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Hydrophobic vs hydrophilic: Ionic competition in remacemide salt structures

Lewis, G.R. and Steele, G. and McBride, L. and Florence, A.J. and Kennedy, A.R. and Shankland, N. and David, W.I.F. and Teat, S.J. (2005) Hydrophobic vs hydrophilic: Ionic competition in remacemide salt structures. Crystal Growth and Design, 5 (2). pp. 427-438. ISSN 1528-7483

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Remacemide[2-amino-N-(1-methyl-1,2-diphenylethyl)-acetamide] was developed as a potential antagonist for epilepsy, Parkinsonism, and Huntington's disease. This paper investigates hydrophilic and hydrophobic intermolecular interactions that occur within the series of crystal structures comprising remacemide 1 and six of its salts [2 = chloride; 3 = nitrate; 4 = acetate (C2H3O2-); 5 = hydrogenfumarate (C4H3O4-); 6 = naphthalene-2-sulfonate (napsilate, C10H7O3S-); 7 = 1-hydroxynaphthalene-2-carboxylate (xinafoate, C11H7O3-)]. The hydrophilic interactions are described through graph set analyses of the hydrogen bond motifs and networks. The lattice of 1 comprises unidirectional, one-dimensional chains of molecules parallel to the c-axis. In 2, the cation-anion hydrogen bonding imposes a well-defined hydrophilic stratum structure on the lattice. As the cation itself is amphiphilic, a natural consequence of this is the creation of two-dimensional stacked layers with alternating hydrophilic and hydrophobic character (lattice bilayers). This tendency to form bilayers within the lattice is also observed in structures 3-5 (polar anions) and structures 6-7 (amphiphilic anions). Relatively few well-directed intermolecular interactions are observed between aromatic rings, either in 1 or in the hydrophobic layers of 2-7. Therefore, it is concluded that it is the hydrophilic hydrogen bond interactions that dominate the crystal packing and drive the segregation into lattice bilayers in the salt crystal structures.