Controllable hierarchical self-assembly: systematic study forming metallosupramolecular frameworks on the basis of helical beta-oligoamides

Self-assembly is a key guiding principle for the design of complex nanostructures. Substituted beta oligoamides offer versatile building blocks that can have inherent folding characteristics, offering geometrically defined functionalities that can specifically bind and assemble with predefined morphological characteristics. In this work hierarchical self-assembly is implemented based on metal coordinating helical beta-oligoamides crosslinked with transition metals selected for their favourable coordination geometries, Fe2+, Cu2+, Ni2+, Co2+, Zn2+, and two metalates, MoO42-, and WO42-. The oligoamide Ac-β3Aβ3Vβ3S-αHαHαH-β3Aβ3Vβ3A (3H) was designed to allow crosslinking via three distinct faces of the helical unit, with a possibility of forming three dimensional framework structures. Atomic force microscopy (AFM) confirmed the formation of specific morphologies that differ characteristically with each metal. X-Ray photoelectron spectroscopy (XPS) results reveal that the metal centres can be reduced in the final structures, confirming strong chemical interaction. Time of flight secondary ion mass spectrometry (ToF-SIMS) confirmed the spatial distribution of metals within the self-assembled networks, also revealing molecular fragments that confirm coordination to histidine and carboxyl moieties. The metalates MoO42- and WO42- were also able to induce the formation of specific superstructure morphologies. It was observed that assembly with either of nickel, copper, and molybdate form thin films, while cobalt, zinc, and tungstate produced specific three dimensional networks of oligoamides. Iron was found to form both a thin film and a complex hierarchical assembly with the 3H simultaneously. The design of the 3H substituted beta oligoamide to readily form metallosupramolecular frameworks was demonstrated with a range of metals and metalates with a degree of control over layer thicknesses as a function of the metal/metalate. The results validate and broaden the metallosupramolecular framework concept and establish a platform technology for the design of functional thin layer materials.

Comprehensive multidimensional study of the self-assembly properties of a three residue substituted β3 oligoamide

Substituted β3 oligoamides form a unique self-assembling system where each monomer folds into a helix containing approximately three β3 amino acids per turn, yielding a geometrically well-defined cylindrical building block that, when N-acylated, is able to self-assemble head-to-tail into nanorods that can reach several 100 μm length. It was shown in previous works that self-assembly can be achieved with a three residue long oligoamide as well that lacks any intramolecular H-bonds, yet it crystallizes in a helix-like conformation. The self-assembly properties of these small oligoamides are however elusive, suggesting a more complex system than the self-assembly of the H-bond stabilized helical monomers. Here we focus on the self-assembly behaviour of a three residue oligoamide, Ac-β3[LIA] where the letters denote the side chain of the analogous α amino acid. Ac-β3[LIA] can yield highly inhomogeneous suspensions in water with a broad range of large fibrous structures that seem to be very stable, yet occasionally fibre growth is only observed upon heating. The small size of the monomer suggests a highly dynamic equilibrium yet all previous attempts failed to clearly identify low molecular weight species. Therefore a special methodology was employed in this study to characterize the suspensions at different size ranges: SANS that is optimal to measure the small oligomers and cross sectional diameter of the assemblies, DLS that is sensitive to the large populations and therefore the length of the superstructures, and NMR that is sensitive to monomeric and small oligomeric form, in conjunction with IR spectroscopy to probe the folding and AFM to image the morphology of the assemblies. Temperature ramping was used to perturb the system to probe the dynamicity of the self-assembly. It was found that the anomalous self-assembly behaviour of Ac-β3[LIA] is caused by its two stable conformations, a helix-building “horseshoe” fold and a linear conformer. The latter is exclusively found in monomeric form in solution whereas the horseshoe fold is stable in solid phase and in fibrous assemblies. Small oligomers were absent. Thus the self-assembly of Ac-β3[LIA] is arrested by the activation energy need of the conformation change; fibre growth might be triggered by conditions that allow increased conformational freedom of the monomers. This observation may be used to develop strategies for controlled switchable self-assembly.