The binding of free oligopeptides to cyclodextrins: The role of the tyrosine group

The formation of alpha-cyclodextrin (alpha-CD) and beta-cyclodextrin (beta-CD) inclusion complexes with free tyrosine and the tyrosine residues within two free oligopeptides were investigated using steady-state fluorescence spectroscopy. The oligopeptides consist of five amino acids (pentapeptide) and the tyrosine residues are located at the n-termini. The two peptides used in this study have well-known biological functions and are known to bind selectively to specific cell receptors. Cyclodextrins were used to model this receptor-peptide (protein-ligand) interaction. Equilibrium binding constants and the enthalpy and entropy of binding were recovered. Molecular size of the tyrosine-containing species and pH (7.0 vs. 10.0) were found to have little affect on alpha-CD binding. However, tyrosine binding to beta-CD was dependent on the size (free tyrosine vs. peptide), structure, and pentapeptide conformation.