Coordination crosslinking of helical substituted oligoamide nanorods with Cu(II)

Abstract

Substituted oligoamides are short sequences of unnatural amino acids. Oligoamides made entirely of β 3 amino acids yield helical monomers that, if N-acylated, assemble into nanorod structures via a supramolecular assembly motif. Crosslinking these structures using a secondary binding motif would deliver a unique hierarchical nanostructured material. In this work, this possibility is explored by attempting coordination crosslinking of oligoamides WKLWEL (KE) and WELWEL (EE) (where the letters denote the analogous α-amino acids) with Cu(II). Atomic force microscopy revealed specific morphologic changes in KE but not in EE. Small angle neutron scattering confirmed a specific change in the typical size of the KE assemblies in solution upon Cu(II) addition. Vibration spectroscopy measurements revealed that Cu(II) can coordinate to the amine moieties of the side chains, without direct effect on the backbone amides. Fast drying of the sample lead to oligoamide templated crystallization of CuCl2. It was revealed that metal coordination takes place in dilute solutions, whereas the high dielectric constant of the concentrated salt solution upon drying leads to an ionic interaction between deprotonated sidechain amine moieties, yielding a composite structure defined by charge interactions. The former is the first realization of a metallosupramolecular framework structure.

A two-dimensional metallosupramolecular framework design based on coordination crosslinking of helical oligoamide nanorods

“…The supramolecular three point hydrogen bonding motif that yields the head-to-tail self-assembly of helical b 3 -oligoamide units is very strong, hence fibres are observed in water and even in DMSO, 25 without a noticeable monomeric/small oligomeric population. 26,27 For this reason, traditional characterization methods such as NMR or CD are not feasible to perform, and therefore structure can only be inferred from microscopy imaging and indirect characterization methods.…”

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

“…6,10,13 The 14-helix allows for a greater number of internal hydrogen bonds between amide residues compared to a-helices, shielded from the chemical environment by the high density of side chains and thus leading to greater structural stability and a resistance to hydrolysis, enzymatic degradation and unwanted metal coordination. [14][15][16][17] The distinct faces of the helix can be functionalized by control of sequence. 18 The mechanisms of head-to-tail self-assembly of helical beta oligoamide units were established previously.…”

“…Through knowledge of similar beta-oligoamides we expected the neat 3H to form a fibrous assembly. 16,17,[22][23][24] The addition of metals is expected to form thin films if coordination takes place with the oligoamide, in which individual nanorods are not distinguishable. The presence of fibrous assemblies akin to the neat 3H indicate incomplete coordination or no coordination at all.…”

“…It is important to note that in the nanorods the backbone amide moieties are not accessible to the metals and so the coordination can only take place at side chains. 16,17…”

C-amidation of substituted β3 oligoamides yields novel supramolecular assembly motif

“…The acylation of the N-terminus of the oligoamides leads to the head-to-tail assembly of the helices into long fibrils, also described as nanorods, through an intermolecular H-bonding pattern that mimics the 14-helix core motif, where the amide formed with the acyl group provides the necessary third hydrogen bond donor [10,[23][24][25]. This motif is very strong and selfassembly occurs regardless of the chemistry of the sidechains present and/or residue sequence [23,24,26,27]. The fibrils further assemble into an array of diverse structures ranging from a few nm to hundreds of nm in diameter.…”