BULK AND SOLUTION PROPERTIES OF PEPTIDE-POLYMER CONJUGATES - Biologically-Responsive Hybrid Biomaterials - page 34

the number of repeat PLLys monomers (from 53 to 65) while maintaining a
constant number of PLPhe monomers (12) roughly tripled vesicle diameter
(from 2.2 ± 0.4 Ⱥ m to 6.2 ± 2.0 Ⱥ m), as visualized by AFM. The opposite
operation, in which the number of PLLys monomer was held constant (53) while
the number of PLPhe monomers was increased (from 6 to 12) also increased
vesicle diameter, in this case by a factor of roughly 2 (from to 1.1 ± 0.2 Ⱥ m to
2.2 ± 0.4 Ⱥ m.) AFM data also revealed a significant increase in vesicle size
corresponding to a decrease in solution pH. The diameter of vesicles swelled
from 1.5 Ⱥ m at pH 9 to 2.6 Ⱥ m at pH 2.5 due to the increase in repulsive forces in
the PLLys chain and its gradual transition from ŋ -helical toward random coil
conformation. UV absorbance, fluorescence, and SEM provided supporting
evidence for the complexation of calf thymus DNA upon mixing of the
copolymer with a DNA solution.
3.3. Polypeptide-based hydrogels
Applications for protein-based hydrogels range from food and cosmetic
thickeners to support matrices for drug delivery and tissue replacement. These
materials are usually prepared using proteins extracted from natural resources,
which can give rise to inconsistent properties unsuitable for medical applications.
Recently, Deming et al. [124-127] designed and synthesized diblock
copolypeptide amphiphiles containing charged and hydrophobic segments. It was
demonstrated that gelation depends not only on the amphiphilic nature of the
polypeptides, but also on chain conformation, meaning ŋ -helix, Ȳ -strand or
random coil. Specific rheological measurements were performed to evidence the
self-assembly process responsible for gelation: the rod-like helical secondary
structure of enantiomerically pure PLLeu blocks is instrumental for gelation at
polypeptide concentrations as low as 0.25 wt % [126]. The hydrophilic
polyelectrolyte segments have stretched coil configurations and stabilize the
twisted fibrillar assemblies by forming a corona around the hydrophobic cores
(Figure 14).
Interestingly, these hydrogels can retain their mechanical strength up to
temperatures of about 90 °C and recover rapidly after stress. This new mode
of assembly was found to give rise to polypeptide hydrogels with a unique
combination of properties, such as heat stability and injectability. Such
properties are attractive for applications in food engineering, personal care, and
medicine. More specifically, investigation has been made into biological
application of the polypeptide hydrogels as tissue engineering scaffolds [128].