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Grid-like anion complexes

Final Activity Report Summary - ANION GRIDS (Grid-like anion complexes)

One of the biggest scientific challenges facing chemistry is to uncover mechanisms of spontaneous organisation of matter into highly complex, hierarchical and functional structures. Self-assembly of specific molecular architectures from organic ligands and metal cations is a very interesting playground for this endeavour. The resulting structures combine features of both types of components: i) metal ions with their optical, magnetic and redox properties, potentially sensitive to the environment and ii) ligands bearing functional groups able to recognise other molecules by non-covalent interactions. This last feature may lead to hierarchical self-assembly, where self-assembled structure binds other molecules forming even larger structures representing higher level of complexity.

In the reporting period, two major projects were pursued:
a) self-assembly of L-shaped metal complexes having anion binding sites, able to self-assemble into grid-like structures upon anion binding (hierarchical self-assembly where cation binding is followed by anion binding);
b) self-assembly of sugar-decorated grid-shaped metal complexes, able to bind multiple lectins (sugar-binding proteins) with concomitant formation of hybrid biopolymers (hierarchical self-assembly where cation binding is followed by sugar-lectin interaction).

The major interest in the first project was in the exploration of the potential of anion binding as a driving force for self-assembly of specific structures. Versatile synthetic route has been developed to novel ligands possessing both cation and anion binding sites. The ligands were shown to form L-shaped complexes with transition metal cations, as expected. Unfortunately however, no evidence of anion binding could be obtained for the resulting structures due to both stability problems (sequestering of cations by added anions) and intramolecular hydrogen bonding competing with anion binding.

The second project was focused on properties/functions emerging as a result of self-assembly, that is properties/functions not displayed by constituents, but emerging as the constituents combine into higher order superstructure. To produce simple model system having these characteristics, we have designed and synthesised sugar-decorated ligands able to self-assemble into grid-like complexes upon binding with transition metal cations (so-called [2x2] grids, comprising 4 ligands and 4 cations). The grids were designed to bind up to four sugar binding proteins (multivalency; concanavalin A was used as a model protein) and each concanavalin A can bind up to four grids. Thus, a three dimensional network of supramolecular hybrid biopolymer was expected to form, leading to precipitation.

The desired sugar-decorated grid-like complexes have been successfully obtained via two routes: by self-assembly from ligands (4 ligands and 4 zinc cations) and by component self-assembly (simple mixing of fragments of ligands, which combine by means of chemical bonds forming first the desired ligands, which in turn self-assemble into the grid-like complex). The grids were shown to be stable under conditions close to physiological (aqueous buffer, pH 7.4). Most significantly, under specific conditions one of the grids was shown to agglutinate concanavalin A, whereas neither its ligands nor their components could do so. Thus, the agglutinating ability was demonstrated to be an emergent property which could be 'obtained' by self-assembly.