Structure and conformation of linear peptides VII.

The tripeptide, L-valyl-glycyl-glycine (C₉H₁₇N₃O₄, molecular weight = 231), crystallizes in the monoclinic space group C2, with a = 24.058(3)Å, b = 4.801(1), c = 10.623(2), β = 110.02(1)° and Z = 4. The structure was determined by direct methods and refined to a final R-index of 0.043 for 830 reflections (sinθ/Λ ≤ 0.53 A^-1) with I> 1.0s?. The molecule exists as a zwitterion. The peptide units are trans and one of them shows significant deviations from planarity (Δω₂ = 9.3°). The peptide chain repeat distance, 1C^α-3Cα, is 7.23Å and the molecule displays a highly extended conformation with backbone torsion angles of ψ₁ = 123.1°, ω₁ = - 179.4°, ø₂ = - 155.1°, ψ;₂ = 154.7°, ω₂ = 170.7°, ø₃ = - 146.6° and ψ₃ = 180.0°. For the valyl side chain, χ₁₁ = - 52.5°, χ₁₂ = 174.2°. The packing involves hydrogen-bonded interactions between successive molecules related by the β-translation of the lattice, giving rise to the familiar parallel β-sheet structure which appears to be the most extended one observed to date.