Structure of N-Boc-L-Pro-dehydro-Leu-NHCH₃

Abstract

The peptide N-Boc-L-Pro-dehydro-Leu-NHCH₃ was synthesized to examine the nature of β-bend as a result of dehydro-Leu in the sequence. The peptide crystallizes from methanol-water mixture at 4° in orthorhombic space group P22₁2₁ with a = 5.726(1)Å, b = 14.989(4) Å, c = 24.131(9) Å, V = 2071(1) ų, Z = 4, dm = 1.064(5)gcm⁻³ and dc = 1.0886(5)gcm⁻³. The structure was solved by direct methods using SHELXS 86 and it was refined by full-matrix least-squares procedure to an R value of 0.059 for 957 observed reflections. The peptide is found to adopt a β-bend type II conformation with φ₁=− 51(1)°, ψ₁= 133(1)°, φ₂= 74(2)° and ψ₂= 8(2)°. The β-bend is stabilized by an intra-chain hydrogen bond between the carbonyl oxygen of ith residue and the NH of (i + 3)th residue. The five-membered pyrrolidine ring of Pro-residue adopts an ideal C^γ-exo conformation with torsion angles of χ¹₁= −25(1)°, χ¹₂= 38(1)°, χ₂= −34(1)°, χ⁴₁= 20(1)° and χ⁰₁= 2(1)°. The side chain conformation angles of dehydro-Leu residue are χ₂= 12(2)°, χ₂^2.1= −112(2)° and χ₂^2.2= 136(2)°. The crystal structure is stabilized by a network of hydrogen bonds and van der Waals interactions.