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  • Title: N-(tert-butoxycarbonyl)-α-aminoisobutyryl-α-aminoisobutyric acid methyl ester: two polymorphic forms in the space group P2(1)/n.
    Author: Gebreslasie HG, Jacobsen Ø, Görbitz CH.
    Journal: Acta Crystallogr C; 2011 Aug; 67(Pt 8):o283-7. PubMed ID: 21817793.
    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.
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