Peptidic Hydrogels - In-Situ Gelling Polymers: For Biomedical Applications - page 133

Fig. 8 Serpin molecule with
tertiary order. ʲ -strands are
represented by arrows with
visible ʱ -helices throughout
the entire protein as well as
disordered, random coils and
turns that connect the areas
of more regular secondary
structure [ 86 ]
structures are stabilized by a wide variety of intramolecular interactions in addi-
tion to hydrogen bonding and hydrophobic interactions. These bonds, including
salt-bridges or disulfide cysteine bonds, can also be covalent bonds unlike primary
or secondary structures [ 16 , 33 , 51 ]. Figure 8 shows a protein example of tertiary
structure from the serpin family of proteins naturally found in the body [ 86 ]. In
the diagram there are visible ʱ -helices, one of which is highlighted in purple, and
there are ʲ -strands highlighted in red and blue all within the same molecule.
While there are many proteins that exhibit tertiary order in their globular,
functional form, in peptide hydrogels currently there are very few tertiary order
structures. Inspirations for some synthetic globular proteins come from naturally
occurring tertiary structures such as bovine serum albumin, ʲ -lactoglobulin, or
ovalalbumin [ 18 , 56 , 58 , 60 , 62 ]. Assembling globular proteins into quaternary
structures for hydrogels are examined later in the chapter. As the field of peptide
hydrogels grows and more complex sequences are created, there may be more ter-
tiary order structures for peptide hydrogels on the horizon [ 87 ].
4 Quaternary Structures
While previous sections described molecular structures peptide sequences can
adopt before interacting with other sequences, the actual peptide hydrogel network
itself has an intermolecular quaternary structure. When exposed to the proper trig-
gers in solution, the previously mentioned primary, secondary, and tertiary struc-
tures create quaternary structures. The solution triggers include, but are not limited
to, changes in pH [ 19 , 37 , 42 , 61 , 67 , 68 ] or ionic concentration [ 45 , 48 , 68 ]. The
desired hydrogel characteristics and properties dictate the solution and triggering
conditions sought for gel formation as peptide sequences fold and assemble into