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PubMed1998

Analytical strategies for amperometric biosensors based on chemically modified electrodes.

Various strategies based on the use of chemically modified electrodes for the development of amperometric biosensors are described. Particular emphasis is placed on materials capable of catalyzing the oxidation of NADH and coupling these with enzymatic activities for biosensor construction. In terms of electrocatalysts, the discussion will centre on electrodeposited films of 3,4-dihydroxy benzaldehyde (3,4-DHB) and related analogs as well as on electrodeposited films of transition metal complexes of 1,10-phenanthroline-5,6-dione (phen-dione). Electrodeposited films of these materials have been coupled to the enzymatic activity of aldehyde dehydrogenase and alcohol dehydrogenase for the development of biosensors for aldehydes and ethanol, respectively.

Alcohol Dehydrogenase↗
PubMed2002

Polymer-bound 1-aryl-3-alkyltriazenes as modular ligands for catalysis. Part 2: screening immobilized metal complexes for catalytic activity.

A range of polymer-supported triazenes and their metal-bound analogues were screened for use in catalysis. Fe, Cu and Zr complexes were, together with the polymer-supported triazene ligand alone, screened in the addition of Et(2)Zn to benzaldehyde. A supported Pd triazene complex was screened for activity in Suzuki and Sonogashira reactions and a supported Ru triazene complex screened for transfer hydrogenation.

Amines↗
PubMed2000

Triplet-state energies and substituent effects of excited aroyl compounds in the gas phase.

Triplet-state energy values obtained from the gas phase are still scarce. In this study, the triplet-state energies of 58 aroyl compounds, introduced as gas chromatographic peaks at atmospheric pressure and typically 473 K, have been determined from the 0-0 bands of their n --> pi* type phosphorescence spectra in excited nitrogen. Correlations of those gas-phase triplet-state energies with Hammett constants could be observed for substituted acetophenones, benzaldehydes and benzophenones.

Acetophenones↗
PubMed2006

A sensitive method for the quantification of acrolein and other volatile carbonyls in ambient air.

Acrolein, an unsaturated aldehyde found in both indoor and outdoor air, is considered one of the greatest noncancer health risks of all organic air pollutants. Current methods for determining acrolein often employ sorbent-filled cartridges containing a carbonyl derivatizing agent (e.g., dinitrophenylhydrazine). These methods are of limited use for unsaturated compounds due to the formation of unstable derivatives, coelution of similar compounds, long sample collection times, and ozone interferences that result in poor sensitivity, selectivity, and reproducibility. The goal of this research was to develop an analytical method for determining ppt concentrations of acrolein and other carbonyls in air with short sampling times (10 min). The method uses a mist chamber to collect carbonyls by forming water-soluble carbonyl-bisulfite adducts. The carbonyls are then liberated from the bisulfite, derivatized, and quantified by gas chromatography/electron capture negative ionization mass spectrometry. The method was applied to determine atmospheric acrolein concentrations at three sites in northern California reflecting hemispheric background concentrations, biogenic-dominated regions, and urban environments. The resulting acrolein concentrations were 0.056, 0.089, and 0.29 microg/m3, respectively, which are all above the EPA Reference Concentration of 0.02 microg/m3. The minimum detection limit of 0.012 microg/m3 is below that of other published methods. Methacrolein, methyl vinyl ketone, crotonaldehyde, glyoxal, methyl glyoxal, and benzaldehyde were also quantified.

Acrolein↗
PubMedDate not supplied

Incorporation of an aldehyde function in oligonucleotides.

A nucleotide-like phosphoramidite building block that has the nucleic base replaced by the tert-butyldimethylsilyl-protected styrene glycol was synthesized. After the automatic synthesis of an oligonucleotide incorporating this synthon, the benzaldehyde function was generated by fluoride deprotection and oxidation by sodium periodate. In a similar manner, an oligonucleotide where a nucleic base was replaced by the (CH2)8CH=O chain was synthesized and conjugated with biotin derivatives.

Aldehydes↗
PubMedDate not supplied

Preparation of 18F-human serum albumin: a simple and efficient protein labeling method with 18F using a hydrazone-formation method.

18F-labeling of proteins and peptides is important for positron emission tomography (PET). Although there are many methods for the labeling of proteins with (18)F, most of these are characterized by complicated procedures or low yields. Here, we report a novel and simple method which includes the preparation of [18F]fluorobenzaldehyde ([18F]FBA) and successive conjugation with hydrazinonicotinic acid-human serum albumin conjugate (HYNIC-HSA) via hydrazone formation. HYNIC-HSA, which can also be used for labeling with (99m)Tc, was prepared via reaction with N-hydroxysuccinimide (NHS) or tetrafluorophenyl (TFP) esters of HYNIC with HSA. No-carrier-added [18F]FBA was prepared by the nucleophilic substitution of [18F]fluoride to 4-trimethylammonium benzaldehyde triflate in the presence of tetrabutylammonium bicarbonate. [18F]FBA was purified by passing ion exchange cartridges (IC-H and QMA) and was adsorbed to a C18 Sep-Pak cartridge. The adsorbed [18F]FBA was then eluted with 50% ethanol. HYNIC-HSA was added to the solution and conjugated with [18F]FBA via hydrazone formation. 18F-HSA was purified with a PD10 column. Biodistribution of 18F-HSA, (99m)Tc-HSA, and [18F]FBA in mice were investigated at 10, 20, and 60 min after intravenous injection. The number of conjugated HYNIC molecules per HSA ranged from 5.2 to 23.2 depending on the reaction conditions. The labeling efficiency of 18F-FBA was 67 +/- 15.7%. The radiochemical purity after purification was over 99%. The conjugation efficiency of HYNIC-HSA with [18F]FBA was between 25% and 90%. The conjugation efficiency was observed to increase with increases in the number of conjugated HYNIC, the HYNIC-HSA concentration, or temperature. 18F-HSA exhibited a biodistribution pattern similar to that of (99m)Tc-HSA while [18F]FBA showed much lower blood activity than that of 18F-HSA and (99m)Tc-HSA. We concluded that 18F-HSA was successfully labeled using a novel method which involves hydrazone formation between [18F]FBA and HYNIC-HSA. This method can be applied to the 18F-labeling of other proteins or peptides.

Animals↗
PubMed1982

Proton release during the pre-steady-state oxidation of aldehydes by aldehyde dehydrogenase. Evidence for a rate-limiting conformational change.

A transient release of protons with an amplitude corresponding to one proton per active site has been observed for the oxidation of propionaldehyde, acetaldehyde, and benzaldehyde by sheep liver cytoplasmic aldehyde dehydrogenase at pH 7.6 with phenol red as indicator. At saturating substrate levels, the rate constants for the proton burst are in each case the same, and for acetaldehyde and propionaldehyde show the same dependence on the concentrations of the substrates, as the rate constants for the transient production of NADH reported previously [MacGibbon, A.K.H., Blackwell, L.F., & Buckley, P.D. (1977) Biochem. J. 167, 469-477]. Although, with propionaldehyde as a substrate, a full proton burst is also observed at pH 6.0, no proton burst is observed at pH 9.0. For 4-nitrobenzaldehyde, there is no burst in NADH production, but a burst in proton release is observed, showing that proton release precedes hydride transfer. No protons were released during the binding of the substrate analogues acetone and chloral hydrate nor on reaction of the enzyme with the inhibitor tetraethylthiuram disulfide (disulfiram). A model is proposed in which the rate-limiting step in the pre-steady-state phase of the reaction is a conformational change which occurs after the binding of aldehydes to the enzyme. As a result of the conformational change, the environment of a functional group on the enzyme, which initially has a pKa of about 8.5, is perturbed to give a final pKa value for the group of less than 5. Computer simulations were used to show that the model accurately reproduces all of the experimental data. The lack of observation of a second transient proton release, as required by the overall stoichiometry, argues that its release occurs in a slow step prior to NADH dissociation.

Acetaldehyde↗
PubMed1996

Kinetics and specificity of human liver aldehyde dehydrogenases toward aliphatic, aromatic, and fused polycyclic aldehydes.

Human mitochondrial aldehyde dehydrogenase (ALDH-2) has a Km for acetaldehyde that is 900-fold lower than that for the cytosolic isozyme, ALDH-1. An increase in aliphatic aldehyde chain length decreases the ALDH-2 Km by up to 10-fold but decreases that of ALDH-1 by 5 orders of magnitude. As a consequence, the Km of ALDH-1 for decanal is 8 times lower than that of ALDH-2, i.e. 2.9 +/- 0.4 and 22 +/- 3 nM, respectively. Determination of these low Km values required kinetic analysis of the simultaneous enzymatic conversion of two aldehyde substrates, an approach also applied to aromatic and fused polycyclic aldehydes. For most of these substrates, maximum velocities are 5-100 times lower than those for acetaldehyde. Addition of one of these tight-binding, slow-turnover substrates to a reaction mixture containing ALDH, NAD+, and a "reference" aldehyde substrate (e.g. acetaldehyde) blocks the principal (reference) enzymatic reaction temporarily and reversibly. Once the first substrate is converted to product, the enzyme can act on the reference substrate. In terms of apparent affinity and blocking capacity, naphthalene and phenanthrene aldehydes were the most potent effectors. Other aromatic and fused polycyclic and heterocyclic aldehydes, as well as derivatives of coumarin, quinoline, indole, and pyridine, are tight-binding, slow-turnover substrates for ALDH-2 and relatively weak inhibitors of ALDH-1. The hydrophobicity of substituents of benzaldehydes, and particularly of naphthaldehydes, correlates with their binding constants toward ALDH-2. Vitamin A1 aldehydes are specific natural substrates for ALDH-1; at pH 7.5, for all-trans- and 13-cis-retinal, Km = 1.1 and 0.37 micromolar, respectively, and kcat/Km is 50-100 times higher than that for acetaldehyde. At the same time, the retinals are inhibitors of ALDH-2, all-trans-retinal being a particularly potent inhibitor (competitive Ki = 43 nM, noncompetitive Ki = 316 nM). These properties suggest that all-trans-retinal is a possible regulatory compound for ALDH-2 in vivo. The data in general point to specialized roles for both major human liver ALDH isozymes in the oxidation of bulky/hydrophobic natural compounds, with Km values in the low nanomolar range.

Acetaldehyde↗
PubMedDate not supplied

Solid-phase Mannich condensation of amines, aldehydes, and alkynes: investigation of diverse aldehyde inputs.

Terminal alkynes, secondary amines, and aldehydes undergo "solid-phase Mannich condensation". A set of diverse aldehyde inputs was examined. Aliphatic, aralkyl, aryl, and heteroaryl carboxaldehydes give good yields of Mannich adduct of high purity. Benzaldehydes containing electron-donating substituents that decrease the electrophilicity of the carbonyl center, or heteroaryl aldehydes that are similarly deactivated by resonance effects, do not undergo reaction.

Aldehydes↗
PubMedDate not supplied

Prediction of enantiomeric excess in a combinatorial library of catalytic enantioselective reactions.

A quantitative structure-enantioselectivity relationship was established for a combinatorial library of enantioselective reactions performed by addition of diethyl zinc to benzaldehyde. Chiral catalysts and additives were encoded by their chirality codes and presented as input to neural networks. The networks were trained to predict the enantiomeric excess. With independent test sets, predictions of enantiomeric excess could be made with an average error as low as 6% ee. Multilinear regression, perceptrons, and support vector machines were also evaluated as modeling tools. The method is of interest for the computer-aided design of combinatorial libraries involving chiral compounds or enantioselective reactions. This is the first example of a quantitative structure-property relationship based on chirality codes.

Benzaldehydes↗
PubMedDate not supplied

Physicochemical stereodescriptors of atomic chiral centers.

Physicochemical atomic stereodescriptors (PAS) were implemented that represent the chirality of an atomic chiral center on the basis of empirical physicochemical properties of the ligands. The ligands are ranked according to a specific property, and the chiral center takes an S/R-like descriptor relative to that property. The procedure is performed for a series of properties, yielding a chirality profile. Application of the PAS descriptors to the prediction of enantioselectivity in chemical reactions, from the molecular structures, is illustrated here. The relationship between the molecular structures, represented by the PAS descriptors, and the enantioselectivity was learned by neural networks, decision trees, or random forests. In a first application, a data set was employed with chiral amino alcohols that enantioselectively catalyze the addition of diethylzinc to benzaldehyde. Prediction of the major enantiomer obtained in the reaction, from the molecular structure of the catalyst, was achieved with accuracy up to 90%. The second application investigated the enantiopreference of Pseudomonas cepacia lipase (PCL) toward primary alcohols. The learned models could make correct predictions about the preferred enantiomer, from the molecular structure of the substrate, in up to 93% of the cases. These included substrates with and without O-atoms bonded to the chiral center. The properties automatically selected to build the models can give indications on the relevant factors guiding the observed chemical behavior.

Alcohols↗
PubMed2004

Full-scale chamber investigation and simulation of air freshener emissions in the presence of ozone.

Volatile organic compound (VOC) emissions from one electrical plug-in type of pine-scented air freshener and their reactions with O3 were investigated in the U.S. Environmental Protection Agency indoor air research large chamber facility. Ozone was generated from a device marketed as an ozone generator air cleaner. Ozone and oxides of nitrogen concentrations and chamber conditions such as temperature, relative humidity, pressure, and air exchange rate were controlled and/or monitored. VOC emissions and some of the reaction products were identified and quantified. Source emission models were developed to predict the time/concentration profiles of the major VOCs (limonene, alpha-pinene, beta-pinene, 3-carene, camphene, benzyl propionate, benzyl alcohol, bornyl acetate, isobornyl acetate, and benzaldehyde) emitted bythe air freshener. Gas-phase reactions of VOCs from the air freshener with O3 were simulated by a photochemical kinetics simulation system using VOC reaction mechanisms and rate constants adopted from the literature. The concentration-time predictions were in good agreement with the data for O3 and VOCs emitted from the air freshener and with some of the primary reaction products. Systematic differences between the predictions and the experimental results were found for some species. Poor understanding of secondary reactions and heterogeneous chemistry in the chamber is the likely cause of these differences. The method has the potential to provide data to predict the impact of O3/VOC interactions on indoor air quality.

Air Pollution, Indoor↗
PubMed2001

Solution structures and reactivities of the mixed aggregates derived from n-butyllithium and vicinal amino alkoxides.

Low-temperature (6)Li, (13)C, and (15)N NMR spectroscopies reveal that mixtures of n-BuLi and (1R,2S)-R'(2)NCH(R)CH(Ph)OLi (ROLi; R = Ph or Me; R'(2)N = pyrrolidino or Me(2)N) in THF/pentane afford a (n-BuLi)(3)(ROLi) (3:1) mixed tetramer and a C(2)-symmetric (n-BuLi)(2)(ROLi)(2) (2:2) mixed tetramer depending on the proportions of the reactants. The corresponding (n-BuLi)(ROLi)(3) (1:3) mixed tetramer is not observed. ROLi-mediated additions of n-BuLi to benzaldehyde proceed with up to 21:1 enantiomeric ratios that depend on the n-BuLi/ROLi stoichiometries. The enantioselectivities are considered in light of a previously posited mechanism involving reaction via the C(2)-symmetric 2:2 mixed tetramer.

Alkynes↗
PubMed2002

Optimization of catalyst enantioselectivity and activity using achiral and meso ligands.

The optimization of asymmetric catalysts for enantioselective synthesis has conventionally revolved around the synthesis and screening of enantiopure ligands. In contrast, we have optimized an asymmetric reaction by modification of a series of achiral ligands. Thus, employing (S)-3,3'-diphenyl BINOL [(S)-Ph(2)-BINOL] and a series of achiral diimine and diamine activators in the asymmetric addition of alkyl groups to benzaldehyde, we have observed enantiomeric excesses between 96% (R) and 75% (S) of 1-phenyl-1-propanol. Some of the ligands examined have low-energy chiral conformations that can contribute to the chiral environment of the catalyst. These include achiral diimine ligands with meso backbones that adopt chiral conformations, achiral diimine ligands with backbones that become axially chiral on coordination to metal centers, achiral diamine ligands that form stereocenters on coordination to metal centers, and achiral diamine ligands with pendant groups that have axially chiral conformations. Additionally, we have structurally characterized (Ph(2)-BINOLate)Zn(diimine) and (Ph(2)-BINOLate)Zn(diamine) complexes and studied their solution behavior.

Benzaldehydes↗
PubMed2003

Quantum mechanical predictions of the stereoselectivities of proline-catalyzed asymmetric intermolecular aldol reactions.

Quantum mechanical calculations were employed to predict the ratio of four stereoisomeric products expected from two complex reactions involving the aldol reactions of cyclohexanone with benzaldehyde or with isobutyraldehyde catalyzed by (S)-proline. Experimental tests of these predictions provide an assessment of the state-of-the-art in quantum mechanical prediction of products of complex organic reactions in solution.

Alcohols↗
PubMed2003

Conversion of a PLP-dependent racemase into an aldolase by a single active site mutation.

Alanine racemase (Alr) [EC 5.1.1.1] from Geobacillus stearothermophilus is a pyridoxal 5'-phosphate-dependent enzyme that catalyzes the first committed step in bacterial cell wall biosynthesis. It is converted to an aldolase upon replacement of Tyr265, which normally serves as a catalytic base in the racemase reaction, with alanine. The Y265A mutation increases catalytic efficiency for cleavage of beta-phenylserine to benzaldehyde and glycine by 2.3 x 105 fold as compared to the wild-type racemase, while racemase activity is greatly decreased. Additional mutagenesis suggests that His166 may act as the base that initiates the retroaldol reaction. The Y265A mutant is highly stereoselective for (2R,3S)-phenylserine, a d-amino acid, and does not process its enantiomer. This preference is consistent with the expected binding mode of substrate in the modified active site and supports the proposal that naturally occurring d-threonine aldolases and alanine racemases derive from a common ancestor.

Alanine Racemase↗
PubMed2004

Reduction of an aldehyde by a NADH/Zn2+ -dependent redox active ribozyme.

We report here the ability of an alcohol dehydrogenase (ADH) ribozyme to reduce a benzaldehyde. While the ribozyme was initially evolved in vitro based on the activity for the NAD+-dependent oxidation of the benzyl alcohol, we found that this ADH ribozyme is also capable of reducing the aldehyde in the presence of NADH and Zn2+. The rate acceleration gained by ribozyme catalysis was more than 6 orders of magnitude larger than the spontaneous reaction. Although the reversibility of phosphordiester and acyl transfer reactions catalyzed by ribozymes was known, that of other chemical reactions has not been well established. This study has demonstrated the reversibility of a hydride transfer chemistry catalyzed by the ADH ribozyme. Most interestingly, the ribozyme shares many features with the protein ADHs, e.g., reversibility and NADH/Zn2+ dependence.

Alcohol Dehydrogenase↗
PubMed2004

Mechanistic characterization of aerobic alcohol oxidation catalyzed by Pd(OAc)(2)/pyridine including identification of the catalyst resting state and the origin of nonlinear [catalyst] dependence.

The Pd(OAc)(2)/pyridine catalyst system is one of the most convenient and versatile catalyst systems for selective aerobic oxidation of organic substrates. This report describes the catalytic mechanism of Pd(OAc)(2)/pyridine-mediated oxidation of benzyl alcohol, which has been studied by gas-uptake kinetic methods and (1)H NMR spectroscopy. The data reveal that turnover-limiting substrate oxidation by palladium(II) proceeds by a four-step pathway involving (1) formation of an adduct between the alcohol substrate and the square-planar palladium(II) complex, (2) proton-coupled ligand substitution to generate a palladium-alkoxide species, (3) reversible dissociation of pyridine from palladium(II) to create a three-coordinate intermediate, and (4) irreversible beta-hydride elimination to produce benzaldehyde. The catalyst resting state, characterized by (1)H NMR spectroscopy, consists of an equilibrium mixture of (py)(2)Pd(OAc)(2), 1, and the alcohol adduct of this complex, 1xRCH(2)OH. These in situ spectroscopic data provide direct support for the mechanism proposed from kinetic studies. The catalyst displays higher turnover frequency at lower catalyst loading, as revealed by a nonlinear dependence of the rate on [catalyst]. This phenomenon arises from a competition between forward and reverse reaction steps that exhibit unimolecular and bimolecular dependences on [catalyst]. Finally, overoxidation of benzyl alcohol to benzoic acid, even at low levels, contributes to catalyst deactivation by formation of a less active palladium benzoate complex.

Acetates↗