Effect of a hydrophobic amino acid at position (i −1) on the stability of β-turns in hydrophilic pentapeptides as studied by NMR and molecular mechanics

A detailed NMR study of the peptide NAc-FDEKA-NH2 in aqueous and in CD3OH/H2O solutions as well as the N-acetylpentapeptide amides YDEKA, VDEKA, GDEKA, and the protected tetrapeptide NAc-DEKA-NH2 in methanolic solutions indicates the importance of the first amino acid (at i - 1) on stabilizing the type I beta-turn. The data illustrate the hydrophobic stabilization of this turn, which is present in FDEKA, YDEKA, and VDEKA. For GDEKA and DEKA, the NMR data indicate that this turn is not present. Molecular mechanics calculations support this conclusion and indicate that for FDEKA and GDEKA the type I beta-turn is distorted in both the vacuum and the solvated structures. For the solvated structures, the C-alpha(i)-C-alpha(i + 3) distance is 4.87 Angstrom for FDEKA and 6.00 Angstrom for GDEKA, which are to be compared with the value of 4.64 Angstrom for an ideal type I beta-turn, i.e., the distortion is far greater in GDEKA than in FDEKA. The calculations can be interpreted to indicate the presence of two major conformations in solution.