Chitosan is a weak cationic polysaccharide composed essentially of β(1 → 4) linked glucosamine units together with some N‐acetylglucosamine units. It is obtained by extensive deacetylation of chitin, a polysaccharide common in nature. Chitosan is a biocompatible, biodegradable, and nontoxic natural polymer that exhibits excellent film‐forming ability. As a result of its cationic character, chitosan is able to react with polyanions giving rise to polyelectrolyte complexes. Therefore, because of these interesting properties, it has become the subject of numerous scientific reports and patents on the preparation of microspheres and microcapsules. The techniques employed to microencapsulate with chitosan include, among others, ionotropic gelation, spray drying, emulsion phase separation, simple and complex coacervation, and polymerization of a vinyl monomer in the presence of chitosan. The aim of this work is to review some of the more common techniques used and to put forward the results obtained by our research group in preparing chitosan‐based microcapsules: for taste masking and improving the stability of a nutritional oil, the sustained release of drugs, as well as the preparation of chitosan superparamagnetic microcapsules for the immobilization of enzymes.
Scanning electron micrograph of some superparamagnetic chitosan particles and magnetic hysteresis loop of the microparticles. 相似文献
The bacterial phosphotriesterase has been utilized as a template for the evolution of improved enzymes for the catalytic decomposition of organophosphate nerve agents. A combinatorial library of active site mutants was constructed by randomizing residues His-254, His-257, and Leu-303. The collection of mutant proteins was screened for the ability to hydrolyze a chromogenic analogue of the most toxic stereoisomer of the chemical warfare agent, soman. The mutant H254G/H257W/L303T catalyzed the hydrolysis of the target substrate nearly 3 orders of magnitude faster than the wild-type enzyme. The X-ray crystal structure was solved in the presence and absence of diisopropyl methyl phosphonate. The mutant enzyme was ligated to an additional divalent cation at the active site that was displaced upon the binding of the substrate analogue inhibitor. These studies demonstrate that substantial changes in substrate specificity can be achieved by relatively minor changes to the primary amino acid sequence. 相似文献
Complexes having the stoichiometry [R-NH3+][lasalocid A-] have been prepared and characterized, where R represents a series of organic substituents with diverse steric and electronic properties. The crystalline complexes dissolve readily in solvents of low polarity such as chloroform and are characterized in this solvent by molecular weight data and 13C nmr spectroscopy. NMR data indicate that the cations bind the lasalocid A anion hydrogen bonds to O3, O6, and O8. Molecular weight data show that none of these complexes are appreciably dissociated in chloroform solution. 相似文献
An improved polarization-sensitive optical coherence tomography (OCT) system is developed and used to measure birefringence in porcine myocardium tissue and produce two-dimensional birefringence mapping of the tissue. Signal-to-noise issues that cause systematic measurement errors are analyzed to determine the regime in which such measurements are accurate. The advantage of polarization-sensitive OCT systems over standard OCT systems in avoiding image artifacts caused by birefringence is also demonstrated. 相似文献
A method is proposed in which CuII, AsIII and Asv can be determined in copper arsenite without prior separation. It is based on the fact that CuII and AsIII yield prominent, distinguishable, widely-separated cathodic polarographic waves in a 0.1M LiCl-0.01M EDTA—0.001M LiOH solution using a dropping mercury electrode, whereas Asv does not give a wave in this medium. The Asv is determined by difference after reduction with sulphurous acid. 相似文献