3 Ordering of Polypeptides in Gel Structures
Gels are soft materials comprising a liquid-like phase and a solid network, the latter
preventing the bulk flow of the liquids. They have potential in diverse applications
for tissue engineering, nanoscale electronics, etc. Polymer gels are usually formed
in a moderately concentrated solution, in which physical or chemical crosslinks are
necessary. Both polypeptide homopolymers and copolymers can assemble into gel
structures. Similar to that in LCs, the ordering of polypeptide chains takes important
responsibility for the gelation behavior. In addition to the α-helix conformation, the
β-sheet conformation also supports the formation of polypeptide gels. In this
section, the ordering of polypeptides in gel structures is discussed. The content is
organized into three subsections. The first section describes the organic gels formed
by polypeptide homopolymers, the second polypeptide block copolymer gels in
organic solvents, and the third hydrogels formed by polypeptide block copolymers
in aqueous solutions.
3.1 Polypeptide Homopolymer Gels in Organic Solvents
Polypeptide homopolymers (typically PBLG) with rigid α-helix conformation can
form LC structures at a high concentration and temperature. When the solution
is cooled, a transparent, mechanically self-supporting gel is always observed
[42, 87–91]. The gel formation was found to be concentration and temperature
dependent and completely reversible. It is well known that the physical or chemical
crosslinks are necessary for polymer gels. Flexible polymers can easily form
crosslinking domains with crystalline or semicrystalline structures. However, for
rigid polypeptide chains, it is less clear how the rodlike polypeptides participate
extensively in intermolecular crosslinks.
There are various approaches to explain this effect but a common aspect is that
rigid polypeptide chains aggregate into nanofibers and the interfiber crosslinking
results in the formation of networks. Figure 9a shows a TEM photograph of the
PBLG gels from DMF (concentration 1 wt%) [42]. Random networks were
observed to be formed by branching and rejoining of different strands. The strands
are of diameters ranging from tens to hundreds nanometers and are composed of
bundles of aligned rods (rod diameter ~2 nm, corresponding to the diameter of
PBLG helix). The benzene ring interaction or stack, as well as the dislocated sideby-side packing tendency of PBLG rods, is responsible for the formation of such
strands. Figure 9b shows a scheme for the polypeptide gel structure. As can be seen,
physical crosslinks are formed by the branching and rejoining of different sheaflike aggregates, which stabilize the gel structures in solution.
Usually, gels are physically crosslinked and the ordered structures cannot be
preserved under heating or other treatments because of the breakage of the physical
crosslinks. However, when the gels are chemically crosslinked, the shape and inner
172
C. Cai et al.
Gels are soft materials comprising a liquid-like phase and a solid network, the latter
preventing the bulk flow of the liquids. They have potential in diverse applications
for tissue engineering, nanoscale electronics, etc. Polymer gels are usually formed
in a moderately concentrated solution, in which physical or chemical crosslinks are
necessary. Both polypeptide homopolymers and copolymers can assemble into gel
structures. Similar to that in LCs, the ordering of polypeptide chains takes important
responsibility for the gelation behavior. In addition to the α-helix conformation, the
β-sheet conformation also supports the formation of polypeptide gels. In this
section, the ordering of polypeptides in gel structures is discussed. The content is
organized into three subsections. The first section describes the organic gels formed
by polypeptide homopolymers, the second polypeptide block copolymer gels in
organic solvents, and the third hydrogels formed by polypeptide block copolymers
in aqueous solutions.
3.1 Polypeptide Homopolymer Gels in Organic Solvents
Polypeptide homopolymers (typically PBLG) with rigid α-helix conformation can
form LC structures at a high concentration and temperature. When the solution
is cooled, a transparent, mechanically self-supporting gel is always observed
[42, 87–91]. The gel formation was found to be concentration and temperature
dependent and completely reversible. It is well known that the physical or chemical
crosslinks are necessary for polymer gels. Flexible polymers can easily form
crosslinking domains with crystalline or semicrystalline structures. However, for
rigid polypeptide chains, it is less clear how the rodlike polypeptides participate
extensively in intermolecular crosslinks.
There are various approaches to explain this effect but a common aspect is that
rigid polypeptide chains aggregate into nanofibers and the interfiber crosslinking
results in the formation of networks. Figure 9a shows a TEM photograph of the
PBLG gels from DMF (concentration 1 wt%) [42]. Random networks were
observed to be formed by branching and rejoining of different strands. The strands
are of diameters ranging from tens to hundreds nanometers and are composed of
bundles of aligned rods (rod diameter ~2 nm, corresponding to the diameter of
PBLG helix). The benzene ring interaction or stack, as well as the dislocated sideby-side packing tendency of PBLG rods, is responsible for the formation of such
strands. Figure 9b shows a scheme for the polypeptide gel structure. As can be seen,
physical crosslinks are formed by the branching and rejoining of different sheaflike aggregates, which stabilize the gel structures in solution.
Usually, gels are physically crosslinked and the ordered structures cannot be
preserved under heating or other treatments because of the breakage of the physical
crosslinks. However, when the gels are chemically crosslinked, the shape and inner
172
C. Cai et al.