PEPTIDE DESIGN - INFLUENCE OF A GUEST AIB-PRO SEGMENT ON THE STEREOCHEMISTRY OF AN OLIGO-VAL SEQUENCE - SOLUTION CONFORMATIONS AND CRYSTAL-STRUCTURE OF BOC-(VAL)2-AIB-PRO-(VAL)3-OME

The peptide Boc‐Val‐Val‐Aib‐Pro‐Val‐Val‐Val‐OMe has been synthesized to investigate the effect of introduction of a strong β‐turn promoting guest segment into an oligopeptide with a tendency to form extended structures. 1H‐nmr studies in solution using analysis of NH group solvent accessibility and nuclear Overhauser effects suggest an appreciable solvent dependence of conformations. In chloroform a 310‐helical structure is favored while in dimethylsulfoxide an Aib‐Pro β‐turn with extended arms on either side is suggested. In the crystal, the backbone forms a somewhat distorted 310‐helix despite the presence of a Pro residue in the middle. Among the four possible intrahelical hydrogen bonds three are of the 4 → 1 type and one 5 → 1. Head‐to‐tail NH ⃛OC hydrogen bonds link the helical molecules into continuous columns. The space group is P212121 with a = 11.320(2), b = 19.889(3), and c = 21.247(3) Å. Copyright © 1990 John Wiley & Sons, Inc.