In order to explore the essence of the hydration process of chitin or chitosan in the presence of cation, the cooperativity effects between the H-bonding and Na+···molecule interactions in the 1,4-dimethoxy-D-glucosamine (DMGA) complexes with H2O and Na+ were investigated at the B3LYP/6-311++G(d,p), M06-2X/6-311++G(2df,2p), and &#969;B97X-D/6-311++G(2df,2p) levels. The result shows that the complexes in which Na+ or H2O is bonded simultaneously to the -NH and -OH groups connected to the C3 atom of DMGA are the most stable. The cooperativity and anti-cooperativity effects occur in DMGA···H2O···DMGA and DMGA···Na+···H2O, while only the cooperativities are confirmed in DMGA···Na+···DMGA. The cooperativity occurs in the DMGA···Na+···H2O complexes without the hydration, while the anti-cooperativity occurs in those with the hydration. Furthermore, the cooperativity and anti-cooperativity in DMGA···Na+···H2O are far stronger than those in DMGA···Na+···DMGA or DMGA···H2O···DMGA. Therefore, a deduction is given that the cooperativity and anti-cooperativity effects play an important role in the hydration of chitin or chitosan in the presence of Na+. When only Na+ is linked with -OH and -NH groups of chitosan or chitin, due to the cooperativity effect, the hydration does not occur. When both Na+ and H2O are linked with -OH and -NH groups, the anti-cooperativities are dominant in controlling of the aggregation process of Na+, H2O, chitosan, and chitin, leading to the possible hydration. Atoms in molecules (AIM) analysis confirms the cooperativity and anti-cooperativity effects. Graphical abstract.
Theoretical investigation into the cooperativity effect of 1,4-dimethoxy-D-glucosamine complex with Na+ and H2O