甲壳素／壳聚糖的化学修饰

详述了甲壳素／壳聚糖的主要化学反应所得衍生物的一些性质和应用。     

N-乙基壳聚糖的针状结晶

在N 乙基壳聚糖的甲酸溶液浇铸膜中观察到高分子少有的针状晶体 .从球晶经后结晶得到的针状晶体为长条矩形 ,典型尺寸为～ 5 0 μm× 2～ 5 μm× 1～ 2 μm ,高分子链平行于晶体长轴 .针状晶体首先出现在球晶的核心处 ,继而出现在球晶微纤每个树枝状分叉处 ,最后才遍布球晶各处 .针状晶体可以看成是高分子伸直链结晶的一种 .浇铸膜吸潮实际上形成了超浓溶液 (浓度 >80wt% ) ,从而分子链可以运动而后结晶成针状晶体     

The use of DSC curves to determine the acetylation degree of chitin/chitosan samples

The use of DSC curves is proposed as an alternative method to determine the degree of N-acetylation (DA) in chitin/chitosan samples, based in both peak area and height of the decomposition signal. Samples with DA from 74 to 16% were prepared from a chitin commercial sample and the DA was determined by ¹H NMR, ¹³C CP/MAS NMR and IR spectra. The effect of water content, heating rate, sample mass and gas flow on the DSC peaks were evaluated and optimized. Using optimized conditions a linear relationship between peak area and height with the DA could be achieved with linear correlation coefficients of −0.998 and −0.999 (n = 7), respectively. The calibration graphs were used to determine the DA of a commercial chitosan sample with relative errors ranging from 2 to 3% for both peak area and peak height, when compared with the DA determined by ¹H NMR method.     

甲壳胺的结晶动力学

给出了制备非晶甲壳胺的新方法，并由Ｘ衍射曲线和偏光显微镜照片及ＤＳＣ曲线得到证实。由ＤＳＣ测得玻璃化温度为８２℃，结晶温度为１５２℃，并研究了其结晶动力学。     

Voltammetric sensing of sugar by an electrode covered with wheat germ agglutinin/chitin film

The binding between wheat germ agglutinin (WGA) and N-acetylglucosamine at the electrode covered with chitin film was investigated with voltammetry. Chitin, β-1,4-poly-N-acetylglucosamine, is one of the biolpolymers which have a high biocompatibility. WGA is immobilized to the surface of chitin film by the affinity of WGA to N-acetylglucosamine residue of chitin. To investigate the binding event of WGA on the chitin modified electrode, N-acetylglucosamine labeled with an electroactive compound was prepared. The binding causes the changes in the electrode response of labeled sugar. The peak current of labeled sugar decreased due to the specific binding with WGA on the chitin film modified at the electrode. N-Acetylglucosamine was successfully determined by using the competitive reaction with labeled sugar to WGA on the chitin film electrode.     

Preparation and characterization of chitin whisker-reinforced silk fibroin nanocomposite sponges

For highly porous form such as sponges or scaffolds, the induction of the β-sheet formation of silk fibroin to make the water-stable materials usually results in their high shrinkage leading to a difficulty in controlling shape and size of materials. Thus, the objective of this study was to improve dimensional stability of silk fibroin sponge by incorporating chitin whiskers as nanofiller. Chitin whiskers exhibited the average length and width of 427 and 43 nm, respectively. Nanocomposite sponges at chitin whiskers to silk fibroin weight ratio (C/S ratio) of 0, 1/8, 2/8, or 4/8 were prepared by using a freeze-drying technique. The dispersion of chitin whiskers embedded in the silk fibroin matrix was found to be homogeneous. The presence of chitin whiskers embedded into silk fibroin sponge not only improved its dimensional stability but also enhanced its compression strength. Regardless of the chitin whisker content, SEM micrographs showed that all samples possessed an interconnected pore network with an average pore size of 150 μm. To investigate the feasibility of the nanocomposites for tissue engineering applications, L929 cells were seeded onto their surfaces, the results indicated that silk fibroin sponges both with and without chitin whiskers were cytocompatible. Moreover, when compared to the neat silk fibroin sponge, the incorporation of chitin whiskers into the silk fibroin matrix was found to promote cell spreading.     

Mechanical and viscoelastic properties of chitin fiber reinforced poly(ε-caprolactone)

Poly(ε-caprolactone)/chitin fiber (PCL-CF) composites as potential bone substitutes were prepared using a simple melt-processing method. The results from differential scanning calorimetry and dynamic mechanical thermal analysis (DMTA) showed that there was interaction between PCL and CF. Static mechanical testing showed that tensile strength, Young’s modulus and flexural strength were increased by the addition of CF. The measurements from DMTA and an advanced rheometric expansion system showed that both the storage modulus and loss modulus were enhanced by CF. The PCL-CF composite with CF of 45% by mass had the best properties among all the tested composites.     

Evaluation of Binding Between Electroactive Biotin Derivative and Streptavidin Immobilized on Chitin Film

Evaluation of the streptavidin‐biotin binding at the surface of chitin film was carried out with voltammetry. Immobilization of streptavidin was attempted to the protonated chitin film, based on an electrostatic interaction that hardly causes any change in the protein structure. The streptavidin‐biotin binding was estimated from changes in the electrode response of biotin labeled with an electroactive compound. Although the response of daunomycin as an electroactive compound did not change at an electrode covered with streptavidin/chitin film, the response of the labeled biotin decreased. This observation shows that streptavidin is immobilized on the chitin film and the biotin binds with immobilized streptavidin. Consequently, it was clear that the chitin film is useful as a reaction field for protein‐ligand binding. Generally, a binding event between protein and its ligand in the living body occurs on the cell surface. The electrochemical evaluation of protein‐ligand binding on a natural polysaccharide like chitin membrane surface is important.     

Chitin- and Streptavidin-Mediated Affinity Purification Systems: A Screening Platform for Enzyme Discovery

Affinity purification of recombinant proteins is an essential technique in biotechnology. However, current affinity purification methods are very cost-intensive, and this imposes limits on versatile use of affinity purification for obtaining purified proteins for a variety of applications. To overcome this problem, we developed a new affinity purification system which we call CSAP (chitin- and streptavidin-mediated affinity purification) for low-cost purification of Strep-tag II fusion proteins. The CSAP system is designed to utilize commercially available chitin powder as a chromatography matrix, thereby significantly improving the cost-efficiency of protein affinity purification. We investigated the CSAP system for protein screening in 96-well format as a demonstration. Through the screening of 96 types of purified hemoproteins, several proteins capable of the catalytic diastereodivergent synthesis of cyclopropanes were identified as candidates for an abiotic carbene transfer reaction.     

Selective Synthesis of Oligosaccharides by Mechanochemical Hydrolysis of Chitin over a Carbon-Based Catalyst

Oligosaccharides possess fascinating functions that are applicable in a variety of fields, such as agriculture. However, the selective synthesis of oligosaccharides, especially chitin-oligosaccharides, has remained a challenge. Chitin-oligosaccharides activate the plant immune system, enabling crops to withstand pathogens without harmful agrichemicals. Here, we demonstrate the conversion of chitin to chitin-oligosaccharides using a carbon catalyst with weak acid sites and mechanical milling. The catalyst produces chitin-oligosaccharides with up to 94 % selectivity in good yields. Monte-Carlo simulations indicate that our system preferentially hydrolyzes larger chitin molecules over oligomers, thus providing the desired high selectivity. This unique kinetics is in contrast to the fact that typical catalytic systems rapidly hydrolyze oligomers to monomers. Unlike other materials carbons more strongly adsorb large polysaccharides than small oligomers, which is suitable for the selective synthesis of small oligosaccharides.