A helical peptide containing a majority of valyl residues. Crystal structure of t-butyloxycarbonyl-(L-valyl-α-aminoisobutyryl)₃-L-valyl methyl ester monohydrate

Abstract

The monohydrate of the heptapeptide t-butyloxycarbonyl-(L-valyl-α-aminoiso-butyryl)₃-L-valyl methyl ester crystallizes in the orthorhombic space group P2₁2₁2₁ with four molecules in a unit cell with the dimensions α= 9.375, b = 19.413 and c = 25.878 ÅA. The structure has been solved by direct methods and refined to an R value of 0.059 for 3633 observed reflections. The molecule in the structure exists as a slightly distorted 3₁₀-helix stabilized by five 4 → 1 intramolecular hydrogen bonds, indicating the overwhelming influence of α-aminoisobutyryl (Aib) residues in dictating helical fold even when a majority of residues in the peptide have a low intrinsic propensity to be in helices. Contrary to what is expected in helical structures, the valyl side chains, two of which are disordered, exhibit all three possible conformations. The molecules arrange themselves in a head-to-tail fashion along the c-axis. The columns thus generated pack nearly hexagonally in the crystal.