Molecular conformations at the cellulose–water interface

¹³C-NMR chemical shifts were measured for C-4 and C-6 in a collection of eight crystalline glucoses and glucosides. The influence of the hydroxymethyl conformation was greater at C-4 than at C-6, with mean chemical shifts for gauche–trans molecules displaced 3.1 ppm (C-4) and 2.5 ppm (C-6) relative to gauche–gauche molecules. This information was used to interpret ¹³C-NMR spectra of crystalline celluloses. Chemical shifts for C-4 in the crystallite cores of celluloses I and II differed by just 0.2 ppm, but the corresponding chemical shifts for well-ordered crystallite surfaces differed by 3.0 ppm. The separation between crystallite-surface signals was attributed to different hydroxymethyl conformations at the cellulose–water interface, i.e., gauche–gauche and gauche–trans on crystallites of cellulose I and cellulose II, respectively. A broad C-4 signal in the spectrum of cellulose II indicated gauche–gauche conformations in disordered cellulose. Chemical shifts for C-6 were consistent with these conformations.     

Two ethyl 2-deoxy-α-d-hexo-3,7-pyranoso-3-octulosonate derivatives

In each of the two pyran­oid sugars, ethyl 2-deoxy-4,5,6,8-tetra-O-acetyl-α-d -gluco-3,7-pyran­oso-3-octulosonate, C₁₈H₂₆O₁₂, and ethyl 2-deoxy-4,5,6,8-tetra-O-benzyl-α-d -galacto-3,7-pyranoso-3-octulosonate, C₃₈H₄₂O₈, the anomeric configuration is α. The acetoxy­methyl substituent on the hexo­pyran­ose ring of the former compound and the ethoxy­carbonyl­methyl substituents in both sugars all have the gauche–trans conformation, while the benzyl­oxy­methyl substituent of the galacto­pyran­ose sugar has the trans–gauche conformation. In each structure, the anomeric hydroxy group forms an intramolecular hydrogen bond with the carbonyl O atom of the ethoxy­carbonyl­methyl substituent.     

Oxygen‐barrier properties of oriented and heat‐set poly(ethylene terephthalate)

The improvement in the oxygen‐barrier properties of poly(ethylene terephthalate) (PET) by orientation and heat setting was examined. Orientation was carried out at 65 °C by constrained uniaxial stretching to a draw ratio of about 4. Heat setting was performed at temperatures from 90 to 160 °C with the specimen taut. Orientation decreased the permeability of PET to almost one‐third that of the unoriented, amorphous polymer because of decreases in both the diffusion coefficient and the solubility coefficient. The proposed two‐phase model for oriented PET consisted of a permeable isotropic amorphous phase (density = 1.335 g/cm³) with ethylene linkages predominately in the gauche conformation and an impermeable oriented phase (density = 1.38 g/cm³) with ethylene linkages that had transformed from the gauche conformation to the trans conformation during stretching. Chain segments in the trans conformation did not possess crystalline order; instead, they were viewed as forming an ordered amorphous phase. Crystallization by heat setting above the glass‐transition temperature did not dramatically affect the permeability. However, a decrease in the diffusion coefficient, offset by an increase in the solubility coefficient, indicated that crystallization affected the barrier properties of the permeable amorphous phase. Analysis of the barrier data, assuming a two‐phase model with variable density for both the permeable and impermeable phases, revealed that the impermeable phase density increased during crystallization, approaching a value of 1.476 g/cm³. This value is consistent with previous measurements of the density of the defective crystalline phase in PET. The density of the permeable amorphous phase decreased concurrently to about 1.325 g/cm³, indicating the appearance of additional free volume. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1679–1686, 2000     

Infrared spectra of some isotopomers of isopropylamine: A theoretical study

The normal‐mode frequencies and the corresponding vibrational assignments of some isotopomers of isopropylamine are examined theoretically using the Gaussian 94 set of quantum chemistry codes at the MP2 level of theory using a 6‐311g** basis set. In particular deuteration of the NH₂ hydrogen atoms as well as the deuteration of the CH₃ hydrogen atoms, the α‐CH hydrogen atom, and various combinations of deuteration of isopropylamine are examined. Correction factors for predominant vibrational motions are reported and compared for a number of isotopomers of s‐trans‐ and gauche‐isopropylamine. Comparison of theoretically predicted infrared spectra with experimental spectra is made for isotopomers for which experimental data exist. Enthalpy, entropy, and Gibbs free energy changes are considered for the trans→gauche transformation. An amino torsional potential is deduced and barrier heights are calculated for the trans→gauche, gauche→trans, and gauche→gauche transformations. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 109–126, 1999     

Effect of pH on the rotational conformations of 1,3‐diamino‐2‐hydroxypropane

The effect of pH on the rotational conformations of 1,3‐diamino‐2‐hydroxypropane in aqueous solution was investigated by proton NMR. Both the observed chemical shifts and coupling constants were used to calculate experimental pK_a values. The observed couplings were correlated with the expected couplings for the various possible staggered conformations to try to determine the pattern of conformations for the diamine and its conjugate acids. The best fits suggested a modest preference for the gauche–gauche conformation, especially at low pH, where the diprotonated hydroxydiamine predominates. In methanol, dimethyl sulfoxide and trichloromethane solutions, it was only possible to evaluate the conformational equilibria of the diamine. Slow proton exchange, which caused uncertainties in both chemical shifts and couplings for the monoprotonated and unprotonated diamine, nullified efforts to determine whether or not hydrogen bonding was important for these species in less polar solvents. Copyright © 2002 John Wiley & Sons, Ltd.     

Infrared studies of drawn polyethylene. II. Orientation behavior of highly drawn linear and ethyl-branched polyethylene

Infrared dichroism is employed to study the orientation of chain molecules in linear and ethyl-branched polyethylene in the crystalline and noncrystalline regions during drawing and subsequent annealing. A crystalline (1894 cm^?1) and a noncrystalline (1368 cm^?1) band, as well as the bands at 909 cm^?1 and 1375 cm^?1 resulting from vinyl endgroups and methyl endgroups and sidegroups, are studied. For these bands relative orientation functions are derived and compared as a function of draw ratio and annealing temperature. It is shown that the relative orientation functions as derived from the dichroism of the noncrystalline, vinyl and methyl bands follow the same curve while the orientation function for the crystalline bands does not. These results support a two-phase model for partially crystalline polyethylene and additionally favor segregation of the endgroups and sidegroups in the noncrystalline component during crystallization. It is further shown that shrinkage occurs at the temperature at which the noncrystalline chain molecules start to disorient. From the dichroism of the methyl groups in ethyl-branched polyethylene, a value for the mean orientation of the noncrystalline chain molecules is calculated. We obtain for the orientation function of the noncrystalline regions at highest draw ratios (λ = 15–20), f = 0.35–0.57, while the chain molecules in the crystallites are nearly perfectly oriented (f ≈ 1.0). On the assumption that the noncrystalline component consists of folds, tie molecules, and chain ends, the different contributions of these components to the overall orientation are estimated. From these the relative number of CH₂ groups incorporated into folds, tie molecules, and cilia can be derived. Further, on the basis of a simple structural model, the relative number of chains on the crystal surface contributing to the different noncrystalline components and their average length are estimated.     

Solid‐state NMR analyses of the crystalline–noncrystalline structure and its thermal changes for ethylene ionomers

The crystalline–noncrystalline structure and its structural changes from thermal treatments for ethylene ionomers have been investigated with solid‐state ¹³C and ²³Na NMR spectroscopy. ¹³C spin–lattice relaxation time (T_1C) measurements reveal that as‐received ethylene ionomers have much enhanced molecular mobility in the crystalline region in comparison with conventional polyethylene samples. By appropriate annealing, however, polyethylene‐like morphological features reflecting T_1C behavior can also be observed. ¹³C spin–spin relaxation time (T_2C) measurements for the noncrystalline region reveal the existence of two components with different T_2C values, and these two components have been assigned to the crystalline–amorphous interfacial and rubbery–amorphous components. These results indicate that the structure of the major part of the noncrystalline region in the ethylene ionomers is similar to that of bulk‐crystallized polyethylene samples, regardless of possible ionic aggregates. The origin of the lower temperature endothermic peak in the heating process of the differential scanning calorimetry curve observed for the as‐received sample has also been examined somewhat in detail. As a result, it is proposed that the melting of smaller crystallites produced during storage at room temperature is the origin of the lower temperature peak. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1142–1153, 2002     

Spectroscopic characterization of the rotational conformations in the disordered phase of poly(ethylene terephthalate)

Recently developed data processing techniques have been applied to determine the complete spectra of the trans and gauche isomers of PET and to use these spectra to determine the conformational composition of PET samples isothermally crystallized from the glassy state between 75 and 240°C. A linear correlation was found between normalized absorbance of conformational bands and density measurements, with the trans bands showing a positive slope and the gauche bands a negative slope. Extrapolation of the results from trans bands to zero percent of trans yields the density of the pure gauche isomer, 1.326 ± 0.002 g/cm³. Similarly, from the gauche bands the density of 100% crystalline trans is found to be 1.510 ± 0.014 g/cm³. Meanwhile, the density of the amorphous trans is calculated to be 1.430 ± 0.003 g/cm³. From these values and the combined density and infrared measurements, the content of trans isomers in the amorphous phase is calculated. It is found that the content of amorphous trans is a function of both annealing temperature and time. It is observed that the amorphous trans content decreases as the crystalline trans content increases. At high crystallinity, the amorphous trans content approaches zero. These amorphous trans isomers are associated with the extended units making up the interlamellar links. The loss of these extended trans interlamellar links is reflected in the mechanical properties of PET.     

NMR and theoretical conformational investigations of some 3-substituted propionitriles

The gauche/trans rotational isomerism of the compounds PhXCH₂CH₂CN (X = O, NH, S) is investigated by means of ¹ H NMR, IR, semiempirical MO and molecular mechanics methods. It is shown that in nonpolar solvents the compounds with X = O and X = S possess similar conformational behaviour (favoured trans conformation) whereas that with X = NH appears in the gauche favoured form. This difference is explained by the presence of a NH ... CN hydrogen bond. The ¹³C chemical shift/carbon atomic charge relationship is also investigated.     

FTIR spectroscopic investigation of thermal effects in semi‐syndiotactic polypropylene

The temperature‐dependent behavior of individual components within metallocene‐catalyzed semisyndiotactic polypropylenes (semi‐sPP) with a wide range of stereoregular content (26 to 96% rr) is studied using Fourier transform infrared (FTIR) spectroscopy and temperature‐modulated differential scanning calorimetry (DSC). Changes in sensitive, high‐resolution absorbance spectra are observed as melt‐slow‐cooled thin films are subjected to stepwise temperature increases. In general, spectral bands previously identified as being sensitive to ordered structures (e.g., conformed chains, crystal morphs) appear to follow overall trends of shifting to lower wavenumbers (energies), broadening, and decreasing in peak area intensity as temperature increases. Peaks that appear due to “splitting” (observed in more stereoregular materials) show a trend toward coalescence as temperature increases; this corresponds to a gradual loss of chain conformational order. Gauche‐gauche‐trans‐trans (ggtt)_n helical and all‐trans (tttt)_n planar zigzag‐conformed chains that participate in the crystalline‐amorphous interfacial region (“mesophase”) appear to be more stable (i.e., they do not lose their conformational order as easily) with increasing temperature in materials with a greater degree of syndiotacticity. Moreover, IR data correspond well with modulated DSC endotherms located near 50 °C and 70 °C. At each transition temperature—thought to represent, respectively, a thermally driven chain conformation from planar zigzags to helices, and a dynamic disorder of helices marked by rapid gauche ? trans isomerization—the IR absorbance ratio, A₉₇₈/A₉₆₃, which represents the relative population of helical chains, undergoes an accelerated decrease. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 439–461, 2005     

A conformational study of ethylene glycol

Ab initio calculations were performed for some different conformers of 1,2-ethanediol in order to reveal their relative energies. The equilibrium conformation is of gauche type with a comparatively weak intramolecular hydrogen bond. The energy of the all-trans conformer is 3 kcal/mol above the minimum.     

Quantum chemical analysis of the energetics of the anti and gauche conformers of ethanol

Ethanol displays two stable conformers, the classic anti (or trans) form and a gauche conformation in which the hydroxyl hydrogen points toward one of the methyl hydrogens. Surprisingly, the two forms have nearly equal energies, and in the vapor phase the gauche form predominates because of its twofold degeneracy. An analysis of the energetics of these conformers based on natural bond orbital analysis helps to explain the apparently anomalous near degeneracy of these conformers.     

Metal nanocomposite films prepared in situ from PVA and silver nitrate. Study of the nanostructuration process and morphology as a function of the in situ routes

Cast‐hybrid films composed of polyvinyl alcohol (PVA) and silver nitrate were treated according to three different ways, thermal annealing, UV‐irradiation, and chemical reduction by a borohydride solution, to obtain PVA/silver nanocomposite films. The nanostructuration process was studied as a function of the treatment conditions, and discussed as a function of the mobility state of the polymer chains in the nanocomposite matrix during treatment. A homogeneous dispersion of crystalline silver nanoparticles was obtained by thermal annealing above T_g and below T_m and UV‐lamp irradiation below T_g. For these two treatments, the major processing parameters were the annealing temperature and time and the UV‐exposure time, respectively. For low‐conversion rate in Ag(0), the films evolved upon ageing at room temperature. Totally different morphology and Ag(0) conversion were achieved by chemical reduction in a borohydride solution. All the silver ions were reduced into Ag(0), and crystalline silver nanoparticles layers parallel to the film surface were observed after the treatment. This morphology was related to the high‐swollen state of the polymer matrix during treatment. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2062–2071, 2008     

NMR Conformational Study of Aminoalkylbenzamides,Aminoalkyl-o-anisamides,and Metoclopramide,a Dopamine Receptor Antagonist

The conformational behaviour of metoclopramide, a neuroleptic benzamide, and model compounds was investigated byt ¹ H-NMR spectroscopy. An intramolecular amide-methoxy H-bond is shown to exist in CDCl₃-solution, but not in D₂O-solution, independently of the length and protonation state of the basic side-chain. This H-bond creates a virtual cycle which may be a key feature for the binding of neuroleptic benzamides to the dopamine receptor. The conformational behaviour of the aminoethyl side-chain is shown to be markedly condition-dependent. For metoclopramide and its analogues in their protonated form, the gauche- and trans- rotamers have identical energies in D₂O-as well as in CDCl₃-solutins. For the non-protonated molecules, the trans-rotamer is favoured in D₂O-solutin, while the gauche-rotamer is favoured in CDCl₃-solution (ΔG°?|0.5|kcal/mol in both cases). The side-chain conformation of neuroleptic benzamides is discussed in terms of receptor affinity.     

Unique morphological and thermal behaviors of reorganized poly(ethylene terephthalates)

Bulk poly(ethylene terephthalate) PET has been reorganized both morphologically and conformationally by processing from its inclusion complex (IC) formed with γ‐cyclodextrin (CD). In the narrow channels of its γ‐CD‐IC crystals the included guest PET chains are isolated from neighboring PET chains and the ethylene glycol (EG) units adopt the highly extended g±tg? kink conformations, whose cross‐sectional diameters are ～80% of the diameter of the fully extended, all‐trans crystalline PET conformer, though they are nearly (～95%) as extended. When the highly extended, unentangled guest PET chains are coalesced from their γ‐CD‐IC crystals by exposure to hot water, host γ‐CDs are removed and the PET chains are presumably consolidated into a bulk sample with a morphology and constituent chain conformations not normally found in PET samples solidified from their randomly coiling, possibly entangled, disordered melts and solutions. Observations by polarized light and atomic force microscopies provide visual evidence for widely different semicrystalline morphologies developed in coalesced and as‐received PETs when crystallized from their melts, with possibly chain extended, small crystals and spherulitic, chain‐folded, large crystals, respectively. DSC observations reveal that coalesced PET is rapidly crystallizable from the melt, while as‐received PET is slow to crystallize and is easily quenched into a totally amorphous sample. Analyses of ¹³C‐NMR data strongly indicate that the PET chains in the noncrystalline regions of the coalesced sample remain predominantly in the highly extended kink conformations, with g±tg? EG units, which are required by their inclusion into PET‐γ‐CD‐IC crystals, while the predominantly amorphous PET chains in the as‐received sample have high concentrations of gauche± ? CH₂? CH₂? and trans ? O? CH₂? ,? CH₂? O? EG bond conformations. ¹³C‐NMR T₁(¹³C) and T_1ρ(¹H) relaxation studies show no evidence of a glass transition for coalesced PET, while the as‐received sample shows abrupt changes in both the MHz [T₁(¹³C)] and kHz [T_1ρ(¹H)] motions at T ～ T_g. Preliminary observations of differences in their macroscopic properties are attributed to the very different morphologies and conformations of the constituent chains in these PET samples. Apparently the kink conformers in the noncrystalline regions of coalesced PET are at least partially retained for extended periods even in the melt and are rapidly crystallized upon cooling. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 386–394, 2004     

The gauche-trans transformation of monoethanolamine in gas phase

Monoethanolamine is a model molecule for biological systems and widespread ligand in coordination chemistry; it has a large number of isomers, of which gauche and trans are the most stable in gas and crystalline phases, respectively. The work is devoted to non-empirical study of conformational transformations of different gauche and trans isomers of individual monoethanolamine molecule. These transformations was shown to be determined by the features of formation and cleavage of intramolecular hydrogen bonds O-H?N and N-H?O and the internal rotation of substituted methyl groups. Assessments were obtained for the free energies of conformers and activation energies of their mutual transformations.     

Conformational studies of new pseudotripeptide with pyrazine amidoxime motif and simplified analogs using IR,NMR spectroscopy,and molecular dynamic simulations

Solution structures of new pyrazine-based pseudotripeptide with amidoxime function and simplified pseudodipeptide analogs were determined by a combination of IR and NMR spectroscopic studies and molecular dynamic simulations using explicit chloroform as a solvent. It was found that proline-phenylalanine dipeptide residue and amidoxime moiety in o-position are essential for intramolecular hydrogen bonding including a seven-membered γ-turn formation. In addition, a cis/trans equilibrium study was present for prolyl amides in polar solvents (D₂O and DMSO). A phenylalanine substituent was found to exhibit profound effect on thermodynamic parameters in prolyl peptides. The presence of intramolecular hydrogen bonds dramatically increases the amount of trans isomer in non-hydrogen-bonding CHCl₃ and significantly favor cis isomer in hydrogen-bonding solvents such as DMSO and D₂O. All molecules are not cytotoxic therefore they can be further studied in relation to potent biological activities.

     

Effect of morphology and degree of crystallinity on the infrared absorption spectra of linear polyethylene

The infrared absorption spectra of selected crystalline and noncrystalline bands were studied in bulk-crystallized specimens of linear polyethylene which encompassed the extremely wide density range of 0.92–0.99 g./cm.³. The analysis of the data obtained at room temperature yield degrees of crystallinity by infrared methods which are in very good accord with the values deduced from the density measurements. Studies of the infrared spectra as a function of temperature give fusion curves which are in agreement with those obtained by thermodynamic methods. However, in order to obtain these latter results cognizance must be taken of the large negative temperature coefficient of the specific extinction coefficients of the crystalline bands from room temperature to the melting point. The necessary data to account for this phenomena were obtained from studies of the spectra of the n-paraffin, C₉₄H₁₉₀, where molecular crystals are formed. Analysis of the two gauche bands, at 1352 and 1303 cm.^?1, which are assigned to the noncrystalline regions demonstrate that for bulk-crystallized samples of lowest densities the intensity ratio at room temperature is identical to that expected from the pure melt at this temperature. The conclusion is thus reached that the noncrystalline regions in these cases and the pure melt are structurally very similar. For samples of higher density, where the crystallite size is comparable to the extended chain length, the intensity ratio of the two gauche bands is altered. This change could reflect a change in the sequential distribution of gauche bonds. This intensity ratio for crystals formed from dilute solution is very similar to that for the high-density bulk-crystallized material and indicates a similarity in structure of the noncrystalline regions in the two cases.     

Unusual Folding Propensity of an Unsubstituted β,γ‐Hybrid Model Peptide: Importance of the C?H⋅⋅⋅O Intramolecular Hydrogen Bond

The single‐crystal X‐ray diffraction analysis of a β,γ‐hybrid model peptide Boc‐β‐Ala‐γ‐Abu‐NH₂ revealed the existence of four crystallographically independent molecules ( A , B , C and D conformers) in the asymmetric unit. The analysis revealed that unusual β‐turn‐like folded structures predominate, wherein the conformational space of non‐proteinogenic β‐Ala and γ‐Abu residues are restricted to gauche‐gauche‐skew and skew‐gauche‐trans‐skew orientations, respectively. Interestingly, the U‐shaped conformers are seemingly stabilised by an effective unconventional C? H ??? O intramolecular hydrogen bond, encompassing a non‐covalent 14‐membered ring‐motif. Taking into account the signs of torsion angles, these conformers could be grouped into two distinct categories, A / B and C / D , establishing the incidence of non‐superimposable stereogeometrical features across a non‐chiral one‐component peptide model system, that is, “mirror‐image‐like” relationships. The natural occurrence of β‐Ala and γ‐Abu entities in various pharmacologically important molecules, coupled with their biocompatibilities, highlight how the non‐functionalised β,γ‐hybrid segment may offer unique advantages for introducing and/or manipulating a wide spectrum of biologically relevant hydrogen bonded secondary structural mimics in short synthetic peptides.     

Covalently Linked Bis(Amido-Corroles): Inter- and Intramolecular Hydrogen-Bond-Driven Supramolecular Assembly

Four bis-corroles linked by diamide bridges were synthesized through peptide-type coupling of a trans-A₂B-corrole acid with aliphatic and aromatic diamines. In the solid state, the hydrogen-bond pattern in these bis-corroles is strongly affected by the type of solvent used in the crystallization process. Although intramolecular hydrogen bonds play a decisive role, they are supported by intermolecular hydrogen bonds and weak N−H⋅⋅⋅π interactions between molecules of toluene and the corrole cores. In an analogy to mono(amido-corroles), both in crystalline state and in solutions, the aliphatic or aromatic bridge is located directly above the corrole ring. When either ethylenediamine or 2,3-diaminonaphthalene are used as linkers, incorporation of polar solvents into the crystalline lattice causes a roughly parallel orientation of the corrole rings. At the same time, both NHCO⋅⋅⋅NH corrole hydrogen bonds are intramolecular. In contrast, solvation in toluene causes a distortion with one of the hydrogen bonds being intermolecular. Interestingly, intramolecular hydrogen bonds are always formed between the –NHCO– functionality located further from the benzene ring present at the position 10-meso. In solution, the hydrogen-bonds pattern of the bis(amido-corroles) is strongly affected by the type of the solvent. Compared with toluene (strongly high-field shifted signals), DMSO and pyridine disrupt self-assembly, whereas hexafluoroisopropanol strengthens intramolecular hydrogen bonds.