoindoles.

Direct detection of aromatic amines and observation of intermediates of Schiff-base reactions by reactive desorption electrospray ionization mass spectrometry

The fast, sensitive detection for 6 aromatic amines and direct observation of intermediates of Schiff-base reactions were achieved through reactive desorption electrospray ionization mass spectrometry (reactive DESI-MS). Pure acetone was electrosprayed to impact the aromatic amines on the paper, allowing two reactions, a proton-transfer reaction and a Schiff-base reaction, to occur in these experiments. The former was used to detect the aromatic amines, and the protonated analytes generated were selected for the qualitative and quantitative analysis. The false-positive signals were excluded by tandem mass spectrometry. For the 6 analytes, linear signal responses were obtained, and each had a dynamic range of 5 orders of magnitude. The relative standard deviation (RSD) and limit of detection (LOD) for all the measurements were in 1.5–5.3% and 0.03–0.2pg/mm2 range, respectively. The latter is a nucleophilic addition reaction that occurred between the aromatic amines and the acetone. The carbinolamine intermediates of these reactions were directly detected and identified by reactive DESI-MS. The data show that reactive DESI-MS is not only a reliable, sensitive tool for chemical analysis, but also a valid and promising method for studying organic reactions simultaneously, especially for heterogeneous reactions that occurred at a solution/solid interface.

Non-steady state intermediates: a re-examination of the kinetics of hydrolysis of N-methylisobutylidene and N-isopropylethylidene under acidic conditions

Direct measurement by stopped flow and UV–vis spectroscopy of the decay of two protonated imines, N-methylisobutylidene and N-isopropylethylidene, below a pH 3 resulted in a biphasic kinetic profile involving a rapid decay of the iminium ion to a carbinolamine intermediate. Previously published hydrolysis studies on N-methylisobutylidene did not report observation of non-first order kinetics. Spectroscopic evidence of the carbinolamine intermediate of N-methylisobutylidene was observed by 1H NMR. The rate constants for the pH independent step, k1, were similar for both compounds, 0.94s−1 for N-methylisobutylidene versus 0.87s−1 for N-isopropylethylidene. Following k1, the rate constants, k−1 and k±Ka(OH) differed by a factor of approximately 3 with the carbinolamine intermediate of N-isopropylethylidene being more stable over the carbinolamine of N-methylisobutylidene. An in-depth examination of the rate constants is presented.

Glutamates 78 and 122 in the Active Site of Saccharopine Dehydrogenase Contribute to Reactant Binding and Modulate the Basicity of the Acid-Base Catalysts

Saccharopine dehydrogenase catalyzes the NAD-dependent oxidative deamination of saccharopine to give l-lysine and alpha-ketoglutarate. There are a number of conserved hydrophilic, ionizable residues in the active site, all of which must be important to the overall reaction. In an attempt to determine the contribution to binding and rate enhancement of each of the residues in the active site, mutations at each residue are being made, and double mutants are being made to estimate the interrelationship between residues. Here, we report the effects of mutations of active site glutamate residues, Glu(78) and Glu(122), on reactant binding and catalysis. Site-directed mutagenesis was used to generate E78Q, E122Q, E78Q/E122Q, E78A, E122A, and E78A/E122A mutant enzymes. Mutation of these residues increases the positive charge of the active site and is expected to affect the pK(a) values of the catalytic groups. Each mutant enzyme was completely characterized with respect to its kinetic and chemical mechanism. The kinetic mechanism remains the same as that of wild type enzymes for all of the mutant enzymes, with the exception of E78A, which exhibits binding of alpha-ketoglutarate to E and E.NADH. Large changes in V/K(Lys), but not V, suggest that Glu(78) and Glu(122) contribute binding energy for lysine. Shifts of more than a pH unit to higher and lower pH of the pK(a) values observed in the V/K(Lys) pH-rate profile of the mutant enzymes suggests that the presence of Glu(78) and Glu(122) modulates the basicity of the catalytic groups.

Insights on Co-Catalyst-Promoted Enamine Formation between Dimethylamine and Propanal through Ab Initio and Density Functional Theory Study

The mechanistic details on enamine formation between dimethylamine and propanal are unraveled using the ab initio and density functional theory methods. The addition of secondary amine to the electrophile and simultaneous proton transfer results in a carbinolamine intermediate, which subsequently undergoes dehydration to form enamine. The direct addition of amine as well as the dehydration of the resulting carbinolamine intermediate is predicted to possess fairly high activation barrier implying that a unimolecular process is unlikely to be responsible for enamine formation. Different models are therefore proposed which could explain the relative ease of enamine formation under neat condition as well as under the influence of methanol as the co-catalyst. The explicit inclusion of either the reagent or the co-catalyst is considered in the transition states as stabilizing agents. The participation of the reagent or the co-catalyst as a monofunctional ancillary species is found to stabilize the transition states relative to the unassisted or the direct addition/dehydration pathways. The reduction in enthalpy of activation is found to be much more dramatic when two co-catalysts participate in an active bifunctional mode in the rate-determining dehydration step. The transition structures exhibited characteristic features of a relay proton transfer mechanism. The free energy of activation associated with the two methanol-assisted pathway is found to be 16.7 kcal/mol lower than that of the unassisted pathway. The results are found to be in concurrence with the available reports on the rate acceleration by co-catalysts in the Michael reaction between enamine and methyl vinyl ketone under neat conditions.

Equilibria of formation and dehydration of the carbinolamine intermediate in the reaction of benzaldehyde with hydrazine

Measurement of polarographic limiting currents at equilibria made it possible at pH 3–7 to simultaneously determine concentrations of benzaldehyde, of its hydrazone and of the carbinolamine derivative. The dependence of concentration of carbinolamine at equilibrium on pH indicated presence of its di-, mono-, and unprotonated forms. Acid dissociation constants of the formation (p K a 1≈3.2) of the diprotonated form and of the dissociation of the monoprotonated form of carbinolamine (p K a 2≈4.7) were estimated. The equilibrium constants of formation ( K 1) and dehydration ( K 2) of the carbinolamine intermediate were determined.

Cytochrome P450-Catalyzed Oxidation of N-Benzyl-N-cyclopropylamine Generates Both Cyclopropanone Hydrate and 3-Hydroxypropionaldehyde via Hydrogen Abstraction, Not Single Electron Transfer

The suicide substrate activity of N-benzyl-N-cyclopropylamine (1) and N-benzyl-N-(1'-methylcyclopropyl)amine (2) toward cytochrome P450 and other enzymes has been explained by a mechanism involving single electron transfer (SET) oxidation, followed by ring-opening of the aminium radical cation (protonated aminyl radical) and reaction with the P450 active site. Although the SET oxidation of N-cyclopropyl-N-methylaniline (3) by horseradish peroxidase leads exclusively to ring-opened (non-cyclopropyl) products, P450 oxidation of 3 leads to formation of cyclopropanone hydrate and no ring-opened products, and 3 does not inactivate P450. To help reconcile these discrepant behaviors we have determined the complete metabolic fate of 1 with P450 in vitro. 3-Hydroxypropionaldehyde (3HP), the presumptive "signature metabolite" for SET oxidation of a cyclopropylamine, was observed for the first time in 57% yield, along with cyclopropanone hydrate (34%), cyclopropylamine (9%), benzaldehyde (6%), benzyl alcohol (12%), and benzaldoxime (19%). Unexpectedly, N-benzyl-N-cyclopropyl-N-methylamine (4) was found not to inactivate P450 and not to give rise to 3HP as a metabolite without first undergoing oxidative N-demethylation to 1. These and other observations argue against a role for SET mechanisms in the P450 oxidation of cyclopropylamines. We suggest that a conventional hydrogen abstraction/hydroxyl recombination mechanism (or its equivalent as a one-step "insertion" mechanism) at C-H bonds in 1-4 leads to nonrearranged carbinolamine intermediates and thereby to "ordinary" N-dealkylation products including cyclopropanone hydrate. Alternatively, hydrogen abstraction at the N-H bond of secondary cyclopropylamines 1 gives a neutral aminyl radical which could undergo rapid ring-opening leading either to enzyme inactivation or 3HP formation.

Mechanism of the Class I KDPG aldolase

In vivo, 2-keto-3-deoxy-6-phosphogluconate (KDPG) aldolase catalyzes the reversible, stereospecific retro-aldol cleavage of KDPG to pyruvate and d-glyceraldehyde-3-phosphate. The enzyme is a lysine-dependent (Class I) aldolase that functions through the intermediacy of a Schiff base. Here, we propose a mechanism for this enzyme based on crystallographic studies of wild-type and mutant aldolases. The three dimensional structure of KDPG aldolase from the thermophile Thermotoga maritima was determined to 1.9 Å. The structure is the standard α/β barrel observed for all Class I aldolases. At the active site Lys we observe clear density for a pyruvate Schiff base. Density for a sulfate ion bound in a conserved cluster of residues close to the Schiff base is also observed. We have also determined the structure of a mutant of Escherichia coli KDPG aldolase in which the proposed general acid/base catalyst has been removed (E45N). One subunit of the trimer contains density suggesting a trapped pyruvate carbinolamine intermediate. All three subunits contain a phosphate ion bound in a location effectively identical to that of the sulfate ion bound in the T. maritima enzyme. The sulfate and phosphate ions experimentally locate the putative phosphate binding site of the aldolase and, together with the position of the bound pyruvate, facilitate construction of a model for the full-length KDPG substrate complex. The model requires only minimal positional adjustments of the experimentally determined covalent intermediate and bound anion to accommodate full-length substrate. The model identifies the key catalytic residues of the protein and suggests important roles for two observable water molecules. The first water molecule remains bound to the enzyme during the entire catalytic cycle, shuttling protons between the catalytic glutamate and the substrate. The second water molecule arises from dehydration of the carbinolamine and serves as the nucleophilic water during hydrolysis of the enzyme-product Schiff base. The second water molecule may also mediate the base-catalyzed enolization required to form the carbon nucleophile, again bridging to the catalytic glutamate. Many aspects of this mechanism are observed in other Class I aldolases and suggest a mechanistically and, perhaps, evolutionarily related family of aldolases distinct from the N-acetylneuraminate lyase (NAL) family.

Dioxododecenoic Acid: A Lipid Hydroperoxide-Derived Bifunctional Electrophile Responsible for Etheno DNA Adduct Formation

It has been proposed that 13(S)-hydroperoxy-9Z,11E-octadecadienoic acid [13(S)-HPODE]-mediated formation of 4-oxo-2(E)-nonenal and 4-hydroxy-2(E)-nonenal arises from a Hock rearrangement. This suggested that a 4-oxo-2(E)-nonenal-related molecule, 9,12-dioxo-10(E)-dodecenoic acid (DODE), could also result from the intermediate formation of 9-hydroperoxy-12-oxo-10(E)-dodecenoic acid. A recent report has described the formation of DODE-derived etheno adducts when 13(S)-HPODE was allowed to decompose in the presence of 2'-deoxynucleosides or DNA. However, the regioselectivity of lipid hydroperoxide-derived DODE addition to 2'-deoxyguanosine (dGuo) or other 2'-deoxynucleosides was not determined. The structure of carboxynonanone-etheno-dGuo formed from vitamin C-mediated 13(S)-HPODE decomposition has now been established by a combination of 1H and 13C NMR spectroscopy studies of its bis-methylated derivative. The site of dGuo methylation was first established as being at N-5 rather than at O-9 from NMR analysis of a methyl derivative of the model compound, heptanone-etheno-dGuo. (1)H,(13)C 2D heteronuclear multiple bond correlations were then used to establish unequivocally that the bis-methyl derivative of carboxynonanone-etheno-dGuo was 3-(2'-deoxy-beta-d-erythropentafuranosyl)imidazo-7-(9' '-carboxymethylnona-2' '-one)-9-oxo-5-N-methyl[1,2-a]purine rather than its 6-(9' '-carboxymethylnona-2"-one)-9-oxo-5-N-methyl[1,2-a]purine regioisomer. Therefore, etheno adduct formation occurred by initial nucleophilic attack of the exocyclic N(2) amino group of dGuo at the C-12 aldehyde of DODE to form an unstable carbinolamine intermediate. This was followed by intramolecular Michael addition of the pyrimidine N1 of dGuo to C-11 of the resulting alpha,beta-unsaturated ketone. Subsequent dehydration gave 3-(2'-deoxy-beta-d-erythropentafuranosyl)imidazo-7-(9' '-carboxynona-2' '-one)-9-oxo-[1,2-a]purine (carboxynonanone-etheno-dGuo). An efficient synthesis of DODE was developed starting from readily available 1,8-octanediol using a furan homologation procedure. This synthetic method allowed multigram quantities of DODE to be readily prepared. Synthetic DODE when reacted with dGuo gave carboxynonanone-etheno-dGuo that was identical with that derived from vitamin C-mediated 13(S)-HPODE decomposition in the presence of dGuo.

Regioselectivity for Condensation Reactions of Quinonoid Models of Tryptophan Tryptophylquinone: A Density Functional Theory Study.

The model compounds of tryptophan tryptophylquinone (TTQ), o-benzoquinone (OBQ), 3-methyl-6,7-dihydro-1H-6,7-indoledione (MIQ), and 3-methyl-4-(3-methyl-1H-2-indolyl)-6,7-dihydro-1H-6,7-indoledione (IIQ), all of which are characteristic of o-quinone groups, have been studied with density functional theory. The dihedral angle of the two indole rings (chi) of IIQ is calculated to be 49.6 degrees for the global minimum. Another local minimum, 0.74 kcal/mol higher in energy, with a chi value of 123.5 degrees is also fully optimized. The transition state connecting the two minima, with a chi value of 97.9 degrees, has been located and the rotation barrier is 1.71 kcal/mol. A scan of the potential energy surface along this dihedral angle showed that the difference of the total energy was within 1.0 kcal/mol at a range of the dihedral angle from 30 degrees to 75 degrees. Hence, IIQ is flexible for the rotation of inter-indole rings. The origin of regioselectivity for the condensation reactions of the models MIQ and IIQ with NH(3) has been elucidated. It is shown that the energy difference between the two different types of carbinolamine intermediates (Delta E) and their corresponding transition structures (Delta E(++)) should be responsible for the regioselectivity. To assess the effect of the fused ring on regioselectivity of the condensation reaction, a series of models were designed. A good linear correlation has been found between the energy difference of the two different carbinolamine intermediates (Delta E) and that of the corresponding transition states (Delta E(++)), suggesting that the factors that stabilize the carbinolamine intermediate also favor the stability of the corresponding transition structure. The pair, 6-amino-6-hydroxy-8-methyl-6H-quinolin-5-one and 5-amino-5-hydroxy-8-methyl-5H-quinolin-6-one (7/8), deviates from the correlation and represents some anomalous behavior, which may be due to their structural particularity. It also has been shown that the tricyclic models, which consist of OBQ and two fused heterocyclic rings, represent more regioselectivity in contrast to the bicyclic systems. Moreover, the fused electron-donating pyrrole and the fused electron-withdrawing pyridine or pyrimidine show a somewhat synergistic effect on each other via the medial OBQ molecule. The barrier of the condensation reaction for pyrrolo[2,3-f]quinoline-4,5-dione is calculated to be ca. 22 kcal/mol. This is lower than that for MIQ (ca. 33 kcal/mol) and IIQ (ca. 32 kcal/mol) by as much as 10.0 kcal/mol, explaining reasonably the larger catalytic effect of pyrroloquinolinequinone (PQQ) relative to TTQ.

Regioselectivity for condensation reactions of quinonoid models of tryptophan tryptophylquinone: a density functional theory study.

The model compounds of tryptophan tryptophylquinone (TTQ), o-benzoquinone (OBQ), 3-methyl-6,7-dihydro-1H-6,7-indoledione (MIQ), and 3-methyl-4-(3-methyl-1H-2-indolyl)-6,7-dihydro-1H-6,7-indoledione (IIQ), all of which are characteristic of o-quinone groups, have been studied with density functional theory. The dihedral angle of the two indole rings (chi) of IIQ is calculated to be 49.6 degrees for the global minimum. Another local minimum, 0.74 kcal/mol higher in energy, with a chi value of 123.5 degrees is also fully optimized. The transition state connecting the two minima, with a chi value of 97.9 degrees, has been located and the rotation barrier is 1.71 kcal/mol. A scan of the potential energy surface along this dihedral angle showed that the difference of the total energy was within 1.0 kcal/mol at a range of the dihedral angle from 30 degrees to 75 degrees. Hence, IIQ is flexible for the rotation of inter-indole rings. The origin of regioselectivity for the condensation reactions of the models MIQ and IIQ with NH(3) has been elucidated. It is shown that the energy difference between the two different types of carbinolamine intermediates (Delta E) and their corresponding transition structures (Delta E(++)) should be responsible for the regioselectivity. To assess the effect of the fused ring on regioselectivity of the condensation reaction, a series of models were designed. A good linear correlation has been found between the energy difference of the two different carbinolamine intermediates (Delta E) and that of the corresponding transition states (Delta E(++)), suggesting that the factors that stabilize the carbinolamine intermediate also favor the stability of the corresponding transition structure. The pair, 6-amino-6-hydroxy-8-methyl-6H-quinolin-5-one and 5-amino-5-hydroxy-8-methyl-5H-quinolin-6-one (7/8), deviates from the correlation and represents some anomalous behavior, which may be due to their structural particularity. It also has been shown that the tricyclic models, which consist of OBQ and two fused heterocyclic rings, represent more regioselectivity in contrast to the bicyclic systems. Moreover, the fused electron-donating pyrrole and the fused electron-withdrawing pyridine or pyrimidine show a somewhat synergistic effect on each other via the medial OBQ molecule. The barrier of the condensation reaction for pyrrolo[2,3-f]quinoline-4,5-dione is calculated to be ca. 22 kcal/mol. This is lower than that for MIQ (ca. 33 kcal/mol) and IIQ (ca. 32 kcal/mol) by as much as 10.0 kcal/mol, explaining reasonably the larger catalytic effect of pyrroloquinolinequinone (PQQ) relative to TTQ.

Polydentate ligand construction: a re-examination of an intramolecular condensation reactionElectronic supplementary information (ESI) available: crystal data tables. See http://www.rsc.org/suppdata/dt/b1/b104976n/

The intramolecular condensation reaction of coordinated aminoacetaldehyde and 1,4,7,10-tetraazadecane (trien) has been reinvestigated. The structure of the product, an imine-containing cobalt(III) complex, has been corrected, and this, together with the isolation and characterisation of carbinolamine intermediates, has allowed a modified mechanism to be proposed for the reaction. The isolated carbinolamines result from condensation of the aldehyde with the most acidic coordinated amine (trans to the chloride ligand). The carbinolamines are unable to dehydrate to form an imine product until after they have undergone a rearrangement of the coordination sphere to a geometry that will allow a planar imine to form.

The biotransformation of nitrogen containing xenobiotics to lactams.

The metabolism of nitrogen heterocyclics may lead to lactam formation. In early studies on xenobiotic metabolism lactams were identified as metabolites of nicotine, cyproheptadine, tremorine and prolintane. Now, because of the increasing availability of powerful analytical techniques, there are many instances of lactams being identified as metabolites. Lactam metabolites are formed from either iminium ions or carbinolamines. These two intermediates may have distinct mechanisms of formation but they can interconvert. There is evidence that the iminium ions are oxidized to lactams by aldehyde oxidases (cytosolic molybdenum hydroxylases). The tissue distribution and enzyme activities of aldehyde oxidase have been studied in several animal species. However, it is also known that iminium ions can undergo spontaneous hydrolysis to the corresponding carbinolamine. If the latter is stable it may undergo oxidation by cytochrome P-450 to form the lactam. Thus, species differences in lactam formation might be caused by differences in the concentrations of either cytochrome P450 isozymes or aldehyde oxidases. It appears that lactam formation is an end stage in the metabolism of N-heterocycles in that it is unlikely that the lactam will undergo hydrolysis to the corresponding amino acid. Such amino acids probably arise from the amino aldehydes that may be produced from ring opening of unstable carbinolamine intermediates. When microsomal preparations are incubated with the appropriate substrate in the presence of sodium cyanide the iminium ion may be trapped to produce a cyano compound. Such reactions have led to the proposal that iminium ions might react with nucleophilic sites of cellular macromolecules and so contribute to both the pharmacology and toxicology of N-heterocyclic compounds. Other pathways for the formation of lactam metabolites involve the internal cyclization of precursor metabolites, e.g. the self-condensation of an aldehyde group (formed during metabolism) with a neighboring amide group. However, spontaneous ring closures of amino acids to form lactams seem unlikely since it would be anticipated that the amino acid residue would exist as a stable zwitterion under physiological conditions. Thus, it is unlikely that lactams will undergo futile metabolism via hydrolytic ring opening followed by ring closure. Under extreme conditions such unanticipated ring closures may occur and the conditions of metabolite isolation may contribute to the occurrence of artifacts.

Tyrosinase-Catalyzed Oxidation of 3,4-Dihydroxyphenylglycine

The oxidation chemistry of dopa in relation to melanin biosynthesis has been extensively studied. However, the oxidation of its lower homolog viz., 3,4-dihydroxyphenylglycine has not been described. Using 3,4-dimethoxybenzaldehyde as the starting material, the chemical synthesis of 3,4-dihydroxyphenylglycine was accomplished by Strecker's synthesis and demethylation reactions. Tyrosinase readily oxidized this unusual amino acid to the expected quinone. The glycyl-o-benzoquinone thus formed was highly unstable like its higher analog, dopaquinone. However, unlike its counterpart, it failed to exhibit intramolecular cyclization reaction. Rather, glycyl-o-benzoquinone exhibited facile transformation(s) to ultimately generate 3,4-dihydroxybenzaldehyde as an isolatable product. A probable mechanism involving intermediary formation of unstable quinone methide and carbinolamine intermediates is proposed to account for the novel transformation of glycyl-o-benzoquinone to 3,4-dihydroxybenzaldehyde.

Aspartate aminotransferase complexed with erythro-beta-hydroxyaspartate: crystallographic and spectroscopic identification of the carbinolamine intermediate.

The crystal structure of mitochondrial aspartate aminotransferase (mAAT) of chicken complexed with erythro-beta-hydroxyaspartate has been determined at 2.4 A resolution. Pregrown crystals of mAAT complexed with the inhibitor maleate (closed enzyme conformation, orthorhombic space group C222(1)) were soaked in solutions of erythro-beta-hydroxyaspartate. The ligand exchange was monitored by microspectrophotometry. The active site turned out to be predominantly occupied by the carbinolamine intermediate. The carbinolamine is a true intermediate of the catalytic cycle forming the last covalently bound enzyme:substrate complex before release of the keto acid product. Occupancies of approximately 80% for the carbinolamine and of approximately 20% for the quinonoid intermediate were obtained. Two hydrogen bonds were identified that are potentially relevant for the accumulation of the carbinolamine intermediate: one to the hydroxyl group of Tyr 70* and the other to the epsilon-NH2 group of Lys 258.

Model Studies of TTQ-Containing Amine Dehydrogenases.

The reactions of a TTQ model compound [1, 3-methyl-4-(3'-methylindol-2'-yl)indole-6,7-dione] with several amines have been investigated in organic media to obtain mechanistic information on the action of quinoprotein methylamine and aromatic amine dehydrogenases. It has been found that compound 1 acts as an efficient catalyst for the autorecycling oxidation of benzylamine by molecular oxygen in CH(3)OH. In order to evaluate the oxidation mechanism of amines by 1, the product analyses and kinetic studies have been carried out under anaerobic conditions. In the first stage of the reaction of 1 with amines, 1 is converted into an iminoquinone-type adduct (so-called substrateimine), which was isolated and characterized by using cyclopropylamine as a substrate. The observed NOE of the isolated product indicates clearly that the addition position of the amine is C-6 of the quinone. The molecular orbital calculations suggest that the thermodynamic stability of the carbinolamine intermediate is a major factor to determine such regioselectivity; the C-6 carbinolamine is more stable than the C-7 counterpart by 2.9 kcal/mol. The reactivity of several primary amines and the electronic effect of the p-substituents of benzylamine derivatives in the iminoquinone formation suggest that the addition step of the amine to the quinone is rate-determining. When amines having an acidic alpha-proton such as benzylamine derivatives are employed as substrates, formation of the iminoquinone adduct was followed by rearrangement to the productimine. The kinetic analysis has revealed that this rearrangement consists of noncatalyzed and general base-catalyzed processes. Large kinetic isotope effects of 7.8 and 9.2 were observed for both the noncatalyzed and general base-catalyzed processes, respectively, since these steps involve a proton abstraction from the alpha-position of the substrate. In the reaction with benzhydrylamine, the product imine was isolated quantitatively and well characterized by several spectroscopic data. In the case of benzylamine, the product imine is further converted into the aminophenol derivative by the imine exchange reaction with excess benzylamine. These results indicate clearly that the amine oxidation by compound 1 proceeds via a transamination mechanism as suggested for the enzymatic oxidation of amines by TTQ cofactor.