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: C-alpha,C-alpha-diethylglycine; OtBu: tert-butoxy) adopts in the crystal state a regular, right-handed 3(10)-helical structure stabilized by three N-H...O=C intramolecular 1 <-- 4 (or Clo) H bonds, as determined by an x-ray diffraction analysis. However, a Fourier transform ir absorption and H-1-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 C-5 forms. A comparison with the stable, fully developed, multiple Cg 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. (C) 1994 John Wiley and Sons, Inc.