Electrostatic assembly of a multicomponent peptide/amphiphile nanotube.

The ability to integrate the elements of a multicomponent nanostructure with nanoscale precision by co-assembly provides a versatile strategy to create novel materials with tunable properties. The search for function in these materials will require new strategies to be developed that control the assembly process, especially for structurally dissimilar components, which often have a propensity to self-sort into non-integrated nanostructures. In this work, two components, a peptide (1) and an amphiphile (2), were integratively co-assembled into a multicomponent nanotube. The interaction between the two components at the supramolecular level was driven by the electrostatic complementarity of the components, which was controlled by the pH-dependent charge of 1. Characterization of the co-assembled nanotube, 1-2NT, was achieved using a combination of TEM, AFM, CLSM and SIM techniques, which showed that both components were colocalized within the nanotube. These studies, in conjunction with CD, IR and fluorescence studies, suggested that 1 and 2 were arranged in partially reorganized, self-sorted domains, which were integrated as laminated nanoribbons that coiled together into the final co-assembled nanotube.

Co-assembly of a multicomponent network of nanofiber-wrapped nanotubes.

Strategies to create organized multicomponent nanostructures composed of discrete, self-sorted domains are important for developing materials that mimic the complexity and multifunctionality found in biological systems. These structures can be challenging to achieve due to the required balance of molecular self-recognition and supramolecular attraction needed between the components. Herein, we report a strategy to construct a two-component nanostructure via a hierarchical assembly process whereby two monomeric building blocks undergo self-sorting assembly at the molecular level followed by a supramolecular association to form a nanofiber-wrapped nanotube. The two molecules self-sorted into respective nanofiber and nanotube assemblies, yet assembly of the nanofibers in the presence of the nanotube template allowed for directed integration into a hierarchical multilayer structure via electrostatic interactions. The fiber-wrapped nanotube co-assembly was characterized using transmission electron microscopy (TEM), atomic force microscopy (AFM) and Förster resonance energy transfer (FRET) between the components. Strategies to co-assemble multicomponent nanostructures composed of discrete, spatially sorted domains with controllable higher level interactions will be critical for the development of novel, functionally competent nanomaterials.

Amino Acid Nanofibers Improve Glycemia and Confer Cognitive Therapeutic Efficacy to Bound Insulin.

Diabetes poses a high risk for debilitating complications in neural tissues, regulating glucose uptake through insulin-dependent and predominantly insulin-independent pathways. Supramolecular nanostructures provide a flexible strategy for combinatorial regulation of glycemia. Here, we compare the effects of free insulin to insulin bound to positively charged nanofibers comprised of self-assembling amino acid compounds (AACs) with an antioxidant-modified side chain moiety (AAC2) in both in vitro and in vivo models of type 1 diabetes. Free AAC2, free human insulin (hINS) and AAC2-bound-human insulin (AAC2-hINS) were tested in streptozotocin (STZ)-induced mouse model of type 1 diabetes. AAC2-hINS acted as a complex and exhibited different properties compared to free AAC2 or hINS. Mice treated with the AAC2-hINS complex were devoid of hypoglycemic episodes, had improved levels of insulin in circulation and in the brain, and increased expression of neurotransmitter taurine transporter, Slc6a6. Consequently, treatment with AAC2-hINS markedly advanced both physical and cognitive performance in mice with STZ-induced and genetic type 1 diabetes compared to treatments with free AAC2 or hINS. This study demonstrates that the flexible nanofiber AAC2 can serve as a therapeutic platform for the combinatorial treatment of diabetes and its complications.

Amino acid-based compound activates atypical PKC and leptin receptor pathways to improve glycemia and anxiety like behavior in diabetic mice.

Differences in glucose uptake in peripheral and neural tissues account for the reduced efficacy of insulin in nervous tissues. Herein, we report the design of short peptides, referred as amino acid compounds (AAC) with and without a modified side chain moiety. At nanomolar concentrations, a candidate therapeutic molecule, AAC2, containing a 7-(diethylamino) coumarin-3-carboxamide side-chain improved glucose control in human peripheral adipocytes and the endothelial brain barrier cells by activation of insulin-insensitive glucose transporter 1 (GLUT1). AAC2 interacted specifically with the leptin receptor (LepR) and activated atypical protein kinase C zeta (PKCς) to increase glucose uptake. The effects induced by AAC2 were absent in leptin receptor-deficient predipocytes and in Leprdb mice. In contrast, AAC2 established glycemic control altering food intake in leptin-deficient Lepob mice. Therefore, AAC2 activated the LepR and acted in a cytokine-like manner distinct from leptin. In a monogenic Ins2Akita mouse model for the phenotypes associated with type 1 diabetes, AAC2 rescued systemic glucose uptake in these mice without an increase in insulin levels and adiposity, as seen in insulin-treated Ins2Akita mice. In contrast to insulin, AAC2 treatment increased brain mass and reduced anxiety-related behavior in Ins2Akita mice. Our data suggests that the unique mechanism of action for AAC2, activating LepR/PKCς/GLUT1 axis, offers an effective strategy to broaden glycemic control for the prevention of diabetic complications of the nervous system and, possibly, other insulin insensitive or resistant tissues.

Threading carbon nanotubes through a self-assembled nanotube.

Achieving the co-assembly of more than one component represents an important challenge in the drive to create functional self-assembled nanomaterials. Multicomponent nanomaterials comprised of several discrete, spatially sorted domains of components with high degrees of internal order are particularly important for applications such as optoelectronics. In this work, single-walled carbon nanotubes (SWNTs) were threaded through the inner channel of nanotubes formed by the bolaamphiphilic self-assembly of a naphthalenediimide-lysine (NDI-Bola) monomer. The self-assembly process was driven by electrostatic interactions, as indicated by ζ-potential measurements, and cation-π interactions between the surface of the SWNT and the positively charged, NDI-Bola nanotube interior. To increase the threading efficiency, the NDI-Bola nanotubes were fragmented into shortened segments with lengths of <100 nm via sonication-induced shear, prior to co-assembly with the SWNTs. The threading process created an initial composite nanostructure in which the SWNTs were threaded by multiple, shortened segments of the NDI-Bola nanotube that progressively re-elongated along the SWNT surface into a continuous radial coating around the SWNT. The resultant composite structure displayed NDI-Bola wall thicknesses twice that of the parent nanotube, reflecting a bilayer wall structure, as compared to the monolayer structure of the parent NDI-Bola nanotube. As a final, co-axial outer layer, poly(p-phenyleneethynylene) (PPE-SO3Na, M W = 5.76 × 104, PDI - 1.11) was wrapped around the SWNT/NDI-Bola composite resulting in a three-component (SWNT/NDI-Bola/PPE-SO3Na) composite nanostructure.

pH-Controlled Chiral Packing and Self-Assembly of a Coumarin Tetrapeptide.

A coumarin-tetrapeptide conjugate, EFEK(DAC)-NH2 (1), is reported to undergo a pH-dependent interconversion between nanotubes and nanoribbons. An examination of zeta potential measurements, circular dichroism (CD) spectra, and microscopy imaging (transmission electron microscopy and atomic force microscopy) identified three different self-assembly regimes based on pH: (1) pH 2-5, positively charged, left-handed helical nanotubes; (2) pH 6-8, negatively charged, right-handed helical nanoribbons; and (3) pH ≥ 9.0, a monomeric/disassembled peptide. The nanotubes exhibited uniform diameters of 41 ± 5 nm and wall thicknesses of 4.8 ± 0.8 nm, whereas the nanoribbons existed as either flat or twisted sheets ranging in width from 11 to 60 nm with heights of 8 ± 1 nm. The UV-vis and CD spectra of the most common antiparallel, ß-sheet conformation of 1-dimer were simulated at the B3LYP/def2svpd level of theory in implicit water. These studies indicated that the transition from nanotubes to nanoribbons was coupled to an M → P helical inversion of the coumarin packing orientation, respectively, within the nanostructures. The assembly process was driven by ß-sheet aggregation and π-π interactions, leading to the formation of nanoribbons, which progressively wound into helical ribbons and laterally grew into smooth nanotubes as the pH decreased.