Kinetics and mechanism of intramolecular carboxylic acid participation in the hydrolysis of N-methoxyphthalamic acid

The rate of formation and disappearance of phthalic anhydride (PAn) intermediate in the aqueous cleavage of N-methoxyphthalamic acid (NMPA) under acidic pH was studied spectrophotometrically in mixed CH3CN-H2O solvents. The rate of formation of PAn from NMPA is almost independent of the change in acetonitrile content from 20 to 70% v/v in mixed aqueous solvents. The rate constants for the formation of PAn from NMPA are approximately 10-fold smaller than the corresponding rate constants for the formation of PAn from o-carboxybenzohydroxamic acid (OCBA). These observations are ascribed to the consequence of the occurrence of slightly different mechanisms in these reactions.     

Pyridine-sugar conjugates as potent inhibitors of enzyme-catalysed glycoside hydrolysis

A series of O- and N-linked pseudo-disaccharides incorporating simple functionalised pyridines were synthesized and demonstrated potent inhibition of the glucoamylase-catalysed reaction.     

New steroidal sapogenins from the acid hydrolysis product of the whole glycoside mixture of Welsh onion seeds

Two new steroidal sapogenins(1,2) along with five known steroidal sapogenins were isolated from the acid hydrolysis product of the whole glycoside mixture of Welsh onion(Allium fistulosum L.) seeds.Based on comprehensive spectroscopic analyses, including 2D NMR spectroscopy and mass spectrometry,their structures were elucidated as(25R)-19-norspirosta-l,3,5(10)-triene-4 -methyl-2-ol(1),(25R)-spirost-l,4-diene-3-one-2,6-diol(2),(25R)-spirost-l,4-diene-3-one-2-ol(3),(25R)-spirost-4-ene-3-one-2-ol (4),yuccagenin(5),gitogenin(6) and tigogenin(7).     

A glycoside of meristotropic acid

Chemistry of Natural Compounds -     

Remarkable supramolecular catalysis of glycoside hydrolysis by a cyclodextrin cyanohydrin

(6AR,6DR)-6A,6D-di-C-cyano-beta-cyclodextrin (3) was synthesized and shown to catalyze hydrolysis of nitrophenyl glycosides with the reaction following Michaelis-Menten kinetics. At pH 7.4 and 25 degrees C, hydrolysis of 4-nitrophenyl-beta-glucopyranoside (2) was catalyzed with KM = 15 mM, kcat = 8.2 x 10-6 s-1, and kcat/kuncat = 1217. Catalysis was observed with concentration of 3 as low as 10 muM. Hydrolysis of the corresponding alpha-glucoside, alpha-galactoside, alpha-mannoside, and 2-nitrophenyl-beta-galactoside was also catalyzed by 3, with kcat/kuncat ranging from 283 to 2147. A series of analogues of 3 was prepared and investigated for catalysis of the hydrolysis of 2: (6AR,6DR)-6A,6D-di-C-propyl-beta-cyclodextrin (9) was not catalytic, while 6A,6D-di-C-cyano-6A,6D-dideoxy-beta-cyclodextrin (12) had a low catalytic activity (kcat/kuncat = 4). A kcat/kuncat = 48 was found for 6A,6D-dialdehydo-beta-cyclodextrin dihydrate (11). It was proposed that 3 acts by general acid catalysis on the bound substrate.     

Four orders of magnitude rate increase in artificial enzyme-catalyzed aryl glycoside hydrolysis

[reaction: see text] (6AR,6DR)-6A,6D-Di-C-cyano-beta-cyclodextrin (1) and 6A,6D-di-C-cyano-alpha-cyclodextrin (2) were synthesized and shown to catalyze hydrolysis of aryl glycosides into glucose and phenol with a reaction following Michaelis-Menten kinetics. At pH 8.0 and 59 degrees C hydrolysis of 4-nitrophenyl alpha-glucopyranoside was catalyzed by 1 with KM = 10.5 +/- 1.5 mM, kcat = 1.42(+/-0.09) x 10(-4) s(-1), and kcat/kuncat = 7922. Catalysis was observed with a concentration of 1 as low as 10 microM. Hydrolysis of the other aryl glycosides containing stereochemical variation in the sugar-moiety and 4-nitro-, 2-nitro-, 2-aldehydo-, and 2,4-dinitro- were also catalyzed by 1 and 2 with kcat/kuncat ranging from 4 to 7100. Hydrolysis of a phenyl beta-d-glucoside or the thioglycoside tolylthio beta-D-glucoside was also catalyzed. From a series of prepared analogues of 1 it was found that the catalysis was associated with the hydroxyl groups alpha to the nitril groups. The monocyanohydrin 6-C-cyano-beta-cyclodextrin (3) was also found to catalyze the hydrolysis of 4-nitrophenyl beta-glucopyranoside with kcat/kuncat = 1356. It was proposed that the cyclodextrin cyanohydrins 1-3 catalyze the hydrolysis by general acid catalysis on the bound substrate.     

Carbonyl participation during the hydrolysis of aryl benzenesulphonates

Rate enhancement to the extent of about 10⁶ for the hydrolysis of aryl benzenesulphonates due to participation by the neighbouring carbonyl group is reported.     

Mechanism for hydrolysis of double six-membered ring tetraborate anion

[B₄O₅(OH)₄²⁻] is a representative borate anion with a double six-membered ring structure, but there is limited knowledge about the hydrolysis mechanisms of [B₄O₅(OH)₄²⁻]. Density functional theory-based calculations show that the tetraborate ion undergoes three-step hydrolysis to form [B(OH)₄⁻] and an ring intermediate, [B₃O₂(OH)₆⁻]. Other new structures, such as linear trimer, branched tetraborate, analogous linear tetraborate, are observed, but they are not stable in neutral systems and change to ring structures. [B₃O₂(OH)₆⁻] hydrolyzes to [B(OH)₄⁻] and [B(OH)₃] in the last two steps. The structure of borate anion and the coordination environment of the bridge oxygen atom control the hydrolysis process. [B₄O₅(OH)₄²⁻] always participates in the hydrolysis reaction, even with a decrease in concentration. [B₃O₃(OH)₄⁻], [B(OH)₄⁻], and [B(OH)₃] have different roles in “water-poor” and “water-rich” zones. Concentration and pH of solution are the key factors that affect the distribution of borate ions.     

Direct potentiometric determination of urea after urease hydrolysis

Summary A rapid potentiometric method for the determination of urea based on the electrode response of ammonium ion liberated in the ureasecatalyzed hydrolysis is described.

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Zusammenfassung Es wird eine rasch durchführbare Harnstoffbestimmung beschrieben, die auf der durch Urease katalysierten Hydrolyse und potentiometrischer Bestimmung des dabei gebildeten Ammoniaks beruht.

     

Dilute acid hydrolysis of softwoods

Whole tree chips obtained from softwood forest thinnings were converted to ethanol via a two-stage dilute acid hydrolysis followed by yeast fermentation. The chips were first impregnated with dilute sulfuric acid, then pretreated in a steam explosion reactor to hydrolyze, more than 90% of the hemicellulose and approx 10% of the cellulose. The hydrolysate was filtered and washed with water to recover the sugars. The washed fibers were then subjected to a second acid im pregnation and hydrolysis to hydrolyze as much as 45% of the reamining cellulose. The liquors from both hydrolysates were combined and fermented to ethanol by a Saccharomyces cerevisiae yeast that had been adapted to the inhibitors. Based on available hexose sugars, ethanol yields varied from 74 to 89% of theoretical. Oligosaccharide contents higher than about 10% of the total available sugar appear to have a negative impact on ethanol yield.