Conformational study of two linear hexapeptides by two-dimensional NMR and computer-simulated modeling: implication for peptide cyclization in solution.
Two linear peptides, D-leucyl-L-prolyl-L-isoleucyl-L-valyl-L-alanyl-beta-alanine (I) and D-leucyl-L-prolyl-L-isoleucyl-L-valyl-N-methyl-L-alanyl-beta-alanine (II), whose sequences were designed from protodestruxin and desmethyldestruxin B by replacing D-leucic acid with D-leucine, two cyclic hexadepsipeptides with insecticidal and immunodepressant activities, have been found to be cyclized in unusually high yields (>85%). In order to gain insight into the conformation and the relative flexibility of different constituent residues in these linear peptides, we have applied various techniques of 2D-NMR spectroscopy coupled with dynamic simulated annealing by computer modeling to establish the solution conformations of these two linear peptides. Based on the derived structures, it is found that the distances between N- and C-terminal residues of both peptides are short enough to facilitate the cyclization, thus collaborating the observation of favorable cyclization yields for both linear peptides.