Crystal structure and conformation of methionyl dipeptides: Structure of L-methionyl-L-asparagine: Preferred conformations of peptides containing methionyl and asparaginyl residues

Abstract

Crystals of L-methionyl-L-asparagines (C₉N₃H₁₇O₄S) are orthorhombic, space group P2₁2₁2₁, with cell parameters at 294 K of a = 5.248(1), b = 13.251(2), c = 18.028(2) Å, V = 1251.6(4) ų, Z = 4, D_m = 1.40 Mg/m³, and D_x = 1.397 Mg/m³. The crystal structure was solved by direct methods and refinded by full-matrix least-squares method to a final R value of 0.059 for 1804 reflections. The peptide is slightly nonpalnar [ω = 172.0(5)^∘]; the other torsion angles ψ₁, ϕ₂, ψ₂¹, and ψ₂² are 155.6(5), −95.8(5), 152.5(5), and −29.4(6)^∘ respectively. The methionyl side chain is disordered with two possible position for its C^γ, S^δ, and C^ε atoms. The methionly side chain is twisted with χ¹ = −86.4(7), χ² = 177.5(9), and χ³ = 116.6(2)^∘ for the major conformer A. The asparaginyl side chain is also twisted with χ¹ = −76.3(5) and χ² = 102.3(6)^∘. The crystal structure is stabilized by an intermolecular network of hydrogen bonds involving the N-terminal amino group and the ε-terminal amino group of asn residue as donors and the carboxyl oxygens of the C-terminal and the peptide carbonyl oxygens as acceptor atoms.