Nanometer-sized amino acids for the synthesis of nanometer-scale water-soluble molecular rods of precise length

Sequence effect on the self-assembly of discrete amphiphilic co-oligomers with fluorene-azobenzene semirigid backbones

Sequences can have a dramatic impact on the unique properties and self-assembly in natural macromolecules, which has received increasing interest. Herein, we report a series of discrete amphiphilic co-oligomers with the same composition but different building blocks in a semirigid backbone. These sequence-defined oligomers possess two primary amine groups on the side chain of the azobenzene building block, and hence, they become amphipathic due to quaternization of the amine groups when protonated in acidic aqueous solution. These oligomer isomers assembled into different nanoparticles, including nanofibers, hollow vesicles and spherical micellar complexes, in a THF/water/HCl mixture under the same conditions. UV-vis absorption spectra, differential scanning calorimetry (DSC) and X-ray scattering (XRD) experiments combined with theoretical calculations reveal that the sequence-controlled co-oligomers induce different molecular packing conformations and arrangement modes of building blocks in self-assembly. Furthermore, these self-assembled nanoparticles demonstrate photoresponsive morphological transformation and fluorescence emission under UV light irradiation due to trans-to-cis photoisomerization of azobenzene. This work demonstrates that customizing functional nanoparticles can be achieved by controlling the sequence structure in synthetic co-oligomers.

Evolution of π-Peptide Self-Assembly: From Understanding to Prediction and Control

Supramolecular materials derived from the self-assembly of engineered molecules continue to garner tremendous scientific and technological interest. Recent innovations include the realization of nano- and mesoscale particles (0D), rods and fibrils (1D), sheets (2D), and even extended lattices (3D). Our research groups have focused attention over the past 15 years on one particular class of supramolecular materials derived from oligopeptides with embedded π-electron units, where the oligopeptides can be viewed as substituents or side chains to direct the assembly of the central π-electron cores. Upon assembly, the π-systems are driven into close cofacial architectures that facilitate a variety of energy migration processes within the nanomaterial volume, including exciton transport, voltage transmission, and photoinduced electron transfer. Like many practitioners of supramolecular materials science, many of our initial molecular designs were designed with substantial inspiration from biologically occurring self-assembly coupled with input from chemical intuition and molecular modeling and simulation. In this feature article, we summarize our current understanding of the π-peptide self-assembly process as documented through our body of publications in this area. We address fundamental spectroscopic and computational tools used to extract information regarding the internal structures and energetics of the π-peptide assemblies, and we address the current state of the art in terms of recent applications of data science tools in conjunction with high-throughput computational screening and experimental assays to guide the efficient traversal of the π-peptide molecular design space. The abstract image details our integrated program of chemical synthesis, spectroscopic and functional characterization, multiscale simulation, and machine learning which has advanced the understanding and control of the assembly of synthetic π-conjugated peptides into supramolecular nanostructures with energy and biomedical applications.

Toward the Total Synthesis of Alpkinidine: Synthesis of Haloquinone CE Ring System Synthons and Attempted Nucleophilic Bisannulation

Model chemistry involving the bisannulation of 2,3-dichloro-1,4-naphthoquinone with the ester enolate derived from ethyl o-nitrophenylacetic acid, which rapid assembled the ABCD ring system of a pentacyclic pyrroloacridine, has been applied to the attempted synthesis of the marine natural product alpkinidine. The reaction of ethyl o-nitrophenylacetic acid with 6,7-dichloro-2-methylisoquinoline-1,5,8(2H)-trione, required to extend the model strategy to alpkinidine, was unfruitful, giving only complex mixtures. Efforts to direct the regiochemistry of the key Michael substitution step using 6-bromo-2-methylisoquinoline-1,5,8(2H)-trione afforded an adduct sharing the complete carbon skeleton of alpkinidine, but this could not be elaborated to the natural product.

Ultrathin inorganic molecular nanowire based on polyoxometalates

The development of metal oxide-based molecular wires is important for fundamental research and potential practical applications. However, examples of these materials are rare. Here we report an all-inorganic transition metal oxide molecular wire prepared by disassembly of larger crystals. The wires are comprised of molybdenum(VI) with either tellurium(IV) or selenium(IV): {(NH4)2[XMo6O21]}n (X=tellurium(IV) or selenium(IV)). The ultrathin molecular nanowires with widths of 1.2 nm grow to micrometre-scale crystals and are characterized by single-crystal X-ray analysis, Rietveld analysis, scanning electron microscopy, X-ray photoelectron spectroscopy, ultraviolet-visible spectroscopy, thermal analysis and elemental analysis. The crystals can be disassembled into individual molecular wires through cation exchange and subsequent ultrasound treatment, as visualized by atomic force microscopy and transmission electron microscopy. The ultrathin molecular wire-based material exhibits high activity as an acid catalyst, and the band gap of the molecular wire-based crystal is tunable by heat treatment.

Synthesis of Oligo(spiroketal)s from Naturally Occurring myo-Inositol

Dehydrating polycondensation of tetraol-type monomers derived from naturally occurring myo-inositol with 1,4-cyclohexanedione formed a series of oligo(spiroketal)s. One of the spiroketals had reactive allyl pendants, which were useful for side-chain modifications via thiol-ene chemistry.

Supramolecular construction of optoelectronic biomaterials

Peptide self-assembly is a powerful method to create functional nanoscale materials such as optoelectronically relevant organic nanostructures. The enormous potential that may come from bringing π-conjugated electronic function into biological environments is poised to impact cell and tissue engineering, biosensors, and related biomedical applications. However, very little synthetic guidance is available with respect to uniting these two different materials sets in a generally applicable manner. In this Account, I describe my group's work to synthesize and assemble peptidic nanostructures built around organic electronic elements. The Account begins with a very brief background to the area of supramolecular electronics, followed by a description of areas where these nanomaterials could be useful in biology. I then discuss the synthetic approaches that we utilized to embed a variety of π-electron units directly within peptide backbones. A key supramolecular challenge with respect to subsequent self-assembly of these new molecules is balancing electrostatic contributions within the resulting nanomaterials, because the suitable geometries for stabilizing peptide assemblies may not necessarily correspond to those suitable for maximizing intermolecular π-electron interactions. Regardless of the respective magnitudes of these two major influences, the assembly paradigm is fairly robust. Variation of the π-electron units and the peptide sequences that make up the "peptide-π-peptide" triblock molecules consistently leads to fairly uniform tape-like nanostructures that maintain strong electronic coupling among the component π-electron units. We explored a diverse range of π-electron units spanning fluorescent oligo(phenylene vinylene)s, electron-accepting rylene diimides, and hole-transporting oligothiophenes. I then describe the characterization of the nanomaterials that form after molecular self-assembly in order to understand their internal structures, electronic interactions, and morphologies as existing within self-supporting hydrogel matrices. I also describe how a facile shearing process provided globally aligned macroscopic collections of one-dimensional electronic fibrils in hydrogel matrices. These general assembly processes influence intermolecular π-stacking among the embedded chromophores, and the assemblies themselves can facilitate the covalent cross-linking and polymerization (for example, of reactive diyne units). The latter offers an exciting possibility to create peptidic nanostructures comprised of single polymer chains. Finally, I discuss electronic properties as manifested in the interactions of transition dipoles within the nanomaterials and electrical properties resulting from field-effect gating. The ability to tune the observable electrical properties of the nanostructures externally will allow for their transition to in vitro or in vivo platforms as a powerful new approach to regulating biological interactions at the nanoscale.

Coassembly of enantiomeric amphipathic peptides into amyloid-inspired rippled β-sheet fibrils

Amphipathic peptides composed of alternating hydrophobic and hydrophilic amino acids self-assemble into amyloid-inspired, β-sheet nanoribbon fibrils. Herein, we report a new fibril type that is formed from equimolar mixtures of enantiomeric amphipathic peptides (L- and D-(FKFE)(2)). Spectroscopic analysis indicates that these peptides do not self-sort and assemble into enantiomeric fibrils composed of all-l and all-d peptides, but rather coassemble into fibrils that contain alternating L- and D-peptides in a "rippled β-sheet" orientation. Isothermal titration calorimetry indicates an enthalpic advantage for rippled β-sheet coassembly compared to self-sorted β-sheet assembly of enantiomeric peptides.

A bio-inspired design strategy: Organization of tryptophan-appended naphthalenediimide into well-defined architectures induced by molecular interactions

The chemistry of molecular assemblies involves weak yet complex non-covalent interactions, and the molecular organization of the π-conjugated material is crucial in determining the performance of an organic electronic device. Herein we demonstrate a bioinspired design strategy to tune the self-assembly of naphthalenediimides (NDIs) by minute structural variations, π-π stacking, hydrophobic interactions and metal interactions. We address some of the limitations associated with current design strategies, such as restriction to a specific molecular interaction or the difficulty in controlling the assembly due to several complicated intermolecular interactions. Hydrophobic-effect-induced J-type aggregation and sodium-interaction-induced H-type aggregation of tryptophan-appended NDIs have been illustrated. (1)H NMR spectra further reveal sodium cation-π interactions in tryptophan-appended NDIs, while NMR and IR spectroscopic studies confirm the structural variations associated with the molecular assembly. In summary, the molecular organization has been successfully transformed from nanospheres to particles, nanobelts, fibers and fractals. Such drastic changes in the morphology are clear and striking evidence of the importance of non-trivial weak non-covalent forces.

Nanometer-scale water-soluble macrocycles from nanometer-sized amino acids

This paper introduces the unnatural amino acids m-Abc(2K) and o-Abc(2K) as nanometer-sized building blocks for the creation of water-soluble macrocycles with well-defined shapes. m-Abc(2K) and o-Abc(2K) are homologues of the nanometer-sized amino acid Abc(2K), which we recently introduced for the synthesis of water-soluble molecular rods of precise length (J. Am. Chem. Soc. 2007, 129, 7272). Abc(2K) is linear (180 degrees), m-Abc(2K) creates a 120 degree angle, and o-Abc(2K) creates a 60 degree angle. m-Abc(2K) and o-Abc(2K) are derivatives of 3'-amino-(1,1'-biphenyl)-4-carboxylic acid and 2'-amino-(1,1'-biphenyl)-4-carboxylic acid, with two propyloxyammonium side chains for water solubility. m-Abc(2K) and o-Abc(2K) are prepared as Fmoc-protected derivatives Fmoc-m-Abc(2K(Boc))-OH (1a) and Fmoc-o-Abc(2K(Boc))-OH (1b). These derivatives can be used alone or in conjunction with Fmoc-Abc(2K(Boc))-OH (1c) as ordinary amino acids in Fmoc-based solid-phase peptide synthesis. Building blocks 1a-c were used to synthesize macrocyclic "triangles" 9a-c, "parallelograms" 10a,b, and hexagonal "rings" 11a-d. The macrocycles range from a trimer to a dodecamer, with ring sizes from 24 to 114 atoms, and are 1-4 nm in size. Molecular modeling studies suggest that all the macrocycles except 10b should have well-defined triangle, parallelogram, and ring shapes if all of the amide linkages are trans and the o-alkoxy substituents are intramolecularly hydrogen bonded to the amide NH groups. The macrocycles have good water solubility and are readily characterized by standard analytical techniques, such as RP-HPLC, ESI-MS, and NMR spectroscopy. (1)H and (13)C NMR studies suggest that the macrocycles adopt conformations with all trans-amide linkages in CD(3)OD, that the "triangles" and "parallelograms" maintain these conformations in D(2)O, and that the "rings" collapse to form conformations with cis-amide linkages in D(2)O.

Synthesis of functionalised aromatic oligamide rods

A current goal in synthetic chemistry is the design and synthesis of molecules that adopt well defined conformations-so called foldamers. In this manuscript we describe a modular approach for construction of rod shaped para-oligobenzamide molecules. Our approach permits regiospecific incorporation of side chains through a phenolic ether linkage on the scaffold; a feature that partly restricts the conformation of the rod through intramolecular hydrogen-bonding.