Defect peptide chemistry: Perturbations in the structure of a homopentapeptide induced by a guest residue interrupting side‐chain regularity

The fully blocked pentapeptide Tfa‐(Deg)2‐L‐Abu‐(Deg)2‐OtBu (Tfa:triflouroacetyl; Deg: Cα,α‐diethylglycine; OtBu: tert‐butoxy) adopts in the crystal state a regular, right‐handed 310‐helical structure stabilized by three N  H … O  C intramolecular 1 ← 4 (or C10) H bonds, as determined by an x‐ray diffraction analysis. However, a Fourier transform ir absorption and 1H‐nmr study strongly supports the view that in deuterochloroform solution the four Deg residues at both termini of the peptide main chain are involved in successive, fully extended C5 forms. A comparison with the stable, fully developed, multiple C5 conformation of Tfa‐(Deg)5‐OtBu indicates that incorporation of an Abu guest residue, interrupting the side‐chain uniformity of the host (Deg)5 homopeptide, while altering only marginally the conformation in a solvent of low polarity, is responsible for a dramatic perturbation of the crystal‐state structure. © 1994 John Wiley & Sons, Inc. Copyright © 1994 John Wiley & Sons, Inc.