Lipid-bilayer-mimicking solid-state structures of Cu(ii) and Ni(ii) with l-tryptophan and l-tyrosineSchiff base derivatives

Abstract

New metal-amino acid Schiff bases resembling lipid bilayers have been isolated and established by single-crystal X-ray crystallography. Hydrogen bonding between the solvate and coordination sphere and aromatic stacking of the Schiff base ligands lead to discrete hydrophilic and hydrophobic regions. [Cu₂(WNap)₂(H₂O)₂]·2H₂O, 1 (WNap = N-(2-hydroxy-1-naphthalidene)-L-tryptophan), gives bilayer sheets with width ratio of ca. 3 Å : 5 Å (hydrophilic : hydrophobic). Replacement of Cu²⁺ by octahedral Ni²⁺ gives [Ni(WNap)(H₂O)₂(MeOH)]·H₂O, 2, with a slight increase of hydrophilic layer width (by ca. 2 Å). The additional hydroxyl group in [Ni(YNap)(H₂O)₂(MeOH)]·1.5H₂O, 3 (YNap = N-(2-hydroxy-1-naphthalidene)-L-tyrosine), successfully introduces a separate amphiphilic layer, originated from the hydrogen bond interaction of tyrosine side chain. The coordination environments of 2 and 3 are isostructural whereas 1 is dinuclear with the Cu(II) metal centers showing different geometries (square planar and square pyramid).