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Stability Order (stability + order)
Selected AbstractsReinvestigation of the Mechanism of gem -Diacylation: Chemoselective Conversion of Aldehydes to Various gem -Diacylates and Their Cleavage under Acidic and Basic ConditionsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 2 2005Veerababurao Kavala Abstract The mechanism of gem -diacylate formation has been studied extensively using tetrabutylammonium tribromide (TBATB) as the catalyst. The reaction proceeds by a nucleophilic attack of an anhydride on an aldehydic carbonyl group, nucleophilic attack of the hemiacylate intermediate on a second molecule of the anhydride, followed by an intermolecular attack of a second acetate group to regenerate the anhydride. gem -Diacylates of various aliphatic and aromatic aldehydes were obtained directly from the reaction of a variety of aliphatic and aromatic acid anhydrides in the presence of a catalytic quantity of tetrabutylammonium tribromide (TBATB) under solvent-free conditions. A significant electronic effect was observed during its formation as well as deprotection to the corresponding aldehyde. Chemoselective gem -diacylation of the aromatic aldehyde containing an electron-donating group has been achieved in the presence of an aldehyde containing an electron-withdrawing group. Deprotection of the gem -diacylate to the parent carbonyl compound can be accomplished in methanol in presence of the same catalyst. Here again, chemoselective deprotection of the gem -diacylate of a substrate containing an electrondonating group has been achieved in the presence of a substrate containing an electron-withdrawing group. Both the acid and base stability order of the various gem -diacylates examined follow a similar order. The stability order determined from the present study is: gem -dibenzoate > gemdipivalate > gem -diisobutyrate > gem -diacetate > gem -dipropionate. All the gem -diacylals are more stable under basic conditions than acidic condition. No correlation was found between the stability order and the pKa's of the corresponding acids; rather, the stability order is directly related to the steric crowding around the carbonyl carbon. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Stability of the hydration layer of tropocollagen: A QM studyJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2010K. Pálfi Abstract Collagen is a triple helical protein, highly hydrated in nature. Bella and Berman (J Mol Biol 1996, 264, 734) have reported the structure of the first hydration layer. Water molecules form bridges of different length around the POG repeats and self assemble into left-handed helical water threads. To explore the stability of these specifically hydrated places, we have designed suitable QM models: each comprises a triple helix formed by 18 residues surrounded by 8 to 12 explicit waters. Two sets of amino acids were used, one standing for the core structural subunit of tropocollagen (POG-model) and one for its natural enzyme recognition sites (AAG-model). We have determined the stability order of the water binding places, the strongest being ,8.1 kcal mol,1, while the weakest ,6.1 kcal mol,1 per hydrogen bond. In X-ray structures, each triplet of tropocollagen is shielded by six to nine water molecules. Beside the mandatory six, the "surplus" three water molecules further strengthen the binding of all the others. However, the displacement of selected water molecules turns out to be energy neutral. These water binding places on the surface of the triple helix can provide explanation on how an almost liquid-like hydration environment exists between the closely packed tropocollagens (Henkelman et al., Magn Reson Med 1994, 32, 592). It seems that these water reservoirs or buffers can provide space for "hole conduction" of water molecules and thus contribute to the elasticity of collagen. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 [source] Interaction of Cu+ and Cu2+ ions with ,-alanine.JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 8 2002A density functional study Abstract The Cu+ and Cu2+ preferred binding sites on ,-alanine and their affinity values for this amino acid were determined at the density functional level using three different hybrid exchange correlation potentials and the 6,311++G** basis set. The results demonstrated that the two ions both give stable complexes with ,-alanine but the stability order of the metalated species and the coordination sites are different depending on the nature of the cation. In particular, the Cu+,,-alanine ground-state structure is characterized by an ,2 -N,O coordination with the nitrogen and oxygen atoms belonging to the amino and carbonyl groups, respectively. In contrast, the most stable complex of the Cu2+,,-alanine system has an ,2 -O,O coordination with the cation bonded to the ,CO2, moiety of the zwitterionic form of the amino acid. Comparison with the Cu+ and Cu2+ affinity values for glycine, computed at the same levels of theory, demonstrated that the relative values do not change significantly as different hybrid functionals are used, although the absolute affinities are strongly influenced by the choice of the hybrid potential. Copyright © 2002 John Wiley & Sons, Ltd. [source] Hydrolytic Reactions of Thymidine 5,- O -Phenyl- N -Alkylphosphoramidates, Models of Nucleoside 5,-Monophosphate ProdrugsCHEMISTRY - A EUROPEAN JOURNAL, Issue 30 2007Mikko Ora Dr. Abstract To obtain detailed data on the kinetics of hydrolytic reactions of triester-like nucleoside 5,- O -aryl- N -alkylphosphoramidates, potential prodrugs of antiviral nucleoside monophosphates, the hydrolysis of diastereomeric (RP/SP) thymidine 5,-{O -phenyl- N -[(1S)-2-oxo-2-methoxy-1-methylethyl]phosphoramidate} (3), a phosphoramidate derived from the methyl ester of L -alanine, has been followed by reversed-phase HPLC over the range from H0=0 to pH,8 at 90,°C. According to the time-dependent product distributions, the hydrolysis of 3 proceeds at pH<4 by two parallel routes, namely by nucleophilic displacement of the alaninyl ester moiety by a water molecule and by hydrolysis of the carboxylic ester linkage that allows intramolecular attack of the carboxy group on the phosphorus atom, thereby resulting in the departure of either thymidine or phenol without marked accumulation of any intermediates. Both routes represent about half of the overall disappearance of 3. The departure of phenol eventually leads to the formation of thymidine 5,-phosphate. At pH>5, the predominant reaction is hydrolysis of the carboxylic ester linkage followed by intramolecular displacement of a phenoxide ion by the carboxylate ion and hydrolysis of the resulting cyclic mixed anhydride into an acyclic diester-like thymidine 5,-phosphoramidate. The latter product accumulated quantitatively without any indication of further decomposition. Hydroxide-ion-catalyzed POPh bond cleavage of the starting material 3 occurred as a side reaction. Comparative measurements with thymidine 5,-{N -[(1S)-2-oxo-2-methoxy-1-methylethyl]phosphoramidate} (4) revealed that, under acidic conditions, this diester-like compound is hydrolyzed by PN bond cleavage three orders of magnitude more rapidly than the triester-like 3. At pH>5, the stability order is reversed, with 3 being hydrolyzed six times as rapidly as 4. Mechanisms of the partial reactions are discussed. [source] | ||||||||||