Effect of amino acid coinclusion on the complexation of pyrene with β-cyclodextrin

The possibility of amino acids acting as ternary complexation agents for the binding of pyrene with β-cyclodextrin (β-CD) was investigated. UV - vis absorption measurements combined with steady-state and time-resolved fluorescence studies were employed to characterize ternary complex formation. From the 18 amino acids tested it was determined that tryptophan, leucine, phenylalanine, methionine, and isoleucine form ternary complexes. The first three also significantly enhance the association constant of pyrene with β-CD. Amino acids are less efficient than alcohols in enhancing the association constants of host - guest complexes, but a higher degree of steric constraint in the complexes including these zwitterionic molecules was observed. In addition, two types of pyrene - β-CD complexes were shown to exist, which were assigned to complexes with 1:1 and 1:2 pyrene:β-CD stoichiometries.