The Experts below are selected from a list of 3993 Experts worldwide ranked by ideXlab platform
Jesus T Vazquez - One of the best experts on this subject based on the ideXlab platform.
-
experimental evidence on the Hydroxymethyl Group conformation in alkyl β d mannopyranosides
Tetrahedron-asymmetry, 2004Co-Authors: Carlos Mayato, Rosa L Dorta, Jesus T VazquezAbstract:Abstract A rotational population study of the Hydroxymethyl Group of alkyl β- d -mannopyranosides was performed by means of CD and NMR spectroscopy. Three different benzyl, acetyl, and p -bromobenzoyl series of alkyl β- d -mannopyranosides with different chiral and nonchiral aglycons were synthesized and analyzed. Different rotational populations were observed for each series by changing the structure of the aglycon. The results showed a clear correlation between the rotational population of the Hydroxymethyl Group around the C5–C6 bond and the p K a of the bonded alcohol (aglycon). The population of the gt rotamer gradually increased as the p K a increased while that of the gg rotamer decreased and the population of the tg rotamer remained almost constant. This is explained by the exo -anomeric effect. For chiral alkyl derivatives, the results also showed a close dependence on the absolute configuration of the aglycon. Comparison of rotational population anomers revealed the dependence of the Hydroxymethyl Group on the anomeric configuration and a greater dependence on the aglycon structure in the β anomers.
-
conformational study of the Hydroxymethyl Group in α d mannose derivatives
Tetrahedron-asymmetry, 2003Co-Authors: Chaxiraxi Nobrega, Jesus T VazquezAbstract:Abstract A study of the dependence of the Hydroxymethyl Group in α- d -mannose derivatives on the aglycon and its absolute configuration was performed by means of circular dichroism (CD) and NMR data. Depending mainly on the aglycon present, the gg or the gt rotamer was the most populated, the tg rotamer having a small population. In addition, the study showed a correlation between the rotational populations and the aglycon, the population of the gt rotamer increasing as the p K a of the bonded alcohol (aglycon) increased. Furthermore, the results revealed a strong dependence on the absolute configuration of the aglycon and point to the stereoelectronic exo -anomeric effect being responsible for these rotational dependencies besides nonbonding interactions.
-
alkyl galactopyranosides rotational population dependence of the Hydroxymethyl Group on the aglycon and its absolute configuration and on the anomeric configuration
Journal of Organic Chemistry, 1998Co-Authors: Juan I Padron, Ezequiel Q Morales, Jesus T VazquezAbstract:The rotational populations of the Hydroxymethyl Group in chiral and nonchiral alkyl α- and β-galactopyranosides proved, on the basis of circular dichroism (CD) and 1H NMR data, to be dependent on the structure of the aglycon: the population of the gt and tg rotamers increased and decreased, respectively, as the pKa of the bonded alcohol (aglycon) increased, while the population of the gg rotamer remained practically constant. Furthermore, low-temperature CD measurements proved that the most stable rotamer is the gt, and not the tg. In addition, a clear correlation between the rotational populations and the absolute configuration of the chiral aglycon was observed; namely, higher and smaller gt and tg populations were observed for the (S)-alkyl β-d-galactopyranosides than for their (R)-alkyl β-d-galactopyranoside counterparts, the opposite behavior being observed for the α-anomers. The results point to the exo anomeric effect as being responsible for these rotational dependencies, as well as to nonbonding...
-
rotational population dependences of the Hydroxymethyl Group in alkyl glucopyranosides anomers comparison
Chirality, 1997Co-Authors: Juan I Padron, Jesus T VazquezAbstract:A CD and 1H NMR study of the rotamer population around the C5–C6 bond of alkyl α-d-glucopyranosides proved it to be dependent on the aglycon and its absolute configuration, showing a similar, but not identical, behavior to that of their β-anomers. Thus, the population of the gt rotamer increased as the pKa of the aglycon increased, whereas the population of the tg and gg rotamers decreased, mainly that of the tg rotamer. The results showed the existence of a clear correlation between the stereoelectronic exo-anomeric effect and the rotamer distributions, the endo-anomeric effect not being directly involved. In addition, the 1H NMR chemical shift differences (δD–δROH) of the alkyl α-d-glucopyranosides are characteristic of the absolute configuration of the aglycon, as occurred with the β-anomers. Therefore, the absolute configuration of secondary chiral alcohols or the aglyconic moiety of natural glycosides can be determined independently of the anomeric configuration, a single enantiomer being sufficient. Chirality 9:626–637, 1997. © 1997 Wiley-Liss, Inc.
Akinori Fukuda - One of the best experts on this subject based on the ideXlab platform.
-
polymer materials photopolymerization of vinylester resin having Hydroxymethyl Group
Journal of The Society of Materials Science Japan, 2000Co-Authors: Keiko Ohtsuka, Kiichi Hasegawa, Akihiro Matsumoto, Hajime Kimura, Akinori Fukuda, Mizuho Yoshimoto, Hideki FujiwaraAbstract:Vinylester resin having Hydroxymethyl Group (VEDA) was synthesized by the reaction of epoxy resin having Hydroxymethyl Group with acrylic acid. Effect of Hydroxymethyl Group on photopolymerization and after thermal curing of VEDA were studied. α-isobutyl ketone was used as phtoinitiator. Glass transition temperature (Tg) and storage modulus (E') of cured resin were increased after thermal cure. This result is considered to be due to the increase of crosslinking density of the cured resin by condensation reaction of Hydroxymethyl Group at higher temperature. The cured films of VEDA on glass, steel and poly (methyl methacrylate) plates were found to exhibit good adhesion to those substrates, resulting in introducing Hydroxymethyl Group.
-
synthesis and properties of urethane elastomer modified epoxy resin having Hydroxymethyl Group
Polymer International, 1994Co-Authors: Keiko Ohtsuka, Kiichi Hasegawa, Akinori FukudaAbstract:Synthesis and properties of urethane elastomer-modified epoxy resins were studied. The urethane elastomer-modified epoxy resins were synthesized by the reaction of a 4-cresol type epoxy compound having Hydroxymethyl Groups (EPCDA) with isocyanate prepolymer. The structure was identified by IR, 1H NMR and GPC. These epoxy resins (EPCDATDI) were mixed with a commercial epoxy resin (DGEBA) in various ratios. The mixed epoxy resins were cured with a mixture of 4,4′-diaminodiphenylmethane and 3-phenylenediamine (molar ratio 6:4) as a hardener. The curing behaviour of these epoxy resins was studied by DSC. The higher the concentration of EPCDATDI, the higher the onset temperature and the smaller the rate constant (k) of the exothermic cure reaction were. It was considered that the ratio of Hydroxymethyl Group to epoxide Group was very small and the molecular weight of EPCDATDI was large. Therefore, the accelerating effect of the Hydroxymethyl Group on the epoxide–amine reaction was cancelled by the retardant effect of increased molecular weight and viscosity, and decreased molecular motion. Toughness was estimated by Izod impact strength and fracture toughness (K1C). On addition of 10 wt% EPCDATDI with low molecular weight (Mn 6710, estimated by GPC using polystyrene standard samples), Izod impact strength and K1C increased by 70% and 60%, respectively, compared with unmodified epoxy resin. Glass transition temperatures (Tg) for the cured epoxy resins mixed with EPCDATDI measured by dynamic mechanical spectrometry were the same as those of unmodified epoxy resin. The storage modulus (E′) at room temperature decreased with increasing concentration of EPCDATDI. Toughness and dynamic mechnical behaviour of cured epoxy resin systems were studied based on the morphology.
-
synthesis and curing behaviour of urethane modified epoxy resin having Hydroxymethyl Group
Polymer International, 1993Co-Authors: Keiko Ohtsuka, Kiichi Hasegawa, Akinori FukudaAbstract:Synthesis and curing behaviour of urethane-modified epoxy resins were studied. The epoxy resins were synthesized by the reaction of 4-cresol-type epoxy resin having Hydroxymethyl Group (EPCDA) with isocyanates. First, as a model compound, EPCDAPI was synthesized by the reaction of EPCDA with phenyl isocyanate (PI). EPCDAPI was heated at 180°C and the reaction products were identified and the reaction mechanism was studied. The following reactions occurred: regeneration of Hydroxymethyl Group and PI by thermal dissociation of urethane linkage; reaction of epoxide ring and urethane linkage; reaction of epoxide ring and regenerated PI. Urethane-modified epoxy resin having Hydroxymethyl Group and urethane linkage (EPCDAMDI) was synthesized by the reaction of EPCDA with 4,4′-diphenylmethane diisocynate (MDI). The structure was identified by IR, 1H NMR and 13C NMR. The epoxy resin was cured with a mixture of 4,4′-diaminodiphenylmethane and 1.3-phenylenediamine (molar ratio 6:4) as a hardener. Curing behaviour of the epoxy resin with hardener was examined by differential scanning calorimetry (DSC). It was found that onset and peak temperatures of the exothermic cure reaction of the EPCDAMDI system were lower than those of the 2-cresol novolac-type epoxy resin (EOCN) system, and the rate constant (k) of the cure reaction at 100° C of the former system was about six times larger than that of the latter system and was the same as that of 2-cresol resol-type epoxy resin (EOCDA). Viscoelastic properties of the cured resin were studied by dynamic mechanical analysis. It was found that the tan δ of the EPCDAMDI system had two peaks and the lower temperature peak disappeared after further curing over 200° C.
-
Curing behavior of epoxy resin having Hydroxymethyl Group and different molecular weight distribution
Journal of Applied Polymer Science, 1992Co-Authors: K. Ohtsuka, Kiichi Hasegawa, Akinori Fukuda, Kazuo UedeAbstract:o-Cresol novolac-type epoxy resins having Hydroxymethyl Group were synthesized. These epoxy resins were cured with a mixture of 4,4′-diaminodiphenylmethane and m-phenylenediamine (molar ratio, 6:4) as a hardener. Effects of molecular weight distribution of epoxy resins on curing behavior were studied. Curing behavior of epoxy resins with hardener were examined by differential scanning calorimetery (DSC), and cure reaction parameters were obtained. Viscoelastic properties of the cured epoxy resins were studied by dynamic mechanical analyzer. It was found that the lower the average molecular weight of the epoxy resin, that is, the higher the concentration of Hydroxymethyl Group, the shorter the onset time of exothermal reaction, the higher the rate constant (k), and the lower the activation energy (Ea) were. It was also found that glass transition temperature (Tg) of fully cured epoxy resins was higher than those of fully cured general novolac-type epoxy resins.
-
curing behavior of epoxy resin having Hydroxymethyl Group
Journal of Applied Polymer Science, 1992Co-Authors: Keiko Ohashi, Kiichi Hasegawa, Akinori Fukuda, Kazuo UedeAbstract:A new type of epoxy resin having Hydroxymethyl Group was synthesized. This epoxy resin was mixed with commercial epoxy resin in various ratios. The mixed epoxy resins were cured with a mixture of 4,4′-diaminodiphenylmethane and m-phenylenediamine (molar ratio, 6 : 4) as a hardener. Curing behavior of the epoxy resin systems with the hardener was examined by DSC and TG-DSC, and parameters of cure reaction were obtained. Viscoelastic properties of cured resin were studied by dynamic mechanical analyzer. It was found that the higher the amount of epoxy resin having Hydroxymethyl Group, the lower the activation energy (Ea) and the higher the rate constant (k) were. It was also found that the higher the amount of the epoxy resin having Hydroxymethyl Group, the better heat resistance the fully-cured resin had. These results were explained as follows: Hydroxymethyl Group accelerated an epoxideamine reaction. The crosslinking density of the cured resin was increased because in the Hydroxymethyl Group occurred a condensation reaction above 200°C.
Jakob Wohlert - One of the best experts on this subject based on the ideXlab platform.
-
temperature dependence of Hydroxymethyl Group rotamer populations in cellooligomers
Journal of Physical Chemistry B, 2015Co-Authors: Thibault Angles Dortoli, Nils A Sjoberg, Polina Vasiljeva, Jonas Lindman, Goran Widmalm, Malin Bergenstrahlewohlert, Jakob WohlertAbstract:Empirical force fields for computer simulations of carbohydrates are often implicitly assumed to be valid also at temperatures different from room temperature for which they were optimized. Herein, the temperature dependence of the Hydroxymethyl Group rotamer populations in short oligosaccharides is investigated using molecular dynamics simulations and NMR spectroscopy. Two oligosaccharides, viz., methyl β-cellobioside and β-cellotetraose were simulated using three different carbohydrate force fields (CHARMM C35, GLYCAM06, and GROMOS 56Acarbo) in combination with different water models (SPC, SPC/E, and TIP3P) using replica exchange molecular dynamics simulations. For comparison, Hydroxymethyl Group rotamer populations were investigated for methyl β-cellobioside and cellopentaose based on measured NMR 3JH5,H6 coupling constants, in the latter case by using a chemical shift selective NMR-filter. Molecular dynamics simulations in combination with NMR spectroscopy show that the temperature dependence of the h...
-
temperature dependence of Hydroxymethyl Group rotamer populations in cellooligomers b
The Journal of Physical Chemistry, 2015Co-Authors: Thibault Anglesdortoli, Nils A Sjoberg, Polina Vasiljeva, Jonas Lindman, Goran Widmalm, Malin Bergenstrahlewohlert, Jakob WohlertAbstract:Empirical force fields for computer simulations of carbohydrates are often implicitly assumed to be valid also at temperatures different from room temperature for which they were optimized. Herein, the temperature dependence of the Hydroxymethyl Group rotamer populations in short oligosaccharides is investigated using molecular dynamics simulations and NMR spectroscopy. Two oligosaccharides, viz., methyl β-cellobioside and β-cellotetraose were simulated using three different carbohydrate force fields (CHARMM C35, GLYCAM06, and GROMOS 56Acₐᵣbₒ) in combination with different water models (SPC, SPC/E, and TIP3P) using replica exchange molecular dynamics simulations. For comparison, Hydroxymethyl Group rotamer populations were investigated for methyl β-cellobioside and cellopentaose based on measured NMR ³JH₅,H₆ coupling constants, in the latter case by using a chemical shift selective NMR-filter. Molecular dynamics simulations in combination with NMR spectroscopy show that the temperature dependence of the Hydroxymethyl rotamer population in these short cellooligomers, in the range 263–344 K, generally becomes exaggerated in simulations when compared to experimental data, but also that it is dependent on simulation conditions, and most notably properties of the water model.
Goran Widmalm - One of the best experts on this subject based on the ideXlab platform.
-
temperature dependence of Hydroxymethyl Group rotamer populations in cellooligomers
Journal of Physical Chemistry B, 2015Co-Authors: Thibault Angles Dortoli, Nils A Sjoberg, Polina Vasiljeva, Jonas Lindman, Goran Widmalm, Malin Bergenstrahlewohlert, Jakob WohlertAbstract:Empirical force fields for computer simulations of carbohydrates are often implicitly assumed to be valid also at temperatures different from room temperature for which they were optimized. Herein, the temperature dependence of the Hydroxymethyl Group rotamer populations in short oligosaccharides is investigated using molecular dynamics simulations and NMR spectroscopy. Two oligosaccharides, viz., methyl β-cellobioside and β-cellotetraose were simulated using three different carbohydrate force fields (CHARMM C35, GLYCAM06, and GROMOS 56Acarbo) in combination with different water models (SPC, SPC/E, and TIP3P) using replica exchange molecular dynamics simulations. For comparison, Hydroxymethyl Group rotamer populations were investigated for methyl β-cellobioside and cellopentaose based on measured NMR 3JH5,H6 coupling constants, in the latter case by using a chemical shift selective NMR-filter. Molecular dynamics simulations in combination with NMR spectroscopy show that the temperature dependence of the h...
-
temperature dependence of Hydroxymethyl Group rotamer populations in cellooligomers b
The Journal of Physical Chemistry, 2015Co-Authors: Thibault Anglesdortoli, Nils A Sjoberg, Polina Vasiljeva, Jonas Lindman, Goran Widmalm, Malin Bergenstrahlewohlert, Jakob WohlertAbstract:Empirical force fields for computer simulations of carbohydrates are often implicitly assumed to be valid also at temperatures different from room temperature for which they were optimized. Herein, the temperature dependence of the Hydroxymethyl Group rotamer populations in short oligosaccharides is investigated using molecular dynamics simulations and NMR spectroscopy. Two oligosaccharides, viz., methyl β-cellobioside and β-cellotetraose were simulated using three different carbohydrate force fields (CHARMM C35, GLYCAM06, and GROMOS 56Acₐᵣbₒ) in combination with different water models (SPC, SPC/E, and TIP3P) using replica exchange molecular dynamics simulations. For comparison, Hydroxymethyl Group rotamer populations were investigated for methyl β-cellobioside and cellopentaose based on measured NMR ³JH₅,H₆ coupling constants, in the latter case by using a chemical shift selective NMR-filter. Molecular dynamics simulations in combination with NMR spectroscopy show that the temperature dependence of the Hydroxymethyl rotamer population in these short cellooligomers, in the range 263–344 K, generally becomes exaggerated in simulations when compared to experimental data, but also that it is dependent on simulation conditions, and most notably properties of the water model.
-
Hydroxymethyl Group conformation in saccharides structural dependencies of 2 j hh 3 j hh and 1 j ch spin spin coupling constants
Journal of Organic Chemistry, 2002Co-Authors: Roland Stenutz, Goran Widmalm, Ian Carmichael, Anthony S SerianniAbstract:Experimental and theoretical methods have been used to correlate (2)J(HH) and (3)J(HH) values within the exocyclic Hydroxymethyl Groups (CH(2)OH) of saccharides with specific molecular parameters, and new equations are proposed to assist in the structural interpretation of these couplings. (3)J(HH) depends mainly on the C-C torsion angle (omega) as expected, and new Karplus equations derived from J-couplings computed from density functional theory (DFT) in a model aldopyranosyl ring are in excellent agreement with experimental values and with couplings predicted from a previously reported general Karplus equation. These results confirm the reliability of DFT-calculated (1)H-(1)H couplings in saccharides. (2)J(HH) values depend on both the C-C (omega) and C-O (theta) torsions. Knowledge of the former, which may be derived from other parameters (e.g., (3)J(HH)), allows theta to be evaluated indirectly from (2)J(HH). This latter approach complements more direct determinations of theta from (3)J(HCOH) and potentially extends these more conventional analyses to O-substituted systems lacking the hydroxyl proton. (1)J(CH) values within Hydroxymethyl fragments were also examined and found to depend on r(CH), which is modulated by specific bond orientation and stereoelectronic factors. These latter factors could be largely, but not completely, accounted for by C-C and C-O torsional variables, leading to only semiquantitative treatments of these couplings (details discussed in the Supporting Information). New equations pertaining to (2)J(HH) and (3)J(HH) have been applied to the analysis of Hydroxymethyl Group J-couplings in several mono- and oligosaccharides, yielding information on C5-C6 and/or C6-O6 rotamer populations.
Anthony S Serianni - One of the best experts on this subject based on the ideXlab platform.
-
4jcocch and 4jcccch as probes of exocyclic Hydroxymethyl Group conformation in saccharides
Journal of Organic Chemistry, 2005Co-Authors: Qingfeng Pan, Thomas Klepach, Ian Carmichael, Meredith Reed, Anthony S SerianniAbstract:[structure: see text] 1H NMR spectra of aldohexopyranosyl rings containing 13C-enrichment at either C1 or C3 reveal the presence of long-range 4J(C1,H6R/S) and 4J(C3,H6R/S) whose magnitudes depend mainly on the O5-C5-C6-O6 torsion angle. Using theoretical calculations (density functional theory, DFT; B3LYP/6-31G*) and conformationally constrained experimental model compounds, the magnitudes and signs of 4J(C1,H6R/S) and 4J(C3,H6R/S) have been established, and their dependencies on the geometry of the C1-O5-C5-C6-H6R/S and C3-C4-C5-C6-H6R/S coupling pathways, respectively, were determined. The latter dependencies mimic that observed previously for 4J(HH) in aliphatic compounds such as propane. DFT calculations also showed that inclusion of non-Fermi contact terms is important for accurate predictions of 4J(CH) values. Application to methyl alpha- and beta-D-glucopyranosides reveals different rotameric distributions about their Hydroxymethyl Groups, with the beta-anomer enriched in the gt rotamer, in agreement with recent multi-J redundant coupling analyses. 4J(C1,H6R/S) and 4J(C3,H6R/S) are expected to complement other recently developed J-couplings for the assignment of Hydroxymethyl Group conformation in oligosaccharides containing 1,6-glycosidic linkages.
-
Hydroxymethyl Group conformation in saccharides structural dependencies of 2 j hh 3 j hh and 1 j ch spin spin coupling constants
Journal of Organic Chemistry, 2002Co-Authors: Roland Stenutz, Goran Widmalm, Ian Carmichael, Anthony S SerianniAbstract:Experimental and theoretical methods have been used to correlate (2)J(HH) and (3)J(HH) values within the exocyclic Hydroxymethyl Groups (CH(2)OH) of saccharides with specific molecular parameters, and new equations are proposed to assist in the structural interpretation of these couplings. (3)J(HH) depends mainly on the C-C torsion angle (omega) as expected, and new Karplus equations derived from J-couplings computed from density functional theory (DFT) in a model aldopyranosyl ring are in excellent agreement with experimental values and with couplings predicted from a previously reported general Karplus equation. These results confirm the reliability of DFT-calculated (1)H-(1)H couplings in saccharides. (2)J(HH) values depend on both the C-C (omega) and C-O (theta) torsions. Knowledge of the former, which may be derived from other parameters (e.g., (3)J(HH)), allows theta to be evaluated indirectly from (2)J(HH). This latter approach complements more direct determinations of theta from (3)J(HCOH) and potentially extends these more conventional analyses to O-substituted systems lacking the hydroxyl proton. (1)J(CH) values within Hydroxymethyl fragments were also examined and found to depend on r(CH), which is modulated by specific bond orientation and stereoelectronic factors. These latter factors could be largely, but not completely, accounted for by C-C and C-O torsional variables, leading to only semiquantitative treatments of these couplings (details discussed in the Supporting Information). New equations pertaining to (2)J(HH) and (3)J(HH) have been applied to the analysis of Hydroxymethyl Group J-couplings in several mono- and oligosaccharides, yielding information on C5-C6 and/or C6-O6 rotamer populations.
