Conformations of dehydrophenylalanine containing peptides: nmr studies of an acyclic hexapeptide with two Δ z ‐Phe residues

The conformation of an acyclic dehydrophenylalanine (Δ z ‐Phe) containing hexapeptide, Boc‐Phe‐Δ z ‐Phe‐Val‐Phe‐Δ z ‐Phe‐Val‐OMe, has been investigated in CDCl 3 and (CD 3 ) 2 SO by 270‐MHz 1 H‐nmr. Studies of NH group solvent accessibility and observation of interresidue nuclear Overhauser effects (NOEs) suggest a significant solvent‐dependent conformational variability. In CDCl 3 , a population of folded helical conformations is supported by the inaccessibility to solvent of the NH groups of residues 3–6 and the detection of several N i H ⟷ N i+1 H NOEs. Evidence is also obtained for conformational heterogeneity from the detection of some C   i α H ⟷ N i+1 H NOEs characteristic of extended strands. In (CD 3 ) 2 SO, the peptide largely favors an extended conformation, characterized by five solvent‐exposed NH groups and successive C   i α H ⟷ N i+1 H NOEs for the L ‐residues and C   i β H ⟷ N i+1 H NOEs for the Δ z ‐Phe residues. The results suggest that Δ z ‐Phe residues do not provide compelling conformational constraints.