Search results for "acids"
showing 10 items of 3520 documents
DFT study of N–H···O hydrogen bond between model dehydropeptides and water molecule
2013
The strength of the hydrogen bond formed between a water molecule and two α,β-dehydroalanine derivatives including Ac-ΔAla-NMe2 (1) and Ac-ΔAla-NHMe (2) in comparison with standard amino acid Ac-Ala-NMe2 (3) is studied by density functional theory (with M06-2X and B3LYP functionals). Calculations were conducted for two different conformations of the peptides: extended (C5) and bent (β) with polyproline II backbone dihedral angles. The obtained results show that both dehydro and standard peptides in bent conformation form stronger hydrogen bonds with water than in the extended ones. Moreover, due to higher polarity of the N–H group of α,β-dehydroalanine residues, the H-bond in their complexe…
Bis(oxazoline) Lewis Acid Catalyzed Aldol Reactions of PyridineN-Oxide Aldehydes—Synthesis of Optically Active 2-(1-Hydroxyalkyl)pyridine Derivatives…
2006
A new, short, and simplified procedure for the synthesis of optically active pyridine derivatives from pro-chiral pyridine-N-oxides is presented. The catalytic and asymmetric Mukaiyama aldol reaction between ketene silyl acetals and 1-oxypyridine-2-carbaldehyde derivatives catalyzed by chiral copper(II)-bis(oxazoline) complexes gave optically active 2-(hydroxyalkyl)- and 2-(anti-1,2-dihydroxyalkyl)pyridine derivatives in good yields and diastereoselectivities, and in excellent enantioselectivities-up to 99 % enantiomeric excess. As a synthetic application of the developed method, a full account for the asymmetric total synthesis of a nonnatural indolizine alkaloid is provided.
Preferential localization of Lactococcus lactis cells entrapped in a caseinate/alginate phase separated system.
2013
International audience; This study aimed to entrap bioprotective lactic acid bacteria in a sodium caseinate/sodium alginate aqueous two-phase system. Phase diagram at pH = 7 showed that sodium alginate and sodium caseinate were not miscible when their concentrations exceeded 1% (w/w) and 6% (w/w), respectively. The stability of the caseinate/alginate two-phase system was also checked at pH values of 6.0 and 5.5. Lactococcus lactis subsp. lactis LAB3 cells were added in a 4% (w/w) caseinate/1.5% (w/w) alginate two-phase system at pH = 7. Fluorescence microscopy allowed to observe that the caseinate-rich phase formed droplets dispersed in a continuous alginate-rich phase. The distribution of …
Modelling of proton and metal exchange in the alginate biopolymer.
2005
Acid-base behaviour of a commercial sodium alginate extracted from brown seaweed (Macrocystis pyrifera) has been investigated at different ionic strengths (0.1or=I/mol l(-1)or=1.0) and in different supporting electrolytes (Et4NI, NaCl, KCl, LiCl, NaCl+MgCl2), with the aim of examining the influence of ionic medium on the proton-binding capacity and of quantifying the strength of interaction with light metal ions in the perspective of speciation studies in natural aqueous systems. Potentiometric ([H+]-glass electrode) and titration calorimetric data were expressed as a function of the dissociation degree (alpha) using different models (Henderson-Hasselbalch modified, Högfeldt three parameter…
Chemical-based methodologies approaches to extend the shelf life of fresh fish — a review
2021
Due to its characteristics, fresh fish is a highly perishable food with a very short shelf-life under refrigeration. Several methods have been introduced to slow down its deterioration, such as by means of oxygen depletion of the food package (vacuum packaging), or by changing the natural atmosphere that is in contact with the fresh fish (modified atmosphere packaging), or by the use of chemicals generally recognized as safe: such compounds can be directly applied (by dipping or spraying) or incorporated into packaging materials and slowly migrate to the product, exerting a hurdle effect against microbial development and lipid oxidation (active packaging). This review aims to cover the most…
Synthesis, characterization, and thermal behavior of steroidal dendrons
2004
A series of novel dendritic steroidal polyesters of first and second generation has been synthesized in convergent fashion by the use of 2,2-bis(hydroxymethyl)propionic acid as a repeating unit. The first- and second-generation hydroxy-functionalized dendrons with a variety of surface modifications were produced through the use of four bile acids: lithocholic acid (LCA), ursodeoxycholic acid (UDCA), deoxycholic acid (DCA), and cholic acid (CA). The thermal behavior of the steroidal dendrons was characterized by differential scanning calorimetry (DSC) and by thermogravimetric analysis (TGA). Finally, quantum chemical calculation methods were used to study the geometries of the dendrons and t…
Characterization and quantitation of mixtures of alkyl ether sulfates and carboxylic acids by capillary electrophoresis with indirect photometric det…
2003
The separation, characterization, and determination of mixtures of alkyl ether sulfates (AES) and fatty acids (C10-C16) in background electrolytes (BGEs) containing acetonitrile (ACN)-water mixtures is addressed. Due to inhibition of the ionization of the carboxylate groups, the migration time and the resolution between the fatty acids decreased when the water content of the BGE was reduced, but efficiency and resolution between the AES oligomers improved. The migration times increased and resolution improved by substituting 5% ACN by an equivalent amount of dioxane. A complete separation of the two surfactant classes, up to the AES oligomers with 8 ethylene oxide units (EOs) with respect t…
Improved (photo)catalytic propene hydration in a gas/solid system by using heteropolyacid/oxide composites: Electron paramagnetic resonance, acidity,…
2017
Binary materials composed of the oxides SiO2, TiO2 and N-doped TiO2 and the Keggin heteropolyacid (PW12) were prepared and physicochemically characterized. They were used as catalysts and photocatalysts for the hydration of propene to 2-propanol. The characterization of the samples, particularly the electron paramagnetic resonance (EPR) spectroscopy results and the acidity properties, were useful to explain the key role played by the PW12 in the composite materials in the thermal and photoassisted catalytic processes. The simultaneous pres-ence of heat and UV light improved the activity of PW12 in the thermal process, and the binary materials showed better (photo)catalytic activities than t…
Nucleophilic benzoylation using a mandelic acid dioxolanone as a synthetic equivalent of the benzoyl carbanion. Oxidative decarboxylation of α-hydrox…
2004
The synthesis of alkyl aryl ketones using a mandelic acid dioxolanone as a synthetic equivalent (Umpolung) of the benzoyl carbanion is reported. The methodology involves alkylation of the mandelic acid dioxolanone, hydrolysis of the dioxolanone moiety in the alkylated products and oxidative decarboxylation of the resulting alpha-hydroxyacids. The last step is carried out in a catalytic aerobic way using a Co (III) complex in the presence of pivalaldehyde under very mild conditions.
Unsaturated syn- and anti-1,2-Amino Alcohols by Cyclization of Allylic Bis-trichloroacetimidates. Stereoselectivity Dependence on Substrate Configura…
2014
Disubstituted allylic bis-imidates undergo Lewis acid catalyzed or spontaneous cyclization to oxazolines, which are precursors of unsaturated amino alcohols. Stereoselectivity of the cyclization is mainly determined by the substrate configuration. Highly selective cis-oxazoline formation is achieved starting from anti-E-bis-imidates while trans-oxazoline predominantly forms from anti-Z-bis-imidates. On the basis of DFT calculations, the stereoselectivity trends can be explained by the formation of the energetically most stable carbenium ion conformation, followed by the cyclization via most favorable bond rotations.