title: N-(tert-butoxycarbonyl)-α-aminoisobutyryl-α-aminoisobutyric acid methyl ester: two polymorphic forms in the space group P2(1)/n.
abstract: The title compound (systematic name: methyl 2-{2-[(tert-butoxycarbonyl)amino]-2-methylpropanamido}-2-methylpropanoate), C(14)H(26)N(2)O(5), (I), crystallizes in the monoclinic space group P2(1)/n in two polymorphic forms, each with one molecule in the asymmetric unit. The molecular conformation is essentially the same in both polymorphs, with the α-aminoisobutyric acid (Aib) residues adopting ϕ and ψ values characteristic of α-helical and mixed 3(10)- and α-helical conformations. The helical handedness of the C-terminal residue (Aib2) is opposite to that of the N-terminal residue (Aib1). In contrast to (I), the closely related peptide Boc-Aib-Aib-OBn (Boc is tert-butoxycarbonyl and Bn is benzyl) adopts an α(L)-P(II) backbone conformation (or the mirror image conformation). Compound (I) forms hydrogen-bonded parallel β-sheet-like tapes, with the carbonyl groups of Aib1 and Aib2 acting as hydrogen-bond acceptors. This seems to represent an unusual packing for a protected dipeptide containing at least one α,α-disubstituted residue.
authors.affiliation: Department of Chemistry, University of Oslo, Oslo, Norway.
authors.forename: Hadgu Girmay
authors.initials: HG
authors.lastname: Gebreslasie
authors.forename: Øyvind
authors.initials: Ø
authors.lastname: Jacobsen
authors.forename: Carl Henrik
authors.initials: CH
authors.lastname: Görbitz
mesh_terms: D000595:Amino Acid Sequence; D000621:Aminoisobutyric Acids / Q000737:chemistry*; D003460:Crystallization; D018360:Crystallography, X-Ray; D004151:Dipeptides / Q000737:chemistry*; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008968:Molecular Conformation; D009842:Oligopeptides / Q000737:chemistry*
chemical_list: D000621:Aminoisobutyric Acids; D004151:Dipeptides; C561504:N-(tert-butoxycarbonyl)-alpha-aminoisobutyryl-alpha-aminoisobutyric acid methyl ester; D009842:Oligopeptides