Chemistry Reference
In-Depth Information
cations such as Ca
2
þ
to neutralize the charges on the amino acid side chains
(Hartgerink et al. 2002). For example, the self-assembly of PAs containing Glu
can be triggered by lowering the pH or by adding Ca
2
þ
to the PA solution at a pH
range where the PAs are normally disassembled. Depending on the choice of amino
acid residues incorporated in the sequence, we can obtain PA nanofibers in a wide
pH range. The presence of multiple cysteines in the sequence allows covalent capture
by the formation of intermolecular disulfide bonds after self-assembly, which further
stabilizes the nanofibers. PAs form self-supporting gels that can be reversibly disas-
sembled by pH adjustment. However, covalently captured or divalent cation-neutralized
nanofibers are not sensitive to pH changes and retain their fibrous morphology.
The formation of PA gels was studied by varying the alkyl tail length, cross-
linking region, concentration, and bioactivity (Hartgerink et al. 2002). It was
shown that peptides without an alkyl tail or with only a short tail (6 carbons) did
not have enough hydrophobic character to drive self-assembly into gels. Peptides
connected to C10, C16, and C22 carbon alkyl tails formed gels after pH adjustment.
Cross-linking by cysteines is not required for the formation of nanofibers. However,
when present, cysteines must be completely in the reduced state prior to self-assembly
as random disulfide linkages between PA molecules (or intramolecular cross-links)
prevent the structure needed for fiber formation. The PA starting concentration influ-
enced the ability of the fibers to form bundles and 3-D network, but it did not affect
the nanostructure of the individual fibers. Variation in cell-binding functionality,
including the relatively hydrophobic IKVAV, did not affect the propensity of the
PAs to form gels, but TEM revealed changes in the fiber length and stiffness.
Figure 14.10 is a schematic of a PA and the negatively stained nanofibers
(Hartgerink et al. 2001). Because the peptide head group is oriented toward the
surface of the nanofibers, the chemical functional groups imparted by the amino
acid side chains are accessible to the surrounding environment. Furthermore, the
peptide region of PAs is easily tailored to accommodate its use in a wide variety
of applications. In this example, the PA nanofibers directed the mineralization of
hydroxyapatite (HA). Therefore, this particular PA has several features built into
the sequence: 1) a hydrophobic tail that drives the assembly into nanofibers, 2)
four cysteines for covalent capture, 3) three glycines for flexibility in the molecule,
4) phosphorylated serine for calcium binding, and 5) an Arg-Gly-Asp sequence for
cell adhesion. In the HA mineralization studies, the PAs were self-assembled on a
holey-carbon TEM grid by exposure to HCl vapor followed by immersion in I
2
for
covalent capture. HA mineralization on PA nanofibers was performed by diffusion
of CaCl
2
and NaHPO
4
from opposite sides of the grid. Energy dispersion X-ray
fluorescence spectroscopy analysis confirmed the formation of HA on the surface
of the nanofibers. TEM imaging and electron diffraction revealed the preferential
alignment of the HA crystallographic c-axis with the long axis of the fiber.
Sone and Samuel (2004) continued the studies of mineralization on PA nanofibers
by utilizing the same PA described above to nucleate and grow CdS nanocrystals. In
this case, the negatively charged phosphate and carboxylate groups bind to Cd
2
þ
, and
CdS was formed after diffusion of H
2
S gas. A low Cd
2
þ
to PA ratio led to the for-
mation of CdS nanocrystals that were 3-5 nm in diameter. An intermediate ratio of
