Abstract: | 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). |