Carbonyl Intermediates Research Articles

PIG-S and PIG-T, essential for GPI anchor attachment to proteins, form a complex with GAA1 and GPI8

Many eukaryotic cell surface proteins are anchored to the plasma membrane via glycosylphosphatidylinositol (GPI). The GPI transamidase mediates GPI anchoring in the endoplasmic reticulum, by replacing a protein's C-terminal GPI attachment signal peptide with a pre-assembled GPI. During this transamidation reaction, the GPI transamidase forms a carbonyl intermediate with a substrate protein. It was known that the GPI transamidase is a complex containing GAA1 and GPI8. Here, we report two new components of this enzyme: PIG-S and PIG-T. To determine roles for PIG-S and PIG-T, we disrupted these genes in mouse F9 cells by homologous recombination. PIG-S and PIG-T knockout cells were defective in transfer of GPI to proteins, particularly in formation of the carbonyl intermediates. We also demonstrate that PIG-S and PIG-T form a protein complex with GAA1 and GPI8, and that PIG-T maintains the complex by stabilizing the expression of GAA1 and GPI8. Saccharomyces cerevisiae Gpi16p (YHR188C) and Gpi17p (YDR434W) are orthologues of PIG-T and PIG-S, respectively.

Antioxidant and anti-AGE therapeutics: evaluation and perspectives

Diabetic patients exhibit an oxidative stress status, that is an imbalance between reactive oxygen species and antioxidant defences, in favour of the first ones. This oxidative stress, together with formation of advanced glycation endproducts (AGEs), is involved in diabetic complications. It could thus be of great interest to propose antioxidant and/or anti-AGE therapeutics as complementary treatment in these patients. Antioxidants can be classical molecules such as vitamin E, lipoic acid or N-acetylcysteine. Thus, vitamin E supplementation can improve insulin efficiency and glycemic equilibrium, as shown by the decrease of glycaemia, glycated haemoglobin and fructosamine values. In addition, this kind of supplementation lowers plasma lipid peroxidation and oxidizability of low density lipoproteins, which is involved in the atherogenesis process. Moreover, it allows to fight against complications such as retinopathy. A second category is represented by molecules able to fight against the effects of glycation end-products (AGEs). They can act: either by preventing cellular action of AGEs; this is obtained with soluble receptors of advanced glycation endproducts (sRAGE); or by inhibiting AGE formation (scavenging of reactive carbonyl intermediates). Nucleophilic compounds such as pyridoxamine, tenilsetam, 2,3-diaminophenazone, OPB-9195 or aminoguanidine can act in this way. Aminoguanidine is able to limit the development of the main diabetes-associated complications in animals. A double-blind clinical assay has been conducted in type 2 diabetic patients in the United States and the Canada, in order to determine if aminoguanidine is able to slow down the progression of diabetes-induced nephropathy. We will discuss about another guanidic molecule, i.e. metformin, which is also able to scavenge AGEs, in the last part of this review. A third category of molecules is constituted by oral antidiabetic molecules exhibiting antioxidant properties. They are thiazolidinediones (troglitazone) and sulfonylureas (gliclazide). Troglitazone and gliclazide can thus decrease LDL oxidizability and monocyte adhesion to endothelial cells, which is an early step in the atherogenesis process and which is stimulated by oxidised LDLs. Finally, a prospective way is devoted to oral antidiabetic drugs exhibiting both antioxidant and anti-AGE properties. A very used antidiabetic drug of interest is metformin (dimethylbiguanide), since it can prevent diabetes complications not only by lowering glycaemia, but also by inhibiting AGE formation and by stimulating antioxidant defences. The latter therapeutic approach constitutes a future way in the diabetes area, in order both to obtain a better glycemic control and a least development of diabetic complications.

Gaa1p and gpi8p are components of a glycosylphosphatidylinositol (GPI) transamidase that mediates attachment of GPI to proteins.

Many eukaryotic cell surface proteins are anchored to the membrane via glycosylphosphatidylinositol (GPI). The GPI is attached to proteins that have a GPI attachment signal peptide at the carboxyl terminus. The GPI attachment signal peptide is replaced by a preassembled GPI in the endoplasmic reticulum by a transamidation reaction through the formation of a carbonyl intermediate. GPI transamidase is a key enzyme of this posttranslational modification. Here we report that Gaa1p and Gpi8p are components of a GPI transamidase. To determine a role of Gaa1p we disrupted a GAA1/GPAA1 gene in mouse F9 cells by homologous recombination. GAA1 knockout cells were defective in the formation of carbonyl intermediates between precursor proteins and transamidase as determined by an in vitro GPI-anchoring assay. We also show that cysteine and histidine residues of Gpi8p, which are conserved in members of a cysteine protease family, are essential for generation of a carbonyl intermediate. This result suggests that Gpi8p is a catalytic component that cleaves the GPI attachment signal peptide. Moreover, Gaa1p and Gpi8p are associated with each other. Therefore, Gaa1p and Gpi8p constitute a GPI transamidase and cooperate in generating a carbonyl intermediate, a prerequisite for GPI attachment.

Atmospheric Photochemical Degradation of 1,4-Unsaturated Dicarbonyls

To better understand fates of aromatics hydrocarbon species in the atmosphere, the authors have investigated the transformation chemistry of butenedial (CHOCH{double{underscore}bond}CHCHO), 4-oxo-2-pentenal (CH{sub 3}COCH{double{underscore}bond}CHCHO), and 3-hexene-2, 5-dione (CH{sub 3}COCH{double{underscore}bond}CHCOCH{sub 3}). These 1,4-unsaturated dicarbonyls are known to be products of aromatic photochemical oxidation. Both hydroxyl radical (OH) and ozone (O{sub 3}) initiated smog chamber experiments under atmospheric conditions were conducted in the University of North Carolina outdoor smog chamber. Carbonyl intermediates and products were measured using the O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine derivatization method followed by gas chromatography/ion trap mass spectrometry analysis. Carbonyl products detected and identified by comparison with standards in the OH-initiated photooxidation of butenedial include formaldehyde, acrolein, glycolaldehyde, glyoxal, and malonaldehyde (CHOCH{sub 2}CHO). For 4-oxo-2-pentenal, the carbonyl products were formaldehyde, methyl vinyl ketone, glycolaldehyde, hydroxyacetone, glyoxal, methylglyoxal, and malonaldehyde. for 3-hexene-2,5-dione the products were formaldehyde, acetaldehyde, acetone, hydroxyacetone, and methylglyoxal. Carbonyl products detected in the P{sub 3}-initiated experiments with cyclohexane as the OH scavenger were formaldehyde and glyoxal in butenedial; formaldehyde, glyoxal, methyl-glyoxal, and malonaldehyde in 4-oxo-2-pentenal; and formaldehyde and methylglyoxal in 3-hexene-2,5-dione.

Homogeneous transition metal catalysis: from the water gas shift reaction to nuclear waste vitrification

A US–Hungarian cooperative research project between Professor R. Bruce King at the University of Georgia and Professor László Markó at the University of Veszprém during the period 1977 to 1986 was a major stimulus in extending the scope of Professor King’s research at the University of Georgia from synthetic transition metal organometallic chemistry to homogeneous catalysis. Since that US–Hungarian cooperative research project, a major emphasis of the work on homogeneous catalysis at the University of Georgia has been on applications of homogeneous transition metal catalysis to carbon monoxide, formate, and formic acid chemistry. This started with a joint project of Professor R. Bruce King with Professor Allen D. King Jr., which led to the discovery of an interesting mechanistic duality for the metal carbonyl-catalyzed water gas shift reaction (CO+H2O=CO2+H2) where use of M(CO)6 (M=Cr, Mo, W) catalyst precursors leads to a dissociative mechanism through formato metal carbonyl intermediates whereas use of an Fe(CO)5 catalyst precursor leads to an associative mechanism through metallocarboxylate intermediates. Subsequent scientific interaction of Professor R. Bruce King with Dr Charles M. King, then of the Westinghouse Savannah River Technical Center, led to a study of related formic acid reactions occurring during pre-treatment of nuclear wastes arising from catalytic activities of noble metal species in the wastes generated from uranium fission. Thus research at the University of Georgia during the period 1991–1994, supported through funding from the Savannah River and Hanford nuclear sites, led to the identification of a homogeneous nitrorhodium-catalyzed process for hydrogen liberation from formic acid with an apparent mechanism similar to the dissociative mechanism for the water gas shift reaction.

Modelling intermediates in the catalytic carbonylation of CH 2I 2 to malonate esters; Evidence for a ketene pathway

From model intermediates and from labelling studies, a ketene-based mechanism is proposed for the double carbonylation of CH 2I 2 to malonate esters catalysed by [RhX(CO)(PEt 3) 2] and for the deactivation of the catalyst.

Selective Hydrogenation of α,β-Unsaturated Aldehydes

The synthesis of a large number of fine chemicals, particularly in the field of flavor and fragrance chemistry [1,2] and pharmaceuticals [3], involves the selective hydrogenation of unsaturated carbonyl intermediates as a critical step. The hydrogenation of α,β-unsaturated carbonyls into saturated carbonyls is comparatively easy to achieve because thermodynamics favor the hydro-genation of the C═C bonds; therefore, research efforts were more directed at improving the selectivity to unsaturated alcohols. When a substituent is present on the carbon atom of the carbonyl group (i.e. with ketones), there is no chance to hydrogenate selectively the C═O bond, and saturated ketones are obtained with a high yield. This review is thus mostly restricted to the hydrogenation of α, β-unsaturated aldehydes into the corresponding unsaturated alcohols.

Key Intermediates in Combinatorial Chemistry: Access to Various Heterocycles from α,β-Unsaturated Ketones on the Solid Phase

The value of alpha,beta-unsaturated ketones as key intermediates for the combinatorial assembly of four different templates on the solid phase, namely pyrimidines, dihydropyrimidinones, pyridines, and pyrazoles, was explored with individual syntheses of variably substituted model compounds. Starting from aldehydes grafted on polystyrene support, the Wittig and the Claisen-Schmidt reaction conditions were adapted to efficiently prepare alpha,beta-unsaturated ketones on the solid phase. Further derivatization of the alpha,beta-unsaturated ketones to form pyrimidines succeeded with a number of amidines. In a feasibility study, the potential to obtain, in a modular fashion, other small heterocycles from the same intermediates was assessed. In this solid-phase approach alpha,beta-unsaturated carbonyl intermediates can act as a three-carbon component and a primary enamine is utilized to complement the system for pyridine ring formation. Instead, with N-methylurea a dihydropyrimidinone is obtained. As an alternative, substituted hydrazines are incorporated in one orientation, providing pyrazoles with defined regioisomerism. The study indicates that alpha,beta-unsaturated ketones grafted on the solid phase can take a pivotal role as branching points in a number of synthetic diversity schemes and, therefore, represent versatile intermediates for the efficient preparation of combinatorial small molecule libraries.

Role of protein-bound carbonyl groups in the formation of advanced glycation endproducts

Several mechanisms have been postulated for the formation of advanced glycation endproducts (AGEs) from glycated proteins; they all feature protein-bound carbonyl intermediates. Using 2,4-dinitrophenylhydrazine (DNPH), we have detected these intermediates on bovine serum albumin, lysozyme and β-lactoglobulin after in vitro glycation by glucose or fructose. Carbonyls were formed in parallel with AGE-fluorophores, via oxidative Maillard reactions. Neither Amadori nor Heyns products contributed to the DNPH reaction. Fluorophore and carbonyl yields were much enhanced in lipid-associated proteins, but both groups could also be detected in lipid-free proteins. When pre-glycated proteins were incubated in the absence of free sugar, carbonyl groups were rapidly lost in a first-order reaction, while fluorescence continued to develop beyond the 21 days of incubation. Another unexpected finding was that not all carbonyl groups were blocked by aminoguanidine, although there was complete inhibition of reactions leading to AGE-fluorescence. It is suggested that carbonyls acting as fluorophore precursors react readily with aminoguanidine, while others are resistant to this hydrazine, possibly because they are involved in ring closure. Factors influencing the relative rates of acyclisation and hydrazone formation are discussed, together with possible implications for antiglycation therapy.

Cycloadditions of nitrile oxides to amidoximes. A general synthesis of 3,5-disubstituted 1,2,4-oxadiazole-4-oxides.

The cycloaddition of nitrile oxides to amidoximes is a general method for the sinthesis of 3,5-disubstituted 1,2,4-oxadiazole-4-oxides with the same or different substituents. The yields are only moderate since an equivalent amount of the nitrile oxide is consumed by reaction with the amine released in the fragmentation of the primary cycloadducts and reforms the amidoxime. With excess nitrile oxides the 1,2,4-oxadiazole-4-oxides undergo a disproportionation reaction to yield nitroso carbonyl intermediates and 1,2,4-oxadiazoles.

ChemInform Abstract: A Mild Oxidation of Nitrile Oxides: A New Synthetic Route to Nitroso Carbonyl Intermediates.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

A mild oxidation of nitrile oxides: A new synthetic route to nitroso carbonyl intermediates

Addition of N-methyl morpholine N-oxide (NMO) to a nitrile oxide solution in dichloromethane at room temperature in the presence of a diene affords nitroso carbonyl adducts in fair yields.

The synthesis of 1,5-dideoxy-1,5-(alkyl)imino-2-C-methyl- d-glucitols

The regio- and stereoselective synthesis of the title compounds 3 and 4 has been accomplished starting with deoxynojirimycin. The factors affecting the stereochemical outcome of Grignard addition to the key carbonyl intermediate are discussed.

σ-Bishomoaromatic heterocycles: Reactivity and synthesis of 3-oxatetracyclo[3.3.0.0 2,80 4,6]octane by photolysis at 185 nm

The title compound 5 was produced by 185 nm photolysis of 8. The products of the 185 nm photolysis can be rationalized by the assumption of the two diradical intermediates 16 and 17. At a temperature between 150 and 20 °C 5 reacts with fumaronitrile 23 to give the adducts 7 and 26 in a ratio of about (5 : 1). The kinetic analysis of this reaction shows, that the adduct 7 is formed by 1,3 dipolar cycloaddition of intermediate carbonyl ylide 6.

Photooxidation of Matrix-Isolated Nickel Tetracarbonyl in the Presence of Dioxygen

UV Photolysis (A = ca. 254 nm) of Ni(C0)4 molecules isolated in Ordoped argon matrices at ca. 20 K has been monitored by reference to the IR spectra of the matrices. These spectra indicate that Ni(C0)4 undergoes photooxidation to give four distinct nickel carbonyl intermediates A-D with the release of CO and C02, ultimately to arrive at the final product (v2-02)2Ni. The stoichiometry and likely structure of each intermediate A-D have been deduced by analysis of the number, energies, and relative intensities of its IR absorptions, with particular reference to the response to I3COand lSO-enrichment. Hence, the following identities have been inferred: A, Ni(CO)3; B, (q2-02)Ni(C0)2; C, (02)Ni(C0)3; and D, O=Ni(CO)2. The system exhibits some photoselectivity, and reversal of the reactions leading to A-D can be effected under the action of W radiation with wavelengths near 313 nm or by annealing the deposit to ca. 30-35 K. A partial mechanism is proposed to account for the course of the matrix reactions.

Photochemistry of a benzofuro‐annulated oxanorbornadiene formation of propellanes by trapping of an intermediate carbonyl ylide with CN and CC dipolarophiles

AbstractOn photochemical excitation of the benzofuro‐annulated oxanorbornadiene 10 in the presence of dipolarophiles the 1:1 addition compounds 11, 22, 23a,b, and 24, respectively, are formed as the only monomeric products. The mechanistic interpretation is based on a reaction sequence which involves [2 + 2] photocycloaddition to the oxaquadricyclane 12, 1,3‐dipolar cycloreversion to the carbonyl ylide 13 and subsequent face‐selective trapping by the corresponding dipolarophile. No indication of the transformation of 13 to the highly strained [3]3,6‐oxepinophane 14 was obtained. With the formation of 11 one of the rare examples of the cycloaddition of a nitrile dipolarophile (i.e. acetonitrile) was uncovered.

Structural Characterization of Intermediates in the Rhodium-Catalyzed Reductive Carbonylation of Methanol: Rh(COMe)(I)2(dppp) and [Rh(H)(I)(.mu.-I)(dppp)

X-ray structural analyses of Rh(COCH{sub 3})(I{sub 2})(dppp) (1) and [Rh(H)(I)({mu}-I)(dppp)]{sub 2} (2) are reported. Complex 1 is converted to hydride 2 with H{sub 2}, and this reaction is believed to be the critical step in the conversion of methanol to acetaldehyde, catalyzed by 1. Unsaturated complex 1 possesses a distorted five-coordinate geometry that is intermediate between sbp and thp structures. Complex 2 achieves coordinative saturation via formation of iodo bridges. H{sub 2} activation by 1 is discussed in view of these structural results. Complex 1 further is found to heterolytically activate H{sub 2} in the presence of base. 25 refs., 3 figs., 3 tabs.

Reduced-Pressure UV Photo-Oxidation of Organic Contaminants on Si Surfaces

ABSTRACTA reduced-pressure ultraviolet (UV) photo-oxidation process is described for the removal of organic contamination from Si surfaces with concomitant growth of an ultra-thin oxide passivation layer. As in situ photo-generation of ozone (O3) from dioxygen (O2) is impractical at millitorr pressures, a 2% O3/O2 mixture from an ozone generator is fed to the surface conditioning chamber. UV/O3 surface conditioning employing simultaneous 254/185 nm UV irradiation results in essentially complete removal of carbon contamination in 180 s at 100 mTorr and 100–200°C. Kinetics studies employing cyclohexane-contaminated Si(100) surfaces suggest that carbon removal occurs via two consecutive first-order processes: initial fast photo-oxidation of adsorbed cyclohexane to readily desorbed products and slower photo-oxidation of adsorbed carbonyl intermediates to CO2 and H2O. The activation energies for both processes are 2–3 kcal/mol, consistent with the involvement of photo-generated atomic oxygen species. Self-limiting growth of an ultra-thin oxide passivation layer occurs concomitantly with carbon removal. At saturation, the oxide layer is only 3–4 Å thick, and the growth kinetics are described by a first-order Langmuirian adsorption model.

Photo-, electro-, and thermal carbonylation of alkyl iodides in the presence of group 7 and 8–10 metal carbonyl catalysts

Various transition-metal complexes including Group 7 and 8–10 metal carbonyls are highly active catalyst precursors for the photochemical carbonylation of alkyl iodides having β-hydrogens on saturated sp 3-carbons at room temperature under 1 atm of carbon monoxide. Primary, secondary and tertiary alkyl iodides are smoothly carbonylated by this catalyst system without β-hydride elimination (dehydrohalogenation) to give the corresponding esters or amides in high yields. In employment of Mn 2(CO) 10 as a catalyst, electrochemical carbonylation of alkyl iodides also occurred via generation of an anionic manganese carbonyl intermediate. Actually, anionic manganese carbonyl complexes (pentacarbonylmanganates) showed high catalytic activity for thermal carbonylation of alkyl iodides at room temperature under 1 atm of carbon monoxide without photo-irradiation and electrolysis. Mechanistic studies were performed with kinetics and ESR and it was suggested that the present photo-, electro- and thermal carbonylation would involve a non-chain radical mechanism.

The Characterization of HRh(CO)2I3- and EtCORh(CO)2I3-: Two Key Intermediates in Rhodium-Catalyzed Carbonylation of Ethylene

The Characterization of HRh(CO)2I3- and EtCORh(CO)2I3-: Two Key Intermediates in Rhodium-Catalyzed Carbonylation of Ethylene