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

The conformation of an acyclic dehydrophenylalanine ( z-Phe) containing hexapeptide, Boc-Phe- z-Phe-Val-Phe- z-Phe-Val-OMe, has been investigated in CDCl3 and (CD3)2SO by 270-MHz 1H-nmr. Studies of NH group solvent accessibility and observation of interresidue nuclear Overhauser effects (NOEs) suggest a significant solvent-dependent conformational variability. In CDCl3, 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 NiH Ni+1H NOEs. Evidence is also obtained for conformational heterogeneity from the detection of some C[stack i ]H Ni+1H NOEs characteristic of extended strands. In (CD3)2SO, the peptide largely favors an extended conformation, characterized by five solvent-exposed NH groups and successive C[stack i ]H Ni+1 H NOEs for the L-residues and C[stack i ]H Ni+1H NOEs for the z-Phe residues. The results suggest that z-Phe residues do not provide compelling conformational constraints