Predicted Influence of N-Acetyl Group Content on the Conformational Extension of Chitin and Chitosan Chains

ABSTRACT

Conformational analysis of chitosan molecules has been performed using the MM3(92) force field to investigate the role played by the acetamido groups on the stiffness of these chains. A high dielectric constant value was needed to model an aqueous environment and to reproduce the distribution of the N-acetyl glucosamine group orientation that is observed by NMR. Disaccharidic fragments, differently substituted at C2, were selected as models for chitin and chitosan chains. Their conformational space has been explored by means of adiabatic mapping of the glycosidic Φ,Ψ torsion angles. Although the overall features of all the potential energy surfaces created appear similar, the accessible conformational space of a glycosidic bond is affected by the nature of the substituent at C2 on the non-reducing residue of the disaccharide unit. This is illustrated by the differences in the calculated partition functions together with the predicted average homonuclear and heteronuclear coupling constants. Computed maps were used to predict polymeric unperturbed dimensions, characteristic ratio and persistence length of idealized chitin and chitosan chains, by Monte Carlo methods. Pure chitosan is predicted to be more coiled than pure chitin chains. At low N-acetyl group contents, chain extension appears to be dependent on the degree of substitution. Average chain dimensions increase monotonically for increases in content up to 60% of N-acetyl groups, but show no significant variation at higher contents. For molecules consisting of 50% amino and 50% N-acetylated residues, random, alternate and block patterns of substitution have been investigated. It has also been shown that the spatial extension of the polymer chains is dependent on the primary structure. Comparison with the literature experimental data is difficult because of the extreme diversity of the reported conformationally dependent values. However, such study provides a unique insight into the dependence of these two factors (degree of acetylation and distribution of acetyl groups) on the stiffness and flexibility of different chitin and chitosan chains.     

New practical tools for the implementation and use of ultrafast 2D NMR experiments

Ultrafast (UF) 2D NMR is a very promising methodology enabling the acquisition of 2D spectra in a single scan. In the last few years, the analytical performance of UF 2D NMR has been highly increased, consequently maximizing its range of applications. However, its implementation and use by non‐specialists are far from being straightforward, because of the specific acquisition and processing procedures and parameters characterizing UF NMR. To make this methodology implementable and applicable by non‐specialists, we developed a simple routine capable of translating conventional parameters (spectral widths and transmitter frequencies) into specific UF parameters (gradient and chirp pulse parameters). This macro was subsequently implemented in a Web page, which is available for external users. Although the algorithm was designed for two widely used 2D experiments, COSY and HSQC, it can easily be extended to any other pulse sequence. The robustness of this routine was verified successfully on a variety of small molecules. We believe that this tool will eliminate much of the technical difficulties related to UF 2D NMR and will make the technique accessible to a wider audience of organic and analytical chemists. Copyright © 2013 John Wiley & Sons, Ltd.     

Social paradoxes of majority rule voting and renormalization group

Real-space renormalization group ideas are used to study a voting problem in political science. A model to construct self-directed pyramidal structures from bottom up to the top is presented. Using majority rules, it is shown that a minority and even a majority can be systematically self-eliminated from top leadership, provided the hierarchy has a minimal number of levels. In some cases, 70% of the population is found to have zero representation after six hierarchical levels. Results are discussed with respect to the internal operation of political organizations.     

Fluorinated comblike homopolymers: The effect of spacer lengths on surface properties

A series of polyacrylate monomers with F‐alkylalkyl [F(CF₂)_n(CH₂)_n′] side groups were prepared by free‐radical polymerization. The effect of the chemical structure on the surface properties of poly(ethylene terephthalate)s was evaluated by variations in the relative length of the fluorocarbon and hydrocarbon units in the side group. The resulting polymers were quite surface‐active in the solid state. The surface and bulk organization was investigated by X‐ray photoelectron spectroscopy analysis. A strong correlation between the bulk organization and surface properties of the polymers was established. The outmost layer, formed from trifluoromethyl groups and some ester functions, suggests that the side chain is arranged irregularly in the polymer–air interface. The length of the lateral chain governs this organization: long fluorinated chains and short hydrocarbon spacers are essential elements of the molecular design for such low‐surface‐energy materials. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3737–3747, 2005     

The p-Blocks of the Mackey Algebra

Let p be a prime number. This paper describes the primitive idempotents and prime spectrum of the crossed Burnside algebra of a finite group over a p-local ring. The main application is a formula for the block idempotents of the p-local Mackey algebra of the group, in terms of the corresponding bloks of the group algebra.     

A computer assistant for vehicle dispatching with learning capabilities

In this paper, we describe an expert consulting system for a dispatcher working in a courier service company. The system integrates interactive-graphic features and a learning module to support the dispatcher in his(her) task, and to suggest appropriate decisions when new requests come in. An experiment with a professional dispatcher is also reported.     

Some test statistics for the structural coefficients of the multivariate linear functional relationship model

Summary For the testing problem concerning the coefficients of the multivariate linear functional relationship model, the distribution of a statistic previously proposed by A. P. Basu depends on the unknown covariance matrixV of errors, so limiting its applicability. This article proposes new test statistics with sampling distributions which are independent of the unknown parameters for the cases whereV is either unknown or known only up to a proportionality factor. The exact distributions of the test statistics are also discussed.