Homoconjugated (NH···N)-Hydrogen Bonds with Great Proton PolarizabilityFTIR and NMR Studies

2002 ◽  
Vol 106 (24) ◽  
pp. 5828-5831 ◽  
Author(s):  
Robert Bauer ◽  
Georg Zundel
Keyword(s):  
Hydrogen Bonds ◽  
Nmr Studies

The syntheses of 6-C-alkyl derivatives of methyl α-isomaltoside for a study of the mechanism of hydrolysis by amyloglucosidase

2001 ◽  
Vol 79 (2) ◽  
pp. 238-255 ◽  
Author(s):  
Ulrike Spohr ◽  
Nghia Le ◽  
Chang-Chun Ling ◽  
Raymond U Lemieux
Keyword(s):  
Hydrogen Bonds ◽  
Molecular Model ◽  
Aromatic Amino Acids ◽  
Hydroxyl Group ◽  
Bulk Solution ◽  
Anomeric Effect ◽  
Nmr Studies ◽  
Intermolecular Hydrogen Bonds ◽  
Model Calculations ◽  
Derivatives Of

The epimeric (6aR)- and (6aS)-C-alkyl (methyl, ethyl and isopropyl) derivatives of methyl α-isomaltoside (1) were synthesized in order to examine the effects of introducing alkyl groups of increasing bulk on the rate of catalysis for the hydrolysis of the interunit α-glycosidic bond by the enzyme amyloglucosidase, EC 3.2.1.3, commonly termed glucoamylase (AMG). It was previously established that methyl (6aR)-C-methyl α-isomaltoside is hydrolysed about 2 times faster than methyl α-isomaltoside and about 8 times faster than its S-isomer. The kinetics for the hydrolyses of the ethyl and isopropyl analogs were also recently published. As was expected from molecular model calculations, all the R-epimers are good substrates. A rationale is presented for the catalysis based on conventional mechanistic theories that includes the assistance for the decomposition of the activated complex to products by the presence of a hydrogen bond, which connects the 4a-hydroxyl group to the tryptophane and arginine units. It is proposed that activation of the initially formed complex to the transition state is assisted by the energy released as a result of both of the displacement of perturbed water molecules of hydration at the surfaces of both the polyamphiphilic substrate and the combining site and the establishment of intermolecular hydrogen bonds, i.e., micro-thermodynamics. The dissipation of the heat to the bulk solution is impeded by a shell of aromatic amino acids that surround the combining site. Such shields are known to be located around the combining sites of lectins and carbohydrate specific antibodies and are considered necessary to prevent the disruption of the intermolecular hydrogen bonds, which are of key importance for the stability of the complex. These features together with the exquisite stereoelectronic dispositions of the reacting molecules within the combining site offer a rationalization for the catalysis at ambient temperatures and near neutral pH. The syntheses involved the addition of alkyl Grignard reagents to methyl 6-aldehydo-α-D-glucopyranoside. The addition favoured formation of the S-epimers by over 90%. Useful amounts of the active R-isomers were obtained by epimerization of the chiral centers using conventional methods. Glycosylation of the resulting alcohols under conditions for bromide-ion catalysis, provided methyl (6aS)- and (6aR)-C-alkyl-hepta-O-benzyl-α-isomaltosides. Catalytic hydrogenolysis of the benzyl groups afforded the desired disaccharides. 1H NMR studies established the absolute configurations and provided evidence for conformational preferences.Key words: amyloglucosidase (AMG), exo-anomeric effect, 6-C-alkyl-α-D-glucopyranosides and isomaltosides, mechanism of enzyme catalysis.

NMR Studies of Solid Pentachlorophenol-4-Methylpyridine Complexes Exhibiting Strong OHN Hydrogen Bonds: Geometric H/D Isotope Effects and Hydrogen Bond Coupling Cause Isotopic Polymorphism

2012 ◽  
Vol 116 (46) ◽  
pp. 11370-11387 ◽  
Author(s):  
Brenda C. K. Ip ◽  
Ilya G. Shenderovich ◽  
Peter M. Tolstoy ◽  
Jaroslaw Frydel ◽  
Gleb S. Denisov ◽  
...  
Keyword(s):  
Hydrogen Bond ◽  
Hydrogen Bonds ◽  
Isotope Effects ◽  
Nmr Studies

NMR Studies of Coupled Low- and High-Barrier Hydrogen Bonds in Pyridoxal-5‘-phosphate Model Systems in Polar Solution

2007 ◽  
Vol 129 (19) ◽  
pp. 6313-6327 ◽  
Author(s):  
Shasad Sharif ◽  
Gleb S. Denisov ◽  
Michael D. Toney ◽  
Hans-Heinrich Limbach
Keyword(s):  
Hydrogen Bonds ◽  
Model Systems ◽  
Nmr Studies ◽  
Polar Solution

Dinuclear neodymium and lanthanum bis(2,6-diisopropylphenyl) phosphate complexes bearing a hydroxide ligand: catalytic activity of the Nd complex in 1,3-diene polymerization

2018 ◽  
Vol 74 (6) ◽  
pp. 673-682 ◽  
Author(s):  
Mikhail E. Minyaev ◽  
Sof'ya A. Korchagina ◽  
Alexander N. Tavtorkin ◽  
Andrei V. Churakov ◽  
Ilya E. Nifant'ev
Keyword(s):  
Catalytic Activity ◽  
Hydrogen Bonds ◽  
Molar Ratio ◽  
Vacuum Drying ◽  
Water Molecules ◽  
Nmr Studies ◽  
Space Groups ◽  
Phosphate Complex ◽  
Hydroxide Anion ◽  
Isoprene Polymerization

The reactions of K[(2,6-iPr2C6H3-O)2POO] either with LaCl3(H2O)7 or with Nd(NO3)3(H2O)6 in a 3:1 molar ratio, followed by vacuum drying and recrystallization from alkanes, have led to the formation of diaquapentakis[bis(2,6-diisopropylphenyl) phosphato]-μ-hydroxido-dilanthanum hexane disolvate, [La2(C24H34O4P)5(OH)(H2O)2]·2C6H14, (1)·2(hexane), and tetraaquatetrakis[bis(2,6-diisopropylphenyl) phosphato]-μ-hydroxido-dineodymium bis(2,6-diisopropylphenyl) phosphate heptane disolvate, [Nd2(C24H34O4P)4(OH)(H2O)4]·2C6H14, (2)·2(heptane). The compounds crystalize in the P21/n and P\overline{1} space groups, respectively. The diaryl-substituted organophosphate ligand exhibits three different coordination modes, viz. κ2 O,O′-terminal [in (1) and (2)], κO-terminal [in (1)] and μ2-κ1 O:κ1 O′-bridging [in (1) and (2)]. Binuclear structures (1) and (2) are similar and have the same unique Ln2(μ-OH)(μ-OPO)2 core. The structure of (2) consists of an [Nd2{(2,6-iPr2C6H3-O)2POO}4(OH)(H2O)4]+ cation and a [(2,6-iPr2C6H3-O)2POO]− anion, which are bound via four intermolecular O—H...O hydrogen bonds. The molecular structure of (1) displays two O—H...O hydrogen bonds between OH/H2O ligands and a κ1 O-terminal organophosphate ligand, which resembles, to some extent, the `free' [(2,6-iPr2C6H3-O)2POO]− anion in (2). NMR studies have shown that the formation of (1) undoubtedly occurs due to intramolecular hydrolysis during vacuum drying of the aqueous La tris(phosphate) complex. Catalytic experiments have demonstrated that the presence of the coordinated hydroxide anion and water molecules in precatalyst (2) substantially lowered the catalytic activity of the system prepared from (2) in butadiene and isoprene polymerization compared to the catalytic system based on the neodymium tris[bis(2,6-diisopropylphenyl) phosphate] complex, which contains neither OH nor H2O ligands.

scholarly journals NMR studies of the hydrogen bonds involving the catalytic triad of Escherichia coli thioesterase/protease I

FEBS Letters ◽  
2002 ◽  
Vol 528 (1-3) ◽  
pp. 203-206 ◽  
Author(s):  
Sergiy I Tyukhtenko ◽  
Alexandra V Litvinchuk ◽  
Chi-Fon Chang ◽  
Ruey-Jyh Leu ◽  
Jei-Fu Shaw ◽  
...  
Keyword(s):  
Escherichia Coli ◽  
Hydrogen Bonds ◽  
Catalytic Triad ◽  
Nmr Studies
Sign in / Sign up

Export Citation Format

Share Document