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At least 631 records · Page 35Linked to original sources
PubMed2000

Characterization and esterification of hemicelluloses from rye straw.

Hemicelluloses were extracted with 10% KOH/0.5% Na(2)B(4)O(7). 10H(2)O from delignified rye straw. Esterification of the hemicelluloses with various acyl chlorides was performed in a homogeneous N,N-dimethylformamide and lithium chloride system using 4-(dimethylamino)pyridine catalyst and triethylamine as a neutralizer. The degree of substitution was controlled between 0.37 and 1.65. Under an optimum condition (sample 14, molar ratio 3:1), >90% of the free hydroxyl groups in native hemicelluloses were stearoylated at 75 degrees C for 40 min. Meanwhile, the products were characterized by FT-IR and GPC techniques as well as their solubilities. The molecular mass measurements (31400-123300 g mol(-)(1)) showed only a minimal degradation of the macromolecular hemicelluloses during rapid reactions at 48-75 degrees C for 20-40 min.

Chromatography, Gas↗
PubMed2002

Nucleophilic catalysis with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) for the esterification of carboxylic acids with dimethyl carbonate.

1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) is an effective nucleophilic catalyst for carboxylic acid esterification with dimethyl carbonate (DMC). The reaction pathway of this new class of nucleophilic catalysis has been studied. A plausible, multistep mechanism is proposed, which involves an initial N-acylation of DBU with DMC to form a carbamate intermediate. Subsequent O-alkylation of the carboxylate with this intermediate generates the corresponding methyl ester in excellent yield. In the absence of DBU or in the presence of other bases, such as ammonium hydroxide or N-methylmorpholine, the same reaction affords no desired product. This method is particularly valuable for the synthesis of methyl esters that contain acid-sensitive functionality.

Journal Article↗
PubMed1996

The Effect of Acid Strength on the Mitsunobu Esterification Reaction: Carboxyl vs Hydroxyl Reactivity.

In the presence of carboxylic acids, the adduct formed between triphenylphosphine and diisopropyl azodicarboxylate reacts to form mono- and bis-acylated hydrazides and the carboxylic acid anhydrides. These products are formed via attack of the carboxylate on the triphenylphosphonium group of the adduct, with weaker acids reacting much faster than stronger acids. This provides an explanation for the observation in the literature that acids stronger than acetic acid, such as 4-nitrobenzoic acid and chloroacetic acid, provide better yields in esterification reactions, since reaction of the alcohol with the phosphonium group of the adduct is more rapid than the competing reaction of the carboxylate for the phosphonium group.

Journal Article↗
PubMed2005

Phase separation of the reaction system induced by CO2 and conversion enhancement for the esterification of acetic acid with ethanol in ionic liquid.

The effect of CO2 on the phase behavior of the reaction system and equilibrium conversion for esterification of acetic acid and ethanol in ionic liquid (1-butyl-3-methylimidazolium hydrogen sulfate, [bmim][HSO4]) was studied at 60.0 degrees C up to 15 MPa. It was demonstrated that there was only one phase in the reaction system in the absence of CO2. The reaction system underwent two-phase-->three-phase-->two-phase transitions with increasing pressure. The pressure of CO2 or the phase behavior of the system affected the equilibrium conversion of the reaction markedly. As the pressure was less than 3.5 MPa, there were two phases in the system, and the equilibrium conversion increased as pressure was increased. In the pressure range of 3.5-9.5 MPa, there existed three phases, and the equilibrium conversion increased more rapidly with increasing pressure. As the pressure was higher than 9.5 MPa, the reaction system entered another two-phase region and the equilibrium conversion was nearly independent of pressure. The total equilibrium conversion was 64% without CO2 and could be as high as 80% as pressure was higher than 9.0 MPa. The apparent equilibrium constants (K(x)) in different phases were also determined, showing that the K(x) in the middle phase or top phase was much greater than that in the bottom phase.

Journal Article↗
PubMed2006

Mesoporous silica materials with an extremely high content of organic sulfonic groups and their comparable activities with that of concentrated sulfuric acid in catalytic esterification.

Mesoporous silica materials (HS-JLU-20) with an extremely high content of mercaptopropyl groups have been successfully synthesized using fluorocarbon-hydrocarbon surfactant mixtures through a simple co-condensation approach of tetraethyl orthosilicate (TEOS) and (3-mercaptopropyl)trimethoxysilane (MPTS), which are characterized by X-ray diffraction (XRD), nitrogen adsorption and desorption isotherms, transmission electron microscopy (TEM), CHNS elemental analysis, thermogravimetry analysis (TGA), and (29)Si NMR spectroscopy. The results show that HS-JLU-20 samples with molar ratios of MPTS/(MPTS + TEOS) at 0.5-0.8 in the starting synthetic gels still show their mesostructures, while HS-SBA-15 with the molar ratio of MPTS/(MPTS + TEOS) at 0.50 completely loses its mesostructure in the absence of fluorocarbon surfactant. Possibly, fluorocarbon surfactant containing N(+) species with a positive charge could effectively interact with negatively charged mercapto groups in the synthesis of HS-JLU-20 materials, resulting in the formation of mesoporous silicas with good cross-linking of silica condensation even at an extremely high content of organic mercapto groups. More interestingly, after the treatment with hydrogen peroxide, HSO(3)-JLU-20 materials with an extremely high content of organic sulfonic groups exhibit comparable activity with liquid concentrated sulfuric acid in catalytic esterification of cyclohexanol with acetic acid.

Journal Article↗
PubMed2002

Chemoselective esterification of phenolic acids and alcohols.

[formula: see text] The Mitsunobu reaction can distinguish between alcohol and phenol hydroxyls in esterification reactions, providing an expeditious and broadly applicable entry into various phenolics and polyphenolics of biomedical and nutritional relevance.

Journal Article↗
PubMed2006

Stereoselective synthesis of (E)-alpha,beta-unsaturated esters via carbene-catalyzed redox esterification.

[reaction: see text] Stereoselective, carbene-mediated redox esterification of alkynyl aldehydes provides mild and atom economical access to (E)-configurated, alpha,beta-unsaturated carboxylic esters. The organocatalytic method relies on the generation of activated carboxylates via extended/conjugated umpolung in the presence of catalytic amounts of carbene precursor and base.

Journal Article↗
PubMed1999

Palladium-catalyzed highly diastereoselective cyclic carbopalladation-carbonylative esterification tandem reaction of iododienes and iodoarylalkenes.

[formula: see text] Pd-catalyzed reaction of iododienes and iodoarylalkenes represented by 1, 8, and 10 under 1 atm of CO and a small amount of O2 in the presence of a base, e.g., NEt3, as well as MeOH and H2O in DMF can undergo a highly diastereoselective cyclic carbopalladation-carbonylative esterification tandem process (Type II C-Pd process) to give in high yields the corresponding ester-containing cyclization products, e.g., 2, 9, and 11, in as high as 98% diastereoselectivity.

Alkenes↗
PubMed2002

Enzymatic esterification of an acid with an epoxide using an immobilized lipase from Mucor miehei as catalyst: optimization of the yield and isomeric excess of ester by statistical analysis.

We have developed the methodology for the esterification of an acid with an epoxide using 2-chlorobutyric acid and 1,2-epoxy-5-hexene catalysed by a Mucor miehei-immobilized lipase. Thus, this methodology could be applied to obtain 2-chloroesters. A factorial design of experiments and a central composite design have been used to optimise the synthesis of these esters. The variables chosen were temperature and initial catalyst concentration, while the responses were yield and isomeric excess of the ester. According to this study, temperature was the most important factor, having a positive influence on the yield and a small negative influence on the isomeric excess of the ester. The yield and isomeric excess of the ester show a greater dependence on temperature compared to the catalyst concentration. Although the effect of the catalyst concentration on both responses is smaller than the temperature effect, the higher selectivity presented by the biocatalyst towards the studied ester considerably decreased the final product distribution.

Acids↗
PubMed2004

Chemoselectivity in reactions of esterification.

This review is devoted to the problem of chemoselective formation of ester functions in polyfunctional molecules. The review covers most typical approaches to chemoselective acylation of hydroxy groups in molecules containing an amino, mercapto, or another hydroxy functionality as well as chemoselective esterification of di- and polycarboxylic acids.

Journal Article↗
PubMed1970

Effects of sex hormones on enzymic esterification of 2-(N-hydroxy-acetamido) fluorene by rat liver cytosol.

1. Enzymic esterifications of 2-(N-hydroxyacetamido)fluorene and several other hydroxamic acids by liver cytosol were studied. Determination of 2-acetamido-3-methylthiofluorene was used for the assay. 2. With rat liver cytosol, requirement for ATP, Mg(2+) and SO(4) (2-) suggested formation of phosphate and sulphate esters of 2-(N-hydroxyacetamido)fluorene. 3. Rats showed sex and age differences in their activity. Liver from adult male rat was at least twice as active as liver from adult female rat. No such sex differences were found in mice, hamsters and guinea pigs. 4. Administration of testosterone (300mug/day) subcutaneously for 8 days increased the activity in the female rat by 100%, whereas diethylstilboestrol (100mug/day) had no effect. In the male rat diethylstilboestrol treatment decreased the activity by 60%, whereas testosterone pretreatment was without any effect. 5. Among various endocrine ablations such as adrenalectomy, castration, adrenalectomy-castration and hypophysectomy in the adult male rat, hypophysectomy was found to be the most effective in decreasing the activity of the liver to about 50% of control values. 6. Like 2-(N-hydroxyacetamido)fluorene, various other N-hydroxy derivatives of 2-acetamido-7-fluorofluorene, 2-acetamidophenanthrene, 4-acetamidobiphenyl and 4-acetamidostilbene were also shown to be esterified to different extents by rat liver cytosol.

Adenosine Triphosphate↗
PubMed1973

The relationship between palmitoyl-coenzyme A synthetase activity and esterification of sn-glycerol 3-phosphate by the microsomal fraction of guinea-pig intestinal mucosa.

1. With microsomal fractions of guinea-pig intestinal mucosa the mean specific activity of palmitoyl-CoA synthetase was approx. 1.3-fold the esterification of sn-glycerol 3-phosphate with palmitoyl-CoA generated by the endogenous synthetase. The latter activity was approx. 2.5- and 5-fold that when palmitoyl-CoA was generated from palmitoylcarnitine or when it was added directly to the assay system. 2. There were significant correlations (P<0.001) between the specific activities of palmitoyl-CoA synthetase and glycerolipid synthesis from either palmitate or palmitoylcarnitine. 3. The mean molar composition of glycerolipid synthesized from palmitate or palmitoylcarnitine was approx. 18% lysophosphatidate, 75% phosphatidate and 7% neutral lipid. 4. Glycerolipid synthesis from palmitate was inhibited by 80-90% after preincubation of microsomal fractions at 37 degrees C for 40min and was caused by inactivation of palmitoyl-CoA synthetase. 5. Addition of 100-400mm-KCl inhibited palmitoyl-CoA synthetase activity and glycerolipid synthesis from palmitate but stimulated glycerol phosphate acyltransferase activity. 6. Diversion of palmitoyl-CoA synthesized by the endogenous synthetase to palmitoylcarnitine resulted in an almost stoicheiometric decrease in glycerolipid synthesis. 7. Addition of rac-1-monopalmitin promoted utilization of palmitoyl-CoA by the monoglyceride pathway but did not inhibit phosphatidate biosynthesis. 8. With rate-limiting concentrations of CoA and Mg(2+) the relative decreases in velocity for palmitoyl-CoA synthetase and glycerolipid synthesis from palmitate were almost identical. However, low concentrations of palmitate and ATP produced greater decreases in synthetase activity than in glycerolipid synthesis. 9. There appears to be a fine balance between the activities of palmitoyl-CoA synthetase and glycerol phosphate acyltransferase, with neither activity being in excess with respect to phosphatidate synthesis.

Acyltransferases↗
PubMed1994

Effects of insulin treatment of diabetic rats on hepatic partitioning of fatty acids between oxidation and esterification, phospholipid and acylglycerol synthesis, and on the fractional rate of secretion of triacylglycerol in vivo.

1. The hypothesis that insulin treatment of streptozotocin-diabetic rats does not alter acutely the ability of acylcarnitine synthesis to compete successfully for cytosolic long-chain acyl-CoA [Grantham and Zammit (1988) Biochem. J. 249, 409-414], was tested in vivo by using the technique of selective labelling of hepatic fatty acids in awake unrestrained rats. In the same animals, the partitioning of hepatic fatty acids between acylglycerol and phospholipid synthesis, and of newly labelled triacylglycerol between secretion into the plasma and retention in the liver, was also studied. 2. In untreated diabetic animals, the ratio of fatty acid oxidation to esterification was double that found in normal fed animals, whereas there were no differences in the values of the above-mentioned parameters of glycerolipid metabolism. Thus the insulin status of the rats only has chronic effects on specific aspects of fatty acid metabolism in the liver. 3. Treatment of diabetic rats with insulin resulted in no change in the oxidation/esterification ratio for the first 5 h after the start of insulin administration. Thereafter, there were reciprocal changes in the 14CO2 expired (an index of oxidation) and 14C label recovered in hepatic and plasma glycerolipids. However, the pattern of partitioning observed in normal fed rats was still not re-established after 8 h of insulin treatment. 4. There was a small and transient decrease in the fractional rate of triacylglycerol secretion by the liver at the start of insulin treatment and an increase in the proportion of labelled fatty acid that was utilized for phospholipid synthesis such that phospholipid labelling as a proportion of that of total glycerolipids was doubled after 8 h of insulin treatment. 5. The data are discussed in relation to the roles of insulin in mediating acute changes in hepatic fatty acid metabolism and very-low-density-lipoprotein triacylglycerol secretion by the liver.

Animals↗
PubMed1997

Role of cholesterol synthesis and esterification in the growth of CEM and MOLT4 lymphoblastic cells.

CEM and MOLT4 are human T-cell lines isolated from patients with acute cell leukaemia. In culture they show important differences in cholesterol metabolism, CEM being less efficient at synthesizing cholesterol and having a lower activity of 3-hydroxy-3-methylglutaryl-CoA (HMGCoA) reductase. To investigate further the relationship between regulation of intracellular cholesterol metabolism at various steps and rate of cell growth, cholesterol synthesis, esterification and efflux were evaluated in CEM and MOLT4 cells at different times during exponential and stationary growth in vitro. It was shown that, although CEM cells have a lower rate of cholesterol synthesis, they grow at a faster rate than MOLT4 cells. However, CEM cells exhibit an increased capacity to esterify cholesterol associated with a decreased efflux of newly synthesized cholesterol into the medium. These results provide evidence for an association between the capability to synthesize and retain cell cholesterol esters and the growth rate potential.

Acetates↗
PubMed2000

Increased methyl esterification of altered aspartyl residues in erythrocyte membrane proteins in response to oxidative stress.

Protein-L-isoaspartate (D-aspartate) O-methyltransferase (PCMT; EC 2. 1.1.77) catalyses the methyl esterification of the free alpha-carboxyl group of abnormal L-isoaspartyl residues, which occur spontaneously in protein and peptide substrates as a consequence of molecular ageing. The biological function of this transmethylation reaction is related to the repair or degradation of age-damaged proteins. Methyl ester formation in erythrocyte membrane proteins has also been used as a marker reaction to tag these abnormal residues and to monitor their increase associated with erythrocyte ageing diseases, such as hereditary spherocytosis, or cell stress (thermal or osmotic) conditions. The study shows that levels of L-isoaspartyl residues rise in membrane proteins of human erythrocytes exposed to oxidative stress, induced by t-butyl hydroperoxide or H2O2. The increase in malondialdehyde content confirmed that the cell membrane is a primary target of oxidative alterations. A parallel rise in the methaemoglobin content indicates that proteins are heavily affected by the molecular alterations induced by oxidative treatments in erythrocytes. Antioxidants largely prevented the increase in membrane protein methylation, underscoring the specificity of the effect. Conversely, we found that PCMT activity, consistent with its repair function, remained remarkably stable under oxidative conditions, while damaged membrane protein substrates increased significantly. The latter include ankyrin, band 4.1 and 4.2, and the integral membrane protein band 3 (the anion exchanger). The main target was found to be particularly protein 4.1, a crucial element in the maintenance of membrane-cytoskeleton network stability. We conclude that the increased formation/exposure of L-isoaspartyl residues is one of the major structural alterations occurring in erythrocyte membrane proteins as a result of an oxidative stress event. In the light of these and previous findings, the occurrence of isoaspartyl sites in membrane proteins as a key event in erythrocyte spleen conditioning and hemocatheresis is proposed.

Anion Exchange Protein 1, Erythrocyte↗
PubMed1989

The membrane binding domain of rod cGMP phosphodiesterase is posttranslationally modified by methyl esterification at a C-terminal cysteine.

Retinal rod cGMP phosphodiesterase (3',5'-cyclic-GMP phosphodiesterase; EC 3.1.4.35; PDE), a key regulatory enzyme involved in visual excitation, is one of several outer segment membrane proteins that are carboxyl methylated in the presence of the methyl donor S-adenosyl-L-[3H-methyl]methionine. By chromatographic analyses of the 3H-methyl amino acid generated by exhaustive proteolysis of purified PDE, followed by performic acid oxidation of the digest, we have shown that this modification occurs at a C-terminal cysteine residue of the alpha subunit of this enzyme. When PDE is subjected to limited proteolysis with trypsin, a 3H-methylated fragment of 1000 daltons or less is rapidly removed prior to the degradation of its inhibitory gamma subunit. This small fragment remains membrane bound, whereas the bulk of the enzyme is released, indicating that a domain responsible for anchoring PDE to the membrane is located near the C terminus. Based on the C-terminal amino acid sequence of Cys-Cys-Val-Gln predicted from the alpha cDNA sequence, we conclude that PDE undergoes posttranslational modifications, including the proteolytic removal of two or three terminal amino acids, and methyl esterification of the alpha-carboxyl group of the terminal cysteine residue. We speculate that the sulfhydryl group of the methylated cysteine is also lipidated to mediate membrane binding. These modifications may play an important role in delivering the nascent PDE chains to the membrane and in correctly positioning the PDE molecule in the rod disks for phototransduction.

3',5'-Cyclic-GMP Phosphodiesterases↗