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

Kamwaya, M. E. ; Oster, O. ; Bradaczek, H. ; Ponnuswamy, M. N. ; Parthasarathy, S. ; Nagaraj, R. ; Balaram, P. (1982) Structure of tert-butyloxycarbonyl-L-alanyl-L-proline monohydrate (t-Boc-Ala-Pro) Acta Crystallographica Section B: Structural Science, 38 (1). pp. 172-176. ISSN 0108-7681

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C13H22N205.H20, M r = 304.37, is orthorhombic, space group P21212l, with a = 20.751 (2), b = 13.457 (1), c = 5.875 (1) A, V = 1640.49 A3, Do = 1.20, Dc = 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: Ow··· O = 2.78, Ow···O' = 2.69 and Ow···OH = 2.50 A. Cα 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'CαCγCδgroup in the pyrrolidine ring is fairly planar. Cβ is readily displaced from this best plane of the five-membered ring and deviates by 0.489 A. N' and Cγ are on the same side of this plane in relation to the carboxyl C'. Thus t-Boc-Ala-Pro is Cs-Cγ -endo(Cβ-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'-Cα-C'-O, that is, the dihedral angle ψl Pro, equal to 161°. The Cα atoms of the prolyl and alanyl residues are trans with respect to the peptide bond.

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