Structure and conformation of linear peptides. X. Structure of glycyl-glycyl-L-phenylalanine hydrochloride

The crystal structure of a tripeptide, glycyl-glycyl-L-phenylalanine HCl (C₁₃H₁₈N₃O₄ ± HCl, molecular weight = 316.5) has been determined. The crystals are orthorhombic, space group P2₁2₁2₁, with a = 4.877 (2) Å, b = 9.956(3), c = 32.690(5) and Z = 4. The final R-index is 0.043 for 1325 reflections (sinØ/Λ ± 0.55 Å^-1) with I > 2.0 s? (I). The N-terminal of the molecule is protonated and the C-terminal exists in an un-ionised state. The peptide units are trans and one of them shows significant deviations from planarity (¶Δω₁¶ = 11.3°). The peptide backbone is folded with torsion angles of: ø₁ = 165.5°, ω₁ = — 168.7°, ø₂ = 63.6°, ø₂ = — 153.6°, ω₂ = 176.5°, ø₃ = — 72.2° and Φ₃ = 166.5°. For the side chain of phenylalanine, X₁ = — 79.5° and X₂ = 86.8°. An intramolecular water bridge links the two ends of the molecule.