Peptidic Hydrogels - In-Situ Gelling Polymers: For Biomedical Applications - page 129

2 Secondary Structures
Secondary structures in peptides display a specific conformation of the peptide
chain. The two main secondary structures are the ʱ -helix and the ʲ -sheet. For both
structures, there has been extensive research to define, dictate, and predict struc-
tural and folding behaviors of primary sequences of peptides [ 36 ]. Before a trig-
gering event for secondary structure formation (i.e. intramolecular folding), the
unfolded peptide sequence may be a random coil, lacking specific three-dimen-
sional structure. Figure 3 shows the secondary order structures discussed in this
chapter and their potential quaternary order structures [ 36 ].
As indicated by the name, ʱ -helices form a unimolecular helix when triggered
to fold. Figure 3 b shows an external model of an ʱ -helix and Fig. 4 shows a top
Fig. 3 Basic secondary structures a beta strands; extended peptide chains held together by
hydrogen bonds shown in orange between CO and NH groups in the peptide backbones, and
resultant beta-sheets formed by several or more beta strands (actually an example of quaternary
structure). b An ʱ -helix with orange hydrogen bonds along the length of the helix and a coiled
coil quaternary structure made of two alpha-helices. c A proline helix that goes on to form a col-
lagen helix with other proline helices [ 36 ]
Fig. 4 Diagram of typical alpha helical heptad organization for coiled-coil formation and the
protection of hydrophobic residues at a and d for two coiled-coils. a CD data proving alpha helix
confirmation (two minima at 208 and 222 nm) for the peptide at 37 °C ( solid line ) when cooled
to 37 °C from 85 °C ( dotted line ); 85 °C ( broken line ) shows typical random coil behavior. b
Shows TEM of the resultant larger fibril organization with higher magnification to show stria-
tions [ 13 ]