Molecular Dynamics Study of a Nanotube-Binding Amphiphilic Helical Peptide at Different Water/Hydrophobic Interfaces

Many potential applications of single-walled carbon nanotubes (SWNTs) require that they be isolated from one another. This may be accomplished through covalent or noncovalent SWNT functionalization. The noncovalent approach preserves the intrinsic electrical, optical, and mechanical properties of SWNTs and can be achieved by dispersing SWNTs in aqueous solution using surfactants, polymers, or biomacromolecules like DNA or polypeptides. The designed amphiphilic helical peptide nano-1, which contains hydrophobic valine and aromatic phenylalanine residues for interaction with SWNTs and glutamic acid and lysine residues for water solubility, has been shown to debundle and disperse SWNTs, although the details of the peptide−SWNT interactions await elucidation. Here we use fully atomistic molecular dynamics simulations to investigate the nano-1 peptide at three different water/hydrophobic interfaces: water/oil, water/graphite, and water/SWNT. The amphiphilic nature of the peptide is characterized by its secondary structure, peptide−water hydrogen bonding, and peptide−hydrophobic surface van der Waals energy. We show that nano-1 has reduced amphiphilic character at the water/oil interface because the peptide helix penetrates into the hydrophobic phase. The peptide α-helix cannot match its hydrophobic face to the rigid planar graphite surface without partially unfolding. In contrast, nano-1 can curve on the SWNT surface in an α-helical conformation to simultaneously maximize its hydrophobic contacts with the SWNT and its hydrogen bonds with water. The molecular insight into the peptide conformation at the various hydrophobic surfaces provides guidelines for future peptide design

Spontaneous Exfoliation of Single-Walled Carbon Nanotubes Dispersed Using a Designed Amphiphilic Peptide

We have observed concentration dependent exfoliation of single-walled carbon nanotubes dispersed in solutions of the synthetic peptide nano-1. As the nanotube concentration is reduced, the bundle diameters tend to decrease before saturating at −3 mg/mL. The fraction of individual nanotubes increases with decreasing concentration, saturating at ∼95% at low concentration. This concentration dependent exfoliation happens even if the dispersions are not sonicated on dilution, albeit over a longer time scale. The populations both of individual nanotubes and of bundles are much higher than expected at high concentrations, indicating the presence of repulsive internanotube interactions stabilizing the dispersions

Effect of Electron-Donating and Electron-Withdrawing Groups on Peptide/Single-Walled Carbon Nanotube Interactions

Nano-1, a designed peptide, has been demonstrated to efficiently disperse individual single-walled carbon nanotubes (SWNTs) by folding into an amphiphilic α-helix wherein the phenylalanine (Phe) residues on the hydrophobic face of the helix interact via π-stacking with the aromatic surface of the SWNT. In this study, the ability of electron-donating (hydroxyl) and electron-withdrawing (nitro) groups on the phenyl ring of Phe to affect the interactions between the peptide and SWNTs is examined by substituting the Phe residues in the nano-1 sequence with tyrosine and p-nitro-phenylalanine, respectively. Atomic force microscopy measurements and optical absorption spectroscopy revealed that the ability to disperse individual SWNTs increases with increasing electron density of the aromatic residue on the hydrophobic face of the amphiphilic helical peptides. Scanning tunneling spectroscopy (STS) and Raman analyses were used to examine the effect of noncovalent protein functionalization on the electronic properties of SWNTs. Small shifts in the Raman G band peak for the peptide/SWNT composites, as well as weak features that appear near the Fermi energy (Ef) in the STS dI/dV spectra of the peptide-coated SWNTs, are suggestive of a weak charge-transfer interaction between the peptides and the SWNTs

Importance of Aromatic Content for Peptide/Single-Walled Carbon Nanotube Interactions

We have previously demonstrated that a designed amphiphilic peptide helix, denoted nano-1, coats and debundles single-walled carbon nanotubes (SWNTs) and promotes the assembly of these coated SWNTs into novel hierarchical structures via peptide−peptide interactions. The purpose of this study is to better understand how aromatic content impacts interactions between peptides and SWNTs. We have designed a series of peptides, based on the nano-1 sequence, in which the aromatic content is systematically varied. Atomic force microscopy measurements and optical absorption spectroscopy reveal that the ability to disperse individual SWNTs increases with increasing aromatic residues in the peptide. Altogether, the results indicate that π-stacking interactions play an important role in peptide dispersion of SWNTs

<i>De Novo</i> Design of Mercury-Binding Two- and Three-Helical Bundles

De Novo Design of Mercury-Binding Two- and Three-Helical Bundle

Preparation and Characterization of Individual Peptide-Wrapped Single-Walled Carbon Nanotubes

Two challenges for effectively exploiting the remarkable properties of single-walled carbon nanotubes (SWNTs) are the isolation of intact individual nanotubes from the raw material and the assembly of these isolated SWNTs into useful structures. In this study, we present atomic force microscopy (AFM) evidence that we can isolate individual peptide-wrapped SWNTs, possibly connected end-to-end into long fibrillar structures, using an amphiphilic α-helical peptide, termed nano-1. Transmission electron microscopy (TEM) and well-resolved absorption spectral features further corroborate nano-1's ability to debundle SWNTs in aqueous solution. Peptide-assisted assembly of SWNT structures, specifically in the form of Y-, X-, and intraloop junctions, was observed in the AFM and TEM images