Structure and conformation of linear peptides XIII. Structure of l-phenylalanyl-glycyl-glycine

The crystal structure of a tripeptide, l -phenylalanyl-glycyl-glycine (C₁₃H₁₇,N₃O₄), molecular weight = 279.3, has been determined. The crystals are orthorhombic, space group P 2₁2₁2₁, with a= 5.462(1) A, b= 15.285(5), c= 16.056(4), Z = 4 , and P(calc) = 1.384 g. cm^-3. The final R-index is 0.052 for 866 reflections with θ/λ≤ 0.55 A^-1 and 1 > σ. The molecule exists as a zwitterion, with the N-terminus protonated and the C-terminus in an ionized form. Both the peptide units are in the trans configuration and planar, though one of them shows significant deviations from planarity (|Δ| = 5.1°). The peptide backbone is folded, with the torsion angles of ψ₁= 116.2(5)°, ψ₃₁= 178.8(4), φ₂=?89.7(5), ψ₂=?28.9(6), ω₂=?174.9(4), φ₃= 134.9(5), ψ₃₁= 7.8(6), ψ₃₂=?172.6(4). The terminal glycine adopts a “d -residue” conformation. For the sidechain of phenylalanine, χ₁= 175.5(4), χ₂= - 127.0(6).