Synthesis of N-substituted Pyrido[4,3-d]pyrimidines for the Large-Scale Production of Self-Assembled Rosettes and Nanotubes

Water-Soluble Supramolecular Polymers of Paired and Stacked Heterocycles: Assembly, Structure, Properties, and a Possible Path to Pre-RNA

The hypothesis that RNA and DNA are products of chemical and biological evolution has motivated our search for alternative nucleic acids that may have come earlier in the emergence of life-polymers that possess a proclivity for covalent and non-covalent self-assembly not exhibited by RNA. Our investigations have revealed a small set of candidate ancestral nucleobases that self-assemble into hexameric rosettes that stack in water to form long, twisted, rigid supramolecular polymers. These structures exhibit properties that provide robust solutions to long-standing problems that have stymied the search for a prebiotic synthesis of nucleic acids. Moreover, their examination by experimental and computational methods provides insight into the chemical and physical principles that govern a particular class of water-soluble one-dimensional supramolecular polymers. In addition to efficient self-assembly, their lengths and polydispersity are modulated by a wide variety of positively charged, planar compounds; their assembly and disassembly are controlled over an exceedingly narrow pH range; they exhibit spontaneous breaking of symmetry; and homochirality emerges through non-covalent cross-linking during hydrogel formation. Some of these candidate ancestral nucleobases spontaneously form glycosidic bonds with ribose and other sugars, and, most significantly, functionalized forms of these heterocycles form supramolecular structures and covalent polymers under plausibly prebiotic conditions. This Perspective recounts a journey of discovery that continues to reveal attractive answers to questions concerning the origins of life and to uncover the principles that control the structure and properties of water-soluble supramolecular polymers.

Synthesis of N-Bridged Pyrido[4,3-d]pyrimidines and Self-Assembly into Twin Rosette Cages and Nanotubes in Organic Media

Two N-bridged pyrido[4,3-d]pyrimidine derivatives were synthesized toward realization of a self-assembled bis-rosette cage, in organic media. Starting from commercially available malononitrile dimer and dimethyl 5-aminoisophthalate, the target molecules were synthesized in 11 steps using a convergent approach. The final bridged compounds were characterized by nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry. The hierarchical self-assembly of the nanocages into rosette nanotubes and nanobundles was established by electron microscopy and molecular modelling studies.

Formation of supramolecular assemblies and liquid crystals by purine nucleobases and cyanuric acid in water: implications for the possible origins of RNA

The free nucleobases and mononucleotides of RNA do not form Watson-Crick base pairs in water, a fact that presents several challenges for the prebiotic synthesis of RNA. 2,6-Diaminopurine and adenosine-5'-monophosphate (AMP) are shown to form supramolecular assemblies with cyanuric acid in water. These assemblies and their propensity to form liquid crystals suggest a possible means by which non-covalent structures might have originally selected the shape of the Watson-Crick base pairs.

Hydrogen-Bonded Macrocyclic Supramolecular Systems in Solution and on Surfaces

Cyclization into closed assemblies is the most recurrent approach to realize the noncovalent synthesis of discrete, well-defined nanostructures. This review article particularly focuses on the noncovalent synthesis of monocyclic hydrogen-bonded systems that are self-assembled from a single molecule with two binding-sites. Taking advantage of intramolecular binding events, which are favored with respect to intermolecular binding in solution, can afford quantitative amounts of a given supramolecular species under thermodynamic control. The size of the assembly depends on geometric issues such as the monomer structure and the directionality of the binding interaction, whereas the fidelity achieved relies largely on structural preorganization, low degrees of conformational flexibility, and templating effects. Here, we discuss several examples described in the literature in which cycles of different sizes, from dimers to hexamers, are studied by diverse solution or surface characterization techniques.

Molecular and supramolecular chemistry of rosette nanotubes

Synthetic strategies used to tune the properties of a class of supramolecular 1D nanostructures, the rosette nanotubes, are reviewed herein.

Ultra-sensitive pH control of supramolecular polymers and hydrogels: pKa matching of biomimetic monomers

pK_a-matched monomers self-assemble into supramolecular polymers at pH 7, producing hydrogels with high pH sensitivity.