Monoclinic polymorph of Boc-Trp-Ile-Ala-Aib-Ile-Val-Aib-Leu-Aib-Pro-OMe(anhydrous)

Abstract

The structures of two crystal forms of Boc-Trp-Ile-Ala-Aib-Ile-Val-Aib-Leu-Aib-Pro-OMe have been determined. The triclinic form (PI, Z= l) from DMSO/H₂O crystallizes as a dihydrate (Karle, Sukumar & Balaram (1986) Proc. Natl. Acad. Sci. USA 83, 9284-9288). The monoclinic form (P2¹, Z = 2) crystallized from dioxane is anhydrous. The conformation of the peptide is essentially the same in both crystal systems, but small changes in conformational angles are associated with a shift of the helix from a predominantly α-type to a predominantly 3₁₀-type. The r.m.s. deviation of 33 atoms in the backbone and Cβ positions of residues 2-8 is only 0.29 Å between molecules in the two polymorphs. In both space groups, the helical molecules pack in a parallel fashion, rather than antiparallel. The only intermolecular hydrogen bonding is head-to-tail between helices. There are no lateral hydrogen bonds. In the P2₁ cell, a = 9.422(2)Å, b = 36.392(11)Å, c = 10.548(2)Å, β = 111.31(2)° and V = 3369.3ų For 2 molecules of C⁶⁰H⁹⁷N¹¹O¹0 per cell.