Chiral Recognition Of Amino Acid Enantiomers Based On Self-assembled Binuclear Copper(?)-Cyclodextrin Complexes

Binuclear copper???-cyclodextrin complexes?Cu₂-?-CD and Cu₂-b-CD?were self-assembled on poly?L-glutamic acid??P-L-Glu?for chiral recognition of D-tyrosine?D-Tyr?and D-tryptophan?D-Trp?.Cu²⁺ could form binuclear hydroxy-bridge?Cu?OH?₂Cu?at the wider opening of the cavity of binuclear copper???-cyclodextrin complexes and acted as a cap to prevent high-energy water molecules from being released while forcing Tyr and Trp enantiomers to enter through its narrower opening.Because hydrogen bonds were favored to form between the high-energy water molecules and D-Tyr/D-Trp inside the cavity of b-CD,D-Tyr and D-Trp were thus perferably recognized.The recognition capability of the binuclear copper???-cyclodextrin complexes appeared to depend on the way the binuclear copper???-cyclodextrin complexes self-assembled onto P-L-Glu,preferring via hydrogen bonds rather than via electrostatic interactions,because electrostatic interactions between P-L-Glu and Cu?OH?₂Cu could alter the cavity configuration of Cu₂-?-CD and Cu₂-b-CD,resulting in weakened hydrogen bonds between high-energy water molecules and D-Tyr/D-Trp.Cu₂-?-CD was self-assembled on alkali-pretreated chitosan film?CS?through the coordination between Cu₂-?-CD and the amino groups on CS,and then the CS/Cu₂-?-CD was modified on GCE and applied for chiral recognition of amino acids.During the process of self-assembly,the Cu₂-?-CD molecules interacted with each other through hydrogen bonds,inducing the formation of ordered column structure of CS at micrometers.It is also interesting to find that the structure of CS was influenced by the temperature and time during the self-assembly.Effective and selective recognition of D-tryptophan?D-Trp?was achieved at the CS/Cu₂-?-CD due to the highly ordered array of Cu₂-?-CD.This is the first report that the recognition efficiency of Trp enantiomers depends on the ordered structureof the materials for recognition.Cu₂-?-CD was self-assembled on CS films through coordination,however,the Cu₂-?-CD could not match well with the underlayered CS in configuration due to the relatively small cavity size of Cu₂-?-CD,resulting in an unorder self-assembly of Cu₂-?-CD on CS films.Due to the different steric hindrance in the host-guest system,the enantiorecognition of Tyr enantiomers was achieved via selective formation of intermolecular hydrogen bonds.The synergetic effect of Cu₂-?-CD and CS played a key role in the recognition of Tyr enantiomers.Both current ratio and potential difference could be adopted for the effective recognition of Tyr enantiomers.The temperature-sensitive feature of the chiral system was also investigated in this chapter.The chiral system exhibited the ability of predicting the ratio of L-and D-Tyr in a racemic mixture,showing its broad application prospects in the separation of chiral compounds.