Solid state and solution structure of Boc-L-Ala-Δphe-Δphe-NHMe: a dehydropeptide showing propensity for 310-helices of both screw senses

The crystal and molecular structure of the peptide Boc-l-Ala-Δphe-Δphe-NHMe, containing two consecutive dehydro-phenylalanine (Δphe) residues, has been solved by x-ray diffraction. Two independent molecules, X and Y, are present in the crystallographic unit. Their conformation corresponds approximately to an incipient 310-helix stabilized by two intramolecular hydrogen bonds. The (ϕ, ψ) torsion angles, however, have negative and positive signs in the two molecules X and Y, respectively. Therefore, in spite of the presence of an amino acid residue of the l configuration, the two helical molecules have opposite screw senses, even though the right-handed helix is less distorted than the left-handed one in correspondence of the l-Ala residue. The CD spectra in various solvents exhibit exciton bands originating from dipole–dipole interaction between the Δphe side chains. Addition of DMSO to the chloroform solution produces, as a first step, a strong increasing of the CD bands, which are then progressively canceled by increasing DMSO concentration. The nmr data parallel the behavior observed in the CD spectra. In CDCl3 solution, the temperature coefficients of the NH resonances are consistent with the involvement of the last two amide protons of the sequence in intramolecular hydrogen bonds, but only negligibly small nuclear Overhauser effects (NOE) are observed. Addition of 5% DMSO-d6 allows the observation of diagnostic NOEs. CD and nmr data indicate that the solid state structure is retained in solution, and are consistent with the presence of right-handed and left-handed conformers, with a prevalence of the more stable right-handed one.