Molecular recognition of α-cyclodextrin (CD) to choral amino acids based on methyl orange as a molecular probe

The molecular recognition interaction of α-CD to chiral amino acids was investigated by using spectrophotometry based on methyl orange as a molecular probe. The molecular recognition ability depended on the inclusion formation constants. The molecular recognition of α-CD to aromatic amino acids was the order: dl-tryptophan > l-tryptophan > l-phenylalanine > l-tyrosine ≈ dl-β-3,4-dihydroxy-phenylalanine; whereas for aliphatic amino acids, the order was: l-iso-leucine > l-leucine ≈ l-methionine ≈ dl-mehtionine > d-leucine. The effect of temperature on the inclusion interaction was examined and the thermodynamic parameters of inclusion process, ΔG, ΔH, ΔS, were determined. The experimental results indicated that the inclusion process was an exothermic and enthalpy-driven process accompanied with a negative or minor positive entropic contribution. The inclusion interaction between α-CD and amino acids satisfied the law of enthalpy-entropy compensation. The compensation temperature was 291 K.