Amino acid-modified chitosan

Casettari, L., VUasahu, D., Lam, J.K., Soliman, M., Ilium, L., 2012. Biomedical apphcations of amino acid-modified chitosans a review. Biomaterials 33 (30), 7565-7583. 

In 1996, Gauglitz and coworkers coated surfaces with various amino-and carboxy-substituted polymers [198], The polymers tested were branched poly-(ethyleneimine), a,co-amino-functionalized PEG, chitosan, poly(acrylamide-co-acrylic acid) and an amino-modified dextran. The amino-substituted polymers were immobilized on glass by first immobilizing an aminosilane, followed by succinic anhydride/A-hydroxysuccinimide linker chemistry. Poly(acrylamide-co-acrylic acid) was directly coupled to an aminosilanized surface. When probed with 1 mg mL 1 ovalbumin solution, nonspecific adsorption was lowest for the dextran derivative. Notably, nonspecific adsorption increased in most cases when a hydrophobic hapten (atrazine) was coupled to the polymer-modified surface. 

The enzymatic treatment of chitosan in the presence of tyrosinase and phenol derivatives produced new materials based on chitosan.91 During the reaction, unstable o-quinones were formed, followed by the reaction with the amino group of chitosan to give the modified chitosan. The tyrosinase-catalyzed modification of chitosan with phenols dramatically altered rheological and surface properties of chitosan. The modification with chlorogenic acid onto chitosan conferred the water solubility of chitosan under basic conditions.92 A new water-resistant adhesive was developed by the tyrosinase-catalyzed reaction of 3,4-dihydroxyphenethylamine and chitosan.93 Poly(4-hydroxystyrene) was modified with aniline by using tyrosinase catalyst.94 The incorporated ratio of aniline into the polymer was very low (1.3%). 

Chitin is a natural celluloselike biopolymer consisting predominantly of unbranched chains of A -acetyl-D-glucosamine residues, but the presence of amine and amide groups makes the selectivity of chitin different from cellulose. Laboratory-prepared layers of chitin, its deacetylated derivative chitosan, and chitin modified with metal cations have been used to separate a variety of organic and inorganic compounds, including amino acids (Malinowska and Rozylo, 1991 Rozylo et al., 1989). 

It is well known that the natural polymer cellulose and its derivatives have the ability to separate a racemate into enantiomersConsequently, further work was carried out on the selective sorption of various D, L-amino acids by chemically modified chitosan gels. 

The sorption of the D-isomer is almost independent of the D.S. The L-amino acids are sorbed to a greater extent than their isomers. In our previous work, it was observed that the N-octadecanoyl chitosan gel (D.S. = 0.45) selectively sorbs L-tryptophan from an aqueous 1 1 D,L-tryptophan solution giving a ratio of L/D 60. These results suggest that chemically modified chitosan gels are able to separate D,L-amino acids. 

We conclude from these results that the chiral structure of chitosan gels containing hydrophobic groups is effective in the hydrophobic separation and the resolution of optical isomers of D,L-amino acids. Further work will be applied to the separation of D,L-amino acids and peptides by liquid column chromatography, using chemically modified chitosan as stationary phase. In addition, the information about equilibrium sorption obtained from the present investigation is to be expected for supports of immobilized enzymes and separation membranes. 

Eor CNBE, when the the moore retio of NBS and CNBE was 45.5, the activity was completely lost.However, chitosan showed significant protection on the CNBE, when preincubating with chitosan for 30min before NBS modification,the modified CNBE would retain 93% of its original activity,indicating that Trp was one of the essential catalytic amino acids and at least one mol Trp located in the substrate binding domain of CNBE. 

Later, the same authors [40] investigated the performances of chitin and chitosan beds modified by different metal ions solutions [Cu(II), Co(II), Ni(II), Hg(II), Ag(I)] for the resolution of five racemic amino acids (threonine, alanine, valine, leucine, and phenylglycine). Results showed that only stationary phases of chitin modified by Cu(II) ions may be used for separation of the aforementioned racemates using both binary and ternary mobile phases. In particular, the best results were obtained with methanol/water/acetonitrile mixtures (1 1 0.1, v/v/v) for which D,L-valine showed the highest a-value (a = 12.9) and D,L-phenylglycine the lowest (a = 4.56). 

Malinowska, 1. and Rozylo, J.K., Separation of optical isomers of amino acids on modified chitin and chitosan layers, Biomed. Chromatogr., 11, 272-275, 1997. 

Thiolated polymers, also termed thiomers, are conventional mucoadhesive polymers chemically modified to contain a cysteine residue in the polymer chain and thus establish covalent disulfide bonds with mucin." They can be manufactured to be either cationic (mostly thiolated chitosans) or anionic (carboxylic acid-containing polymers) however, their mucoadhesive extent will mostly be determined by their capacity to covalently bind to mucin. The polypeptide backbone of mucin can be divided into three major subunits tandem repeat array, carboxyl-, and amino-terminal domains. While the amino-terminal domain contains some of the cysteine residues, the carboxyl-terminal domain contains more than 10% of the cysteine residues. These cysteine-rich regions are responsible for forming the large mucin oligomers and ultimately, the groups that allow for the covalent mucoadhesive bond formation with oral mucosal systems." ... 

The poor solubility, low surface area, and porosity of chitin and chitosan are the major limiting factors in their utilization. Chitosan can be modified by physical or chemical processes in order to improve the mechanical and chemical properties. Chemical modification of chitosan has two main aims (a) to improve the metal adsorption properties and (b) to change the solubility properties of chitosan in water or acidic medium. The substitution chemical reactions involve the NH2 group in the C2 position or the OH groups in the C3 and Cg positions of acetylated and deacetylated units. Chitosan membrane is swollen in water the amino groups may be protonated and leave the hydroxide ions free in water, which may contribute to the ionic conduction in the membrane. 

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