Structure of <i>N</i>‐Boc‐L‐Pro‐dehydro‐Leu‐NHCH₃

Abstract

The peptide N ‐Boc‐L‐Pro‐dehydro‐Leu‐NHCH 3 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 P22 1 2 1 with a = 5.726(1)Å, b = 14.989(4) Å, c = 24.131(9) Å, V = 2071(1) Å 3 , 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 φ 1 =– 51(1)°, ψ 1 = 133(1)°, φ 2 = 74(2)° and ψ 2 = 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 χ 1 1 =– 25(1)°, χ 1 2 = 38(1)°, χ 2 =– 34(1)°, χ 1 4 = 20(1)° and χ 1 0 = 2(1)°. The side chain conformation angles of dehydro‐Leu residue are χ 2 = 12(2)°, χ 2 2.1 =– 112(2)° and χ 2 2.2 = 136(2)°. The crystal structure is stabilized by a network of hydrogen bonds and van der Waals interactions.