Regularly alternating L,D-peptides. I. The double-stranded left-handed antiparallel beta-helix in the structure of Boc-(L-Val-D-Val)4-OMe.

The structure of Boc-(L-Val-D-Val)4-OMe has been determined by x-ray single-crystal diffraction analysis. The octapeptide crystallizes in the trigonal system, space group P3(2)21 with a = b = 12.760 A, c = 63.190 A and Z = 6. The independent unit is represented by one octapeptide chain. The structure has been solved by direct methods and it was anisotropically refined by least-squares procedures to a final R value of 0.08 for the 3018 "observed" reflections. One molecule of water was also located in the unit cell. Two octapeptide chains, related by a crystallographic binary axis, wind up around each other giving rise to a double-stranded left-handed antiparallel increases decreases beta 5.6-helix. The dimer, stabilized by 14 interstrand N--H....O = C hydrogen bonds, can be regarded as a cylinder with an hydrophilic inner core represented by the peptide units and an hydrophobic exterior of isopropyl groups. The inner diameter of the cylinder is 5.1 A.