INCLUSION COMPLEXES OF PROTEINS - INTERACTION OF CYCLODEXTRINS WITH PEPTIDES CONTAINING AROMATIC-AMINO-ACIDS STUDIED BY COMPETITIVE SPECTROPHOTOMETRY

The stability constants were measured of inclusion complexes formed from aromatic amino acids and their oligopeptides with alpha- and beta-cyclodextrin, hydroxypropyl beta-cyclodextrin, and partially methylated beta-cyclodextrin. The method of competitive spectrophotometry with p-nitrophenol as a competing reagent was used, and measurements were made at pH 7.4. beta-Cyclodextrin formed complexes of higher stability than the other hosts. The stability of complexes of oligopeptides containing L-phenylalanine was invariably higher than that of L-phenylalanine itself. A model for interaction of proteins with cyclodextrins is proposed, in which the most stable complexes are formed when the native functional form of proteins is unfolded and the nonpolar residues that are buried inside the structure are exposed to water. The complexation of the unfolded structure favors its formation; thus thermal denaturation of proteins is easier in the presence of cyclodextrins. On the other hand, this complexation prevents the intermolecular association of unfolded structures by noncovalent hydrophobic bonding between the exposed nonpolar residues; furthermore, the unfolded complexed forms may revert to the native functional form. This prevention of intermolecular association may explain the stabilizing effect of cyclodextrins on solutions of proteins: a return to the native form is achieved more easily from the complexed, unfolded form than from the unfolded, aggregated forms.