Peptidic Hydrogels - In-Situ Gelling Polymers: For Biomedical Applications - page 132

Fig. 7 Left CD spectra of 1 wt% hydrogels containing (from top to bottom ) pure DMAX1,
3:1DMAX1:MAX1, 1:1DMAX1:MAX1, 1:3DMAX1:MAX1, and pure MAX1. The inset
shows the IR spectra of 1:1DMAX1:MAX to prove beta-strand behavior. Right 1 wt%
1:1DMAX1:MAX1gel as a TEM and b AFM images, with insets of average fibrillar widths.
Scale bar is 100 nm [ 85 ]
by molecular bonds to identify the presence of random coils, ʱ -helices, or ʲ -sheets.
Figure 4 shows CD measurements with typical ʱ -helical characteristics (two minima
at 208 and 222 nm) when the peptide is at 37 °C (solid line) or when cooled to 37 °C
from 85 °C (dotted line). The broken line demonstrates the sequence exhibiting ran-
dom coil behavior at 85 °C [ 13 , 39 ]. Figure 7 shows CD data with an FTIR inset of
the Schneider and Pochan groups' MAX1 along with varying ratios of its stereoiso-
mer DMAX1 [ 85 ]. The FTIR inset confirms ʲ -sheet structure that was not clear in the
CD due to a racemic mixture of both D and L peptide stereoisomers, which eliminates
clear CD signal. The inset shows clear absorption at 1,615 and 1,680 cm − 1 indica-
tive of ʲ -strand behavior. These spectroscopy methods are used for confirming and
analyzing secondary structures, but as the peptide sequences go on to form desired
intermolecular structures, microscopy and rheology techniques are required. These
techniques for examining quaternary structures are discussed later in the chapter.
Most of the peptide hydrogels that currently exist exhibit a specific secondary
structure before and during gelation. While there are many secondary structures
available, only those that are able to induce intermolecular assembly into qua-
ternary structure formation are important to the formation of peptide hydrogels.
In current peptide hydrogels, the secondary structure utilized most frequently in
peptide hydrogel design is the ʲ -sheet due to its propensity to form fibrillar inter-
molecular nanostructure and, thus, entangled and branched fibrillar networks for
hydrogel formation. The examples presented herein contain hydrogels of both
major secondary structures but focus mostly on ʲ -sheet constructions.
3 Tertiary Structures
Tertiary structures, like primary and secondary structures, are observed within sin-
gle peptide chains. Unlike primary or secondary structures, tertiary structures can
exhibit primary and/or secondary structures all within the same molecule. Tertiary