Quantum Chemical Study Of Carboxymethyl Chitosan

In this paper, density functional theory calculations were explored the structural optimization, reaction sites, infrared spectrum and intramolecular hydrogen bond of N-carboxymethyl chitosan （N-CMCS） and O-carboxymethyl chitosan （O-CMCS）. In addition, the metal complex models of carboxymethyl chitosan with Cd（Ⅱ） were established and studied.First of all, the characteristics of O-carboxymethyl chitosan and N-carboxymethyl chitosan had been investigated by means of the B3LYP/6-311 + （d, p） method. Through survey and comparing of total energies, bond length and frequency, we obtained the stable configuration of carboxymethyl chitosan. The results of frontier molecular orbit and charge distribution shown that the reaction active sites of N-carboxymethyl chitosan were focus on -OH and -NHCH₂COOH, while the reaction active sites of O-carboxymethyl chitosan were focus on -NH₂, -OH and -COOH. We knew the Einstein shift H-bonds of O₃₂…H₂₁-O₂₀ was formed in the carboxymethyl chitosan molecules. Following that, we went into details of the water-soluble mechanism of carboxymethyl chitosan.Secondly, the Lanl2DZ basis set for Cd（Ⅱ）atom and the B3LYP/6-311G+（d, p）basis set for the other atoms were used. Through analyzing the molecular geometries,binding energy,electronic charge transferred and infrared spectrum, we knew the interaction mechanism and space structures of metal with carboxymethyl chitosan. Carboxymethyl chitosan provided coordination compounds with kinds of ligands including hydroxyl radicals, carbocycle,amino and carboxyl groups. According to the calculated values of bond length and geometric angle,we believed that coordination compounds of b, c and d were bidentate ligand, a and e were tridentate ligands, while f was quadridentate ligand.This paper explained the reasons that the coordination compounds of O-carboxymethyl chitosan with Cd（Ⅱ） were more stable than the coordination compounds of N-carboxymethyl chitosan with Cd（Ⅱ）. On the one hand, the coordination compounds of O-carboxymethyl chitosan with Cd（Ⅱ） could form more number of chelating rings; on the other hand, the binding energie of O-carboxymethyl chitosan with Cd（Ⅱ） were low when compared with metal complexes of N-carboxymethyl chitosan with Cd（Ⅱ）. In the coordination compounds system of N-carboxymethyl chitosan with Cd（Ⅱ）, the order of binding energies was a>c>b. while, in the coordination compounds system of N-carboxymethyl chitosan with Cd（Ⅱ）, the order of binding energies was f>e>d.