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‐AibOMe (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, ϕ, ψ values for residues 1–8 are −59° and −32°, respectively. Crystal parameters are C 47 H 77 N 9 O 12 , space group P1, a = 10.636(4) Å, b = 11.239(4) Å, c = 12.227(6) Å, α = 101.17(4)°, β = 97.22(4)°, γ = 89.80(3)°, Z = 1, R = 5.95% for 3018 data with | F 0 | > 3α( F ), resolution 0.93 Å. The use of the torsion angle κ = C( i − 1)N( i )C α ( i )C β ( i ), where κ = 68° for D ‐Phe and κ = 164° for L ‐Leu, confirms the opposite configurations of these residues. The ϕ, ψ values of −62° and −32° 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 stereo‐chemical preferences of the Leu residue. © 1993 John Wiley & Sons, Inc.