Chemistry Reference
In-Depth Information
cell adhesion by an RGDS containing an a-helical coiled coil compared to fibronectin
and vitronection (Villard et al. 2006). In addition to the biomedical applications
utilizing fibrous nanostructures, nonfibrous coiled coil assemblies have been reported
for templated nanofabrication that has been realized in two coiled coil based systems
to pattern gold nanoparticles (Ryadnov et al. 2003; Stevens et al. 2004) and construc-
tion of a polynanoreactor derived from a dendrimer-like short
leucine-zippers
sequences (Ryadnov 2007).
14.3. NANOFIBERS SELF-ASSEMBLED FROM
b
-SHEETS
b-Sheets also play an important role in the construction of peptide-based functional
nanofibrous materials. b-sheets are preferred over a-helices as molecular building
blocks in the fabrication of artificial nanostructured materials perhaps because of
the growing interest in understanding the self-assembly of two types of natural
b-sheet products: silk protein and amyloid-like b-sheets. Furthermore, extended
b-sheet conformation is relatively easy to achieve. Indeed, preventing their formation,
particularly in high concentration or at high temperature, can be difficult in both
synthetic and natural constructs.
Whereas a-helices are held together by intramolecular hydrogen bonding,
b-sheets are stabilized by intermolecular hydrogen bonding perpendicular to the
peptide chain. This cross molecular interaction results in a stronger tendency for
such peptides to aggregate (or self-assemble) than that of a-helices. In addition to
the difference seen in the secondary structure, when they are self-assembled into
nanofibers, an additional distinction is made in terms of the orientation of the
peptide chains with respect to the fiber long axis. Almost all of the synthetic nano-
fibers derived from a-helices reported thus far exhibit parallel orientation of individ-
ual peptide helices to the fiber axis (an exception is for the fibrous structures formed
by apolipoprotein I mimetics) whereas in b-strand nanofibers the peptide chains lie
approximately perpendicular to the fiber axis and stack into axially aligned extended
b-sheets, a structure well known as a “cross-b” spine.
14.3.1. Helix-Sheet Conversion and Nanofiber Formation
Amyloid-like structures represent a major class of self-assembled b-sheet nanofibers
that have common properties, such as elongated unbranched fiber morphology, high
resistance toward thermal and chemical unfolding, and tendency to form highly
entangled fibrous aggregation. The aggregation formed in the naturally occurring
amyloid peptides may be associated with fatal diseases. Although the mechanism
of amyloid formation is not fully understood, it has been suggested that a confor-
mational transition from the native helical and globular form to a b-sheet rich struc-
ture plays a critical role. Several groups have addressed the issue of the relationship
between secondary structural conversion and nanofiber formation with rationally
designed model peptides (Takahashi et al. 1998; Ciani et al. 2002; Kammerer
et al. 2004; Dong and Hartgerink 2007). This investigation has helped in the
