Accommodation of a D-Phe residue into a right-handed 3(10)-helix: structure of Boc-D-Phe-(Aib)4-Gly-L-Leu-(Aib)2-OMe, an analogue of the amino terminal segment of antiamoebins and emerimicins.

The crystal structure of the nonapeptide Boc-D-Phe-Aib-Aib-Aib-Aib-Gly-Leu-Aib-Aib-OMe (I), which is an analogue of the N-terminal sequence of antiamoebins and emerimicins, establishes a completely 3(10)-helical conformation with seven successive intramolecular 4-->1 hydrogen bonds. The average, phi,psi values for residues 1-8 are -59 degrees and -32 degrees, respectively. Crystal parameters are C47H77N9O12, space group P1, a = 10.636 (4) A, b = 11.239 (4) A, c = 12.227 (6) A, alpha = 101.17 (4) degrees, beta = 97.22 (4) degrees, gamma = 89.80 (3) degrees, Z = 1, R = 5.95% for 3018 data with magnitude of F0 > 3 sigma(F), resolution 0.93 A. The use of the torsion angle kappa = C(i-1)N(i)C alpha(i)C beta(i), where kappa = 68 degrees for D-Phe and kappa = 164 degrees for L-Leu, confirms the opposite configurations of these residues. The phi,psi values of -62 degrees and -32 degrees at D-Phe are unusual, since this region is characteristic of residues with L configurations. Peptide I possesses only two chiral residues of opposing configuration. The observed right-handed 3(10)-helical structure suggests that helix sense has probably been determined by the stereochemical preferences of the Leu residue.