Synthesis, structural study and topological analysis of Zn/Aib and Aib-based small peptide complexes (H-Aib-OH = α-aminoisobutyric acid)

Abstract

The systematic investigation of the coordination chemistry of α-aminoisobutyric acid (H-Aib-OH) and Aib-based small peptides is continued. The solid complexes [Zn 3(H-Aib-O) 6]·MeCOOH·H 2O ( 1·MeCOOH·H 2O), {[Zn(H-Aib-L-Ala-O) 2]·H 2O} n ( 2·H 2O) and [Zn(H 2-Aib-Aib-Aib-O) 4](ClO 4) 2·5.8H 2O ( 3·5.8H 2O) have been isolated and characterized by single-crystal X-ray crystallography. In the structure of complex 1·MeCOOH·H 2O, three Zn II ions and six H-Aib-O − ligands have assembled to form a trinuclear cluster. All three Zn II centers are in a very distorted trigonal bipyramidal coordination. The trinuclear units assemble through a network of hydrogen bonds to form a 2D framework with a (3.6.3.6) topology, while the lattice acetic acid and water molecule connect the layers to create a 3D framework with a fcu topology. Complex 2·H 2O is a two-dimensional coordination polymer. The deprotonated dipeptide behaves as a η 1: η 1: η 1: μ 2 ligand binding one Zn II atom through its amino nitrogen and peptide oxygen, and an adjacent Zn II atom through one of its carboxylate oxygen. In the crystal lattice, the layers are connected in the third direction through hydrogen bonds and the resulting framework conforms to a tfa net. Complex 3·5.8H 2O consists of mononuclear [Zn(H 2-Aib-Aib-Aib-O) 4] 2+ cations, CLO 4 - and lattice water molecules. The tripeptide ligands are in their zwitterionic form and coordinate through one of the carboxylate oxygen atom to the metal ion, while they are participating in a network of intra- and intermolecular hydrogen bonds forming a 3D framework that adopts the bcu network.