A HOST-GUEST SET OF TRIPLE-HELICAL PEPTIDES - STABILITY OF GLY-X-Y TRIPLETS CONTAINING COMMON NONPOLAR RESIDUES

Host-guest peptide sets have been useful in evaluating the propensity of different amino acids to adopt an alpha-helical or beta-sheet form, and this concept is applied here to the triple-helical conformation. A set of host-guest peptides of the form ro-Hyp)(3)-Gly-X-Y-(Gly-Pro-H yp)(4)-Gly-GlyCONH(2) designed to evaluate the contribution of an isol ated Gly-X-Y triplet to triple-helix stability in a defined environmen t. Peptides were synthesized to include guest triplets with the X and Y positions occupied by the most common nonpolar residues found in col lagen: Pro (X position) and Hyp (Y position): Ala; Leu, the most frequ ent hydrophobic residue; and Phe, the only commonly occurring aromatic residue. The guest triplets of the 12 peptides synthesized represent 35% of tile sequence found in the alpha 1 chain of type I collagen. Al l peptides formed stable triple-helical structures, and the peptides s howed a range of thermal stabilities (T-m = 21-44 degrees C), dependin g on the identity of the guest triplet, Thermodynamic calculations ind icate these peptides have a range of free energy values (Delta Delta G = 9 kcal/mol) and suggest that favorable entropy is the dominant fact or in increased stability. Replacement of Ala by Leu in the X position did not affect the thermal stability, while an Ala to Leu change in t he Y position was destabilizing. These data provide experimental evide nce that hydrophobic residues do not stabilize the triple helical conf ormation. Although Leu and Phe are found almost exclusively in tile X position in collagens, peptides with Leu and Phe in the Y position for med stable triple-helices. This supports the hypothesis that the X pos itional preference of these residues relates to their increased potent ial for intermolecular hydrophobic interactions rather than their dest abilization of the triple-helical molecule. These studies establish th e utility of host-guest peptides in defining a scale of triple-helix p ropensities and in clarifying the interactions stabilizing the triple- helical conformation.