Solution structure of peptides containing two dehydro-phenylalanine residues: a CD investigation

The peptides Ac-delta Phe-Ala-delta Phe-NH-Me (1), Ac-delta Phe-Val-delta Phe-NH-Me (2), Ac-delta Phe-Gly-delta Phe-Ala-OMe (3), and Boc-Ala-delta Phe-Gly-delta Phe-Ala-OMe (4), containing two dehydro-phenylalanine (delta Phe) residues, were synthesized and the solution structure investigated in various solvents. The nmr and CD measurements indicate that all the dehydropeptides examined adopt 3(10)-helical conformations in solution. The tripeptides 1 and 2 exhibited an intense negative CD exciton couplet, which was assigned to the right-handed screw sense, while the tetrapeptide 3 displayed a CD couplet having opposite sign, which was assigned to the left-handed helical sense. In the pentapeptide 4 the sense of the helix was found to vary with solvent and temperature, as demonstrated by the sign reversal of the CD spectrum. The right-handed sense dominates in hexafluoro-2-propanol, whereas a left-handed helix prevails in chloroform, acetonitrile and methanol. A crucial role for this behavior is likely to be played by the two alanine residues positioned respectively at the head and tail of the sequence, which favor conformations having opposite screw senses.