The ability of lower peptides to form co-crystals: inclusion compounds of Leu-Leu-Leutripeptide with pyridine and picolines

Abstract

Co-crystals of L-leucyl-L-leucyl-L-leucine tripeptide (LLL) with pyridine (Py), α-picoline (2-MePy), β-picoline (3-MePy) and γ-picoline (4-MePy) have been prepared and studied. Single-crystal XRD analyses reveal the following formulas: LLL*(Py) (1), LLL*(2-MePy)*0.12(H₂O) (2), LLL*(3-MePy) (3), and LLL*(4-MePy)*0.14(H₂O) (4). The compounds may be described as inclusion compounds of LLL (host), pyridine or picolines (guest) and water (secondary guest). The host LLL molecules are arranged in anti-parallel β-sheet layers and have conformations consistent with this type of peptide organization. The layers are packed by van der Waals forces with an interlayer distance of 11.5–11.8 Å. The isobutyl side groups of leucyl residues protrude into the interlayer space and form large channels on one side of the layer. The guest molecules reside in the channels and form H-bonds to the host NH₃⁺ groups. The crystal structures have two host molecules in the asymmetric unit, which in most cases, exhibit rarely observed whole-molecule disorder. Three of the compounds invariably form as twinned crystals. TGA measurements of bulk products reveal the following compositions: LLL*1.04(Py) (1), LLL*0.99(2-MePy) (2), LLL*0.96(3-MePy) (3), and LLL*0.92(4-MePy) (4), in good agreement with the XRD results. The character of thermal dissociation implies the preservation of the host layered structure as guest molecules diffuse out of the channels. Compound 2 dissociates with the formation of an intermediate with the stoichiometry of LLL*0.5(2-MePy).