Regularly alternating L,D-peptides. II. The double-stranded right-handed antiparallel beta-helix in the structure of t-Boc-(L-Phe-D-Phe)4-OMe

The crystal structure of Boc-(L-Phe-D-Phe)4-OMe has been determined by x-ray diffraction analysis. The peptide crystallizes in the triclinic system, space group P1 with a = 15.290 A, b = 15.163 A, c = 19.789 A, alpha = 102.49 degrees, beta = 96.59 degrees, gamma = 74.22 degrees, and Z = 2. The structure has been solved by coupling of the molecular replacement technique and expansion by tangent formula refinement of the set of known phases. Several cycles of Fourier calculations and least-squares refinement led to the location of 194 atoms of the two independent octapeptide chains and few molecules of cocrystallized solvent (chloroform, water, and methanol). The isotropic refinement converged to R = 0.13 for the 3077 "observed" reflections. The two independent octapeptide molecule form a dimer in the solid state: the two chains are associated by interstrand hydrogen bonds (12 of the type N-H ... O = C) with the formation of a double-stranded antiparallel right-handed -- beta 5.6-helix. These double helices can be represented as a cylinder with a hydrophilic inner core represented by the peptide units and an hydrophobic exterior constituted by the aromatic moieties. The dimensions of the cylinder are equal to those observed for Boc-(L-Val-D-Val)4-OMe. In the solid state the dimers pack with each other in an hexagonal fashion with the formation of layers; between the layers, solvent molecules fill empty spaces.