Chemistry Reference
In-Depth Information
successful one at alignment and patterning of the nanofibers into long-range ordered
arrays. In this procedure, PAs were self-assembled by solvent evaporation on the sub-
strate and capped with a poly(dimethylsiloxane) stamp containing patterned grooves
while being subjected to continuous sonication for 1 h. After overnight drying, the
stamp was removed to reveal fiber bundles oriented in the direction of the grooves,
as imaged by AFM.
It is clear from these examples that peptide amphiphiles have great potential. Their
tolerance to modifications of their chemical functionality while maintaining fibrous
morphology and their capability to form gels are beneficial in many applications.
Although most of these applications have focused on utilizing the surface of the nano-
fibers, several others turned to the hydrophobic core. For instance, PAs have been
shown to enhance the solubility of carbon nanotubes (Arnold et al. 2005) in
aqueous solution through hydrophobic interactions between their alkyl tail and the
surface of carbon nanotubes. In another study, several PAs containing a tryptophan
or pyrene chromophore were synthesized to investigate the solvation of different
parts of a PA within the nanofiber (Tovar et al. 2005), which is directly related
to their accessibility to small molecules. The chromophore was placed at the C- or
N-terminus, and in one of the PAs the long alkyl tail was replaced with pyrene.
Fluoresence spectroscopy experiments showed that PA nanofibers maintain a high
degree of free volume, and individual PA molecules were well solvated when
assembled in the nanofiber. Interactions of tryptophan and pyrene probes with fluore-
scent quenchers in solution show a gradual decrease in response from the exterior to
the interior of the nanofiber to the very center of the hydrophobic core.
Nanofibers with a parallel b-sheet character have been found in other systems,
most notably in pathogenic Alzheimer's amyloid (Ab) peptide. Over the years,
researchers have studied the self-assembly of Ab peptide into fibrils. In particular,
Ab peptide segments were synthesized and evaluated with solid-state NMR to
study their secondary structure. Depending on their sequence, the peptides form
either anti-parallel (Lansbury et al. 1995; Balbach et al. 2000) or parallel b-sheet
fibers (Benzinger et al. 1998; Gregory et al. 1998; Antzutkin et al. 2000; Balbach
et al. 2002). Peptides with hydrophobic residues concentrated in one of their
termini, thereby introducing amphiphilic character to the molecule, tend to form
parallel b-sheets (Soreghan et al. 1994). This behavior is reminiscent of peptide-
amphiphile systems in which the peptides have hydrophobicity toward one of their
termini. However, the hydrophobic character in PAs comes from aliphatic chains
or other hydrophobic molecules. In this case, the overall amphiphilicity is inherent
in the peptide sequence. Peptides with no pattern of amphiphilicity between their
terminals form anti-parallel b-sheets. Without a hydrophobic segment, peptides gen-
erally prefer to be in anti-parallel orientation because it produces the most optimum
hydrogen bonding interaction between the peptide strands. Gordon et al. (2004)
demonstrated that an Ab peptide derivative that forms anti-parallel b-sheet fibers
would self-assemble in parallel orientation after acylation of its N-terminus.
The parallel b-sheet is also the secondary structural motif of the pathogenic form
of ataxin-3, a 42-kDa peptide expressed in the brain (Bevivino and Loll 2001). The
peptide becomes harmful when the number of glutamine residues near its C-terminus
