The Structure of Cellulose by Conformational Analysis. Part 4. Crystalline Cellulose II

The Kolpack, Weih, and Blackwell antiparallel central chain model was found to be the most energetically stable for the crystalline structure of cellulose II. Computational results indicated that the two most probable Sarko models are considerably less stable and less probable. The conformations of the monomers terminating the crystalline zone have been described. The predominance of van der Waals and H-bond interactions in the crystalline zone have been confirmed, and the H-bond values, locations, and distribution in the crystalline zone have been accurately mapped. The cellulose II crystallite has been confirmed to be considerably more stable than the cellulose I crystallite. Positions of atomic groups from the ab initio conformational analysis are consistent with the x-ray data for the Kolpack, Weih, and Blackwell model.     

Kinetics of Antibiotic Sorption by Monocarboxyl Cellulose

Kinetics of the sorption of lincomycin and gentamicin from aqueous solutions of their salts and bases, as well as a cephalosporin group antibiotic cephalexin having the zwitterionic nature, by monocarboxyl cellulose is studied. It is disclosed that the sorption of the studied antibiotics is characterized by a combined diffusion type of kinetics. Effective diffusion coefficients (D _eff) are determined, and it is shown that they correspond to the times (t _1/2) of half-equilibrium establishment and increase with passing from lincomycin to gentamicin and, further, to cephalexin. The D _eff and t _1/2 values are found to depend on solution pH and the degree of cellulose sorbent swelling.     

Study of the Structure and Sorption Properties of Some Types of Cellulose

The pulsed NMR, isopiestic and pycnometric methods are applied to investigate the physicochemical properties of softwood [GOST (State Standard) 9571-89] and hardwood [GOST 28172-89] celluloses, as well as flax cellulose [GOST 10330-76, 9394-76], both in the initial state and subjected to chemical treatment. Possible changes occurring in the supramolecular structure of a polymer are analyzed. The structural and sorption characteristics of the investigated samples are calculated. Their water absorption capacities are determined.     

Molecular Dynamics Simulations of Cellulose Oligomers: Conformational Analysis

Summary: The results of classical molecular simulations of cellulose oligomers are presented here. The conformations of the chains in the high temperature melt, room temperature quenched melt and gas phase are compared with respect to various geometrical parameters including square end‐to‐end distances, glycosidic link torsion correlations, ring puckering and hydrogen bonding. The cellulose oligomer melts were relaxed at 800 K with molecular dynamics, and then cooled down in three different ways to obtain dense amorphous systems at 500 K and at room temperature. The sample resulting from the quench (step) shows too much similarity with the melt at 800 K. The two other cooling schemes (ramp, 2ramps) give very similar results for all quantities investigated. The relevance of previous single molecule calculations with respect to the dense amorphous systems is called into question. Comparisons between the chains in the dense systems and those in the gas phase reveal that, even for these relatively short stiff chains, differences exist in the preferred conformations. At high temperatures, where both systems are in equilibrium, the distribution of square end‐to‐end distances are both fairly smooth, but the gas phase clearly prefers more compact conformations. At 300 K, the differences are exacerbated as the equilibrium distribution for the gas phase shows a high proportion of folded conformers, whereas the nonequilibrium quenched systems necessarily retain the extended envelope of the higher temperature. Differences are also evident in the puckering, the rotation of the hydroxymethyl groups and the pattern of hydrogen bonds.

The probability density distribution for the square end‐to‐end distance for octaose in the gas phase (light line) and in the dense phase (dark line) at 300 K.     

Structure and Conformational Analysis of 5-Methyl-2,2-diphenyl-1,3-dioxane

Russian Journal of Organic Chemistry - The structure of 5-methyl-2,2-diphenyl-1,3-dioxane has been studied by1H and 13C NMR spectroscopy and X-ray analysis. Its molecules in crystal, as well as in...     

Water Sorption Properties and Antimicrobial Action of Zinc Oxide Nanoparticles-Loaded Cellulose Acetate Films

In this work, ZnO nanoparticles loaded cellulose acetate (ZOLCA) films have been prepared and characterized by XRD, SPR and SEM analysis. The moisture permeation properties of the films have been investigated. The GAB isotherm model has been found to fit well on the moisture uptake data obtained at different temperatures. The monolayer sorption capacity χ_m was found to decrease from 0.059 to 0.0079 g water/g dry film with increase in temperature from 20 to 37°C. The isosteric heat of sorption, when studied in the lower water activity range of 0.04 to 0.10, was evaluated to be 46.55 to 87.29 kJ/mol. The water vapor permeability across the ZOLCA films was found to increase with temperature and activation energy of moisture sorption process was found to be 48.57 kJ/mol. These films have shown excellent antibacterial action against model bacteria E-Coli when investigated by qualitative and quantitative methods. Films exhibit great potential to be used as edible films to protect food stuff against microbial infections.     

Conformational studies by dynamic NMR. Part XV. The rotational barrier of N-methyl aniline

Low temperatures C-13 NMR spectra allowed the determination of the rotational barrier of N-methyl aniline in solution. The shifts of the anisochronous ortho and meta carbons were assigned by $\text{ab initio}$ calculations.     

Sorption of Water Vapor by Keratin

The word “keratin” is usually applied to that group of fibrous proteins characterized by the presence of the amino acid cystine which enables the formation of disulfide bridges between neighboring polypeptide chains. In nature, the hardened form of keratin plays a protective role and appears in many forms of animal outer covering, such as wool, hair, nails, horn, feather, and quill, where a high degree of physical strength and chemical resistance are desirable. This variety provides a wide selection of geometrical forms and subtle differences in composition between keratins, which can be employed in the study of sorption mechanisms for this material. All forms of keratin contain hydrophilic amino acid residues and peptide groups which ensure significant adsorption of water vapor. Keratin differs from the soluble proteins because the presence of cross-links renders it insoluble in water; it is only soluble in reagents capable of rupturing the disulfide bridges. However, in common with other biopolymers and synthetic polymers having an affinity for water, keratin swells on adsorption over the entire humidity range.     

Conformational analysis of spiranes. Part III

Spiro[5.5]undecane,1, and its oxa analogues,2, exemplify ambiguities in the domain of chirality. Low temperature¹H and¹³C NMR measurements of tetraoxaspiroundecanes3–5 provide an unusual opportunity for detection of largely neglected effects of the chirality of the frozen conformations of these molecules. For the trispiro compound,5, two dynamical processes are detected upon temperature lowering: cyclohexane and dioxane ring inversion. At low temperatures, it is possible to detect diastereoisomerism of5 due to the existence of three elements of chirality in the molecule.For Parts I and II see ref. [1].     

Sorption Properties of Gel Films of Bacterial Cellulose

The structure and sorption properties of gel films of bacterial cellulose were studied in its interaction with polyvinylpyrrolidone and silver nanoparticles stabilized by this compound.     

Effect of Cellulose Suspension Agent Structure on the Particle Morphology of PVC. Part II. Interfacial Properties

The particle structure of polyvinyl chloride is controlled by the shear field imposed on the monomer droplet and the interfacial behavior of the vinyl chloride/water phases during polymerization. The inter facial tension in the presence of hydroxypropyl methylcellulose (HPMC) was measured as a function of concentration and temperature. The molecular weight distribution of HPMC was determined by coupled GPC-LALLS (low angel laser light scattering) technique. By monitoring the concentration of HPMC in the aqueous phase during polymerization, the coverage powers of HPMC were calculated and compared with the theoretical value based on Langmuir layer consideration. The effects of agitation on resin porosity were also examined. These results are discussed with respect to the particle structure.     

The Preparation and Properties of Acrylic and Methacrylic Acid Grafted Cellulose Prepared by Ceric Ion Initiation. Part I. Preparation of the Grafted Cellulose

The ceric ion method has been used to graft acrylic acid directly onto cellulose with a minimum amount of homopolymer. The method utilizes the pretreatment of the cellulose with ceric ammonium nitrate followed by washing out any excess of the catalyst. Oxygen can be present with the pretreatment step, but must be excluded during the grafting reaction itself. The process, which is entirely aqueous in nature, would appear to be quite practical on a large scale. Wet strengthened papers which are essential for the use of the grafted products as ion-exchange media can also be grafted but with adequate but lower yields than with the untreated paper. Apart from ion exchange, the products are of considerable potential value as water sorbing agents. Methacrylic acid gave lower grafting yields than acrylic acid, but these were much improved at higher temperatures.     

The Structure of Nickel(Ii) and Copper(Ii) Complexes with 1,4,8,11-Tetraazacyclotetradecanein Aqueous Solution as Studied by the X-Ray Diffraction Method

The structure of 1,4,8,11-tetraazacyclotetradecane (cyclam) complexes with nickel(II) and copper(II) ions in aqueous solution has been determined by the x-ray diffraction method at 25°C. The [Ni-(cyclam)]²⁺ complex has a square-planar structure with four nitrogen atoms of the cyclam, and the Ni-N bond length has been determined to be 198 pm. Upon the addition of ammonia, the color of the nickel(II)-cyclam solution turns to deep purple and the [Ni(NH₃)₂(cyclam)]²⁺ complex is formed. The complex has a regular octahedral structure with an additional two NH₃ molecules along the axis vertical of the cyclam plane, and the Ni-N (NH₃ and cyclam) bond lengths are 209 pm. The copper(II)-cyclam complex in the aqueous solution is a distorted octahedron with two water molecules along the elongated axis. The axial Cu—O and equatorial Cu—N bond lengths are 277 and 210 pm, respectively.     

Conformational Analysis of 5-Ethyl-5-hydroxymethyl-2,2-dimethyl-1,3-dioxane

Russian Journal of Organic Chemistry - Computer simulation of conformational transformations of 5-ethyl-5-hydroxymefhyl-2,2-dimefhyl-1,3-dioxane in the gas phase, as well as in mixtures with...     

Conformational behaviour of non-fused biheterocycles. Part XV. Isomeric phenylisoxazoles

The conformational behaviour of isomeric phenylisoxazoles has been investigated at the STO-3G level optimizing the most significant geometrical parameters. The energy minima of the two isomers having an heteroatom in an ortho position correspond to planar structures, whereas the 4-isomer has a twisted equilibrium conformation. This confirms that H? H non-bonded repulsions are stronger than (lone-pair)-H ones. The similar conformational behaviour of corresponding phenylfurans and phenylisoxazoles shows that the second heteroatom has a negligible effect and that the conformational behaviour of non-fused biheterocycles is dictated by the nature of the ortho substituents.     

Conformational analysis. Part 16 Conformational free energies in substituted piperidines and piperidinium salts

Summary The conformational free energies (-G^o) of a number of 4-substituted piperidines and piperidinium salts have been determined by the J-value method. For the 4-substituted piperidines (R=Me, Phenyl, CO₂Et, Br, OH, F) the relative conformer energies are almost identical to those of the analogous cyclohexanes.The methyl and phenyl compounds showed no change in the couplings on protonation, implying no change in the conformer energies. In constrast, in the remaining compounds with polar 4-substituents an almost constant stabilisation of the axial conformer of ca. 0.7–0.8 kcal mol^-1 was observed on protonation. In three cases (R=F, OH and Br) the conformational preferences is reversed on protonation and the axial form is favoured.The conformer energies of both the free bases and the piperidinium salts can be quantitatively predicted by molecular mechanics calculations using the COSMIC force-field, in which the electrostatic interactions are calculated by a simple Coulombic model with the partial atomic charges in the molecules given by the CHARGE2 routine, and an effective dielectric constant of five. The precise agreement obtained demonstrates conclusively that the electrostatic interactions between the substituents and the protonated nitrogen are the cause of the conformational changes on protonation, and that these can be modelled successfully using existing force-fields.For Part 15, see Ref. 1.     

Conformational studies by dynamic nmr. Part 18. Restricted rotation in nitrosobenzene and phenylhydrazine

The barriers to Ph-N rotation in the title compounds have been determined by C-13 NMR. The values are 7.6 and 6.9 kcal mol^?1 respectively.