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

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) Å3, 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 Cγ‐exo conformation with torsion angles of χ11=– 25(1)°, χ12= 38(1)°, χ2=– 34(1)°, χ14= 20(1)° and χ10= 2(1)°. The side chain conformation angles of dehydro‐Leu residue are χ2= 12(2)°, χ22.1=– 112(2)° and χ22.2= 136(2)°. The crystal structure is stabilized by a network of hydrogen bonds and van der Waals interactions.