β-Cyclodextrin Inclusion Complexes with Model Pentapeptides: Role of the Tyrosine Position within the Peptide Chain

Peptide's applications frequently present problems of solubility, stability, activity, or membrane permeability. To overcome these issues, cyclodextrins (CDs) can be used to form inclusion complexes with peptide hydrophobic parts; alkyl-chains or aromatic-rings inclusion strongly influences the interacting peptide properties. The study of model tripeptides has revealed that, among the three aromatic amino acids, tyrosine is the best suited to be included within CDs. The interaction with β-CD of five model peptides (Tyr1-5), each constituted by one tyrosine and four alanines, is reported: the tyrosine occupies one of the five position within each peptide chain. Among natural CDs, β-CD has been chosen as it is the most economic, used, and only moderately toxic; its cavity size is the best suited to accommodate the tyrosine ring. Stoichiometry and affinity of each complex are evaluated and in silico and experimental data to describe the molecular determinants of each interaction are combined. The data further defines the role of the aromatic ring position in dictating the stability of formed complexes and demonstrates Tyr3, with its central Tyr, as the most stable complex. Noteworthy, the interaction with β-CD induces Tyr3 to assume a U-shaped conformation representing a nice example of conformation stabilization upon formation of inclusion complexes.