Investigation of the interaction of α- and β-cyclodextrins with a series of aromatic amino acids in aqueous solutions by UV spectroscopy method
The results of experimental data obtained using UV spectroscopy, due to a comparative study on the interaction of α- and β-cyclodextrins with alpha aromatic amino acids differing in molecular size and substituent structure at the α-carbon atom in aqueous solutions with various pH (L-phenylalanine, L-tyrosine and L‑tryptophan) have been analyzed.
The features of the interaction of selected amino acids with cyclodextrins have been clarified. Molecular cationic and zwitterionic forms of amino acids at the same content of components in the solution and an excess of cyclodextrin form 1: 1 host-guest inclusion complexes, regardless of the pH value of the medium. This is manifested by a long-wavelength shift and an increase in the intensity of the characteristic absorption band in the UV spectra of solutions.
It has been proved that inclusion complexes are formed due to the van der Waals interactions by partial displacing water molecules and incomplete (in the case of α-cyclodextrin) immersion of the amino acid aromatic radical into the internal hydrophobic cavity of α-cyclodextrin and of β-cyclodextrin. Stabilization of the complexes is due to hydrogen bonds, and the geometric parameter makes the main contribution to the formation of inclusion complexes.
It has been found that the hydrogen bond is formed between the nitrogen atom of a-amino group or the oxygen atom of a-carboxyl group of the amino acid molecule with hydrogen of the primary hydroxyl group located on the surface of the wide edge of the torus, as well as between the nitrogen atom of the imino group of the L‑tryptophan molecule and glycoside hydrogen atom within the cyclodextrin cavity.
The values of the stability constants (Ks) of the complexes and the thermodynamic parameters of the formation of inclusion compounds (ΔG, ΔH, and ΔS) have been calculated. The use of a smaller macrocycle (a-CD) leads to a decrease in the stability of the inclusion complexes. Negative values of the change in the free Gibbs energy indicate a spontaneous complexation reaction between a- or b-cyclodextrins and alpha aromatic aminoacids. Negative values of the enthalpy DH and entropy DS indicate that the complexes are mostly stabilized due to the entropy contribution.
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