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abstracts
"One of major approaches in the design of cavity space in the solids utilizes non-self-complementary molecules[1]. The irregular shape of the molecules and/or specific directionality of potential H-bonds prevent close packing of the molecules and yields various architectures hosting a second component, from inclusion compounds and co-crystals to complex non-crystalline patterns in biology. The strategy of non-self-complementary molecules has been extended in our studies to 2D supramolecular polymers based on short peptides[2]. The formation of the peptide layer with a desired overall geometry is controlled by strong, charge-assisted H-bonds (arrows in the Figure) in a β-sheet-like network as well as the segregation of hydrophobic amino acid residues into the interlayer space. The H-bonds add stability to the whole architecture while the hydrophobic groups keep the stacking layers at a distance that generates a cavity space available to a second component (encircled ""G"" in the Figure). A wide range of inclusions and co-crystals have been prepared in our group based on a series of dipeptides and higher peptide oligomers. For example, the incorporation of various organic solvents and bioactive molecules have been demonstrated for leucyl-alanine and similar dipeptides: alcohols, amides, phenols, pyridines, polyols, vitamins, scents and flavors. The crystal structure studies reveal a surprisingly persistent structural motif that can be used for engineering of crystalline materials with a specific property. We believe this type of peptide matrix may be utilized in the solid state organic synthesis [3] as reactive molecules of the second component can be oriented in a predictable way with respect to each other. "
abstracts
Although the ability of different molecules to crystallize in a single solid has been known for a long time [1], some of these materials became a subject of intense studies in the last decade due to pharmaceutical and other applications [2]. Short peptides may become a very useful class of co-crystallization agents due to their wide diversity, eco-friendliness and biocompatibility [3]. In this work, we screened the dipeptide leucyl-alanine (LA, see Figure) for the ability to form co-crystals with a variety of solid bioactive compounds. Solvent-assisted grinding of two compounds was followed by a powder X-ray diffraction test. The tested bioactive compounds were found to co-crystallize successfully with the peptide when they possessed both a hydrophobic part and strong hydrogen-bonding functionality. They were primarily derivatives of benzene, phenol, pyridine, pyrazine, quinoline and isoquinoline. Nearly all compounds with an amine or amide group formed a co-crystal, whereas most carboxylic acids did not form a new phase. For the successful combinations, single crystals were obtained when possible and studied using the single-crystal X-ray diffraction analysis. To our surprise, many of the co-crystals formed contained more than the two intended components due to the incorporation of the organic solvent and/or water. For example, one of the co-crystals studied displayed a complex hydrogen bonding framework built by four types of molecules: LA, 8-quinolinecarboxylic acid, ethanol and water in a 1:0.5:0.5:0.5 ratio.
