Extra hydrogen bonding interactions by peripheral indole groups stabilize benzene-1,3,5-tricarboxamide helical assemblies

Expanding the Scope of Metastable Species in Hydrogen Bonding-Directed Supramolecular Polymerization

We reveal unique hydrogen (H-) bonding patterns and exploit them to control the kinetics, pathways and length of supramolecular polymers (SPs). New bisamide-containing monomers were designed to elucidate the role of competing intra- vs. intermolecular H-bonding interactions on the kinetics of supramolecular polymerization (SP). Remarkably, two polymerization-inactive metastable states were discovered. Contrary to previous examples, the commonly assumed intramolecularly H-bonded monomer does not evolve into intermolecularly H-bonded SPs via ring opening, but rather forms a metastable dimer. In this dimer, all H-bonding sites are saturated, either intra- or intermolecularly, hampering elongation. The dimers exhibit an advantageous preorganization, which upon opening of the intramolecular portion of the H-bonding motif facilitates SP in a consecutive process. The retardation of spontaneous self-assembly as a result of two metastable states enables length control in SP by seed-mediated growth.

Hierarchical self-assembly of aromatic peptide conjugates into supramolecular polymers: it takes two to tango

Supramolecular polymers are self-assembled materials displaying adaptive and responsive "life-like" behaviour which are often made of aromatic compounds capable of engaging in π-π interactions to form larger assemblies. Major advances have been made recently in controlling their mode of self-assembly, from thermodynamically-controlled isodesmic to kinetically-controlled living polymerization. Dynamic covalent chemistry has been recently implemented to generate dynamic covalent polymers which can be seen as dynamic analogues of biomacromolecules. On the other hand, peptides are readily-available and structurally-rich building blocks that can lead to secondary structures or specific functions. In this context, the past decade has seen intense research activity in studying the behaviour of aromatic-peptide conjugates through supramolecular and/or dynamic covalent chemistries. Herein, we review those impressive key achievements showcasing how aromatic- and peptide-based self-assemblies can be combined using dynamic covalent and/or supramolecular chemistry, and what it brings in terms of the structure, self-assembly pathways, and function of supramolecular and dynamic covalent polymers.

Higher-order interfiber interactions in the self-assembly of benzene-1,3,5-tricarboxamide-based peptides in water

Mimicking the complexity of biological systems with synthetic supramolecular materials requires a deep understanding of the relationship between the structure of the molecule and its self-assembly pattern. Herein, we report a series of water-soluble benzene-1,3,5-tricarboxamide-based di- and tripeptide derivatives modified with small non-bulky terminal amine salt to induce self-assembly into twisted one-dimensional higher-order nanofibers. The morphology of nanofibers strongly depends on the nature, order, and quantity of amino acids in the short peptide fragments and vary from simple cylindrical to complex helical. From observations of several fiber-splitting events, we detected interfiber interactions that always occur in a pairwise manner, which implies that the C3 symmetry of benzene-1,3,5-tricarboxamide-based molecules in higher-order fibers becomes gradually distorted, thus facilitating hydrophobic contact interactions between fibrils. The proposed mechanism of self-assembly through hydrophobic contact allowed the successful design of a compound with pH-responsive morphology, and may find use in the future development of complex hierarchical architectures with controlled functionality.

Experimental and computational diagnosis of the fluxional nature of a benzene-1,3,5-tricarboxamide-based hydrogen-bonded dimer

Precise characterization of the hydrogen bond network present in discrete self-assemblies of benzene-1,3,5-tricarboxamide monomers derived from amino-esters (ester BTAs) is crucial for the construction of elaborated functional co-assemblies. For all ester BTA dimeric structures previously reported, ester carbonyls in the side chain acted as hydrogen bond acceptors, yielding well-defined dimers stabilized by six hydrogen bonds. The ester BTA monomer derived from glycine (BTA Gly) shows a markedly different self-assembly behaviour. We report herein a combined experimental and computational investigation aimed at determining the nature of the dimeric species formed by BTA Gly. Two distinct dimeric structures were characterized by single-crystal X-ray diffraction measurements. Likewise, a range of spectroscopic and scattering techniques as well as molecular modelling were employed to diagnose the nature of dynamic dimeric structures in toluene. Our results unambiguously establish that both ester and amide carbonyls are involved in the hydrogen bond network of the discrete dimeric species formed by BTA Gly. The participation of roughly 4.5 ester carbonyls and 1.5 amide carbonyls per dimer as determined by FT-IR spectroscopy implies that several conformations coexist in solution. Moreover, NMR analysis and modelling data reveal rapid interconversion between these different conformers leading to a symmetric structure on the NMR timescale. Rapid hydrogen bond shuffling between conformers having three (three), two (four), one (five) and zero (six) amide carbonyl groups (ester carbonyl groups, respectively) as hydrogen bond acceptors is proposed to explain the magnetic equivalence of the amide N-H on the NMR timescale. When compared to other ester BTA derivatives in which only ester carbonyls act as hydrogen bond acceptors, the fluxional behaviour of the hydrogen-bonded dimers of BTA Gly likely originates from a larger range of energetically favorable conformations accessible through rotation of the BTA side chains.