Structure of tert-butyloxycarbonyl-L-alanyl-L-proline monohydrate (t-Boc-Ala-Pro)

Abstract

C₁₃H₂₂N₂0₅.H₂0, M r = 304.37, is orthorhombic, space group P2₁2₁2_l, with a = 20.751 (2), b = 13.457 (1), c = 5.875 (1) A, V = 1640.49 A³, D_o = 1.20, D_c = 1.231 Mg m^-3; Z = 4; λ(Cu Ka) = 1.5418 A; Z = 4; F(000) = 656. Final R = 3.9% for 1664 observed reflexions. There is one molecule of water as well as an N-H···O hydrogen bond, rendering high stability to the crystal packing. The water-bridge bond is of the same type as that of a triple helix: O_w··· O = 2.78, O_w···O' = 2.69 and O_w···OH = 2.50 A. C^α is in the trans configuration. The absolute configuration of the non-centrosymmetric structure and, therefore, of the molecular conformation was determined by anomalous dispersion. The N'C^αC^γC^δgroup in the pyrrolidine ring is fairly planar. C^β is readily displaced from this best plane of the five-membered ring and deviates by 0.489 A. N' and C^γ are on the same side of this plane in relation to the carboxyl C'. Thus t-Boc-Ala-Pro is C_s-C^γ -endo(C^β-exo). This derivative belongs to conformation B, since the dihedral angle χ₁ = 28.55° takes a positive value, and it has collagen-like characteristics, with N'-C^α-C'-O, that is, the dihedral angle ψ_l Pro, equal to 161°. The C^α atoms of the prolyl and alanyl residues are trans with respect to the peptide bond.