homopolymers and copolymers assemble into distinct structures. Great efforts have
been made to construct polypeptide-based assemblies with ordered domains formed
by polypeptide chains.
LCs were the earliest studied structures, in which polypeptide homopolymer
rods pack in an ordered manner to form smectic, nematic, and cholesteric phases.
The smectic LCs are mainly formed by polypeptide homopolymers with identical
polymer length. The cholesteric phase can be prepared by synthetic polypeptides
with polydisperse chain length. The nematic phase can be regarded as a special
example of the cholesteric phase with an infinite cholesteric pitch. The cholesteric
pitch and chirality in the polypeptide LCs are dependent on many factors, such as
temperature, polymer concentration, solvent nature, and polypeptide conformation.
Deep understanding of such phenomena is necessary for preparation of ordered
polypeptide assembles with delicate structures. The addition of denaturing solvent
to polypeptide solution can lead to an anisotropic–isotropic reentrant transition at
low temperatures where the intramolecular helix–coil transformation occurs. However, the helical structure is more stable in LC phase than in dilute solution due to
the conformational ordering effect.
Gels have attracted considerable attention for a long time. In polypeptide
homopolymer gels, polypeptides assemble into fiber structures with dislocated
side-by-side packing of rods. Thus, they possess a similar structure to LC
structures; however, the order parameter is relatively lower. For polypeptide
block copolymers, gels are formed with packing of polypeptide chains, while the
other flexible chains are spread out into the surrounding to stabilize the gels in
solution. Both the α-helix and β-sheet conformation of polypeptides support the
formation of gels through inter- and intramolecular attractions. The gels formed
from β-sheet polypeptides are found to have higher strength than those formed by
α-helix polypeptides. However, most of the gels, especially the hydrogels, are still
not strong enough for practical applications. Modified hydrophilic polypeptides are
promising for the preparation of strong hydrogels. It is fundamentally important to
construct gels with controlled structures and morphologies for diverse applications
in the fields of template synthesis, functional materials, and tissue engineering
scaffolds.
The ordered packing of polypeptides can be also found in copolymer micelles
self-assembled in dilute solutions. The ordering tendency of polypeptide segments
is favorable for the formation of cylindrical micelles, large vesicles, and hierarchical structures. The formed structures show higher stabilities due to the ordering
within domains in the assemblies. Introducing a second component, including
polymers and nanoparticles, is an effective way to adjust the self-assembly behavior
of parent block and graft copolymers. The conformation transition of the polypeptide
chains is also an important factor affecting the assembly behavior of polypeptide
copolymers. The study of hierarchical structures of polypeptides is an especially
attractive and promising topic because polypeptides are the fundamental building
blocks for fabricating hierarchical structures in living organisms. Such hierarchical
structures usually show chirality. Further work eliciting the relationship between
handedness of the polypeptides and the chirality of the formed hierarchical
192
C. Cai et al.
been made to construct polypeptide-based assemblies with ordered domains formed
by polypeptide chains.
LCs were the earliest studied structures, in which polypeptide homopolymer
rods pack in an ordered manner to form smectic, nematic, and cholesteric phases.
The smectic LCs are mainly formed by polypeptide homopolymers with identical
polymer length. The cholesteric phase can be prepared by synthetic polypeptides
with polydisperse chain length. The nematic phase can be regarded as a special
example of the cholesteric phase with an infinite cholesteric pitch. The cholesteric
pitch and chirality in the polypeptide LCs are dependent on many factors, such as
temperature, polymer concentration, solvent nature, and polypeptide conformation.
Deep understanding of such phenomena is necessary for preparation of ordered
polypeptide assembles with delicate structures. The addition of denaturing solvent
to polypeptide solution can lead to an anisotropic–isotropic reentrant transition at
low temperatures where the intramolecular helix–coil transformation occurs. However, the helical structure is more stable in LC phase than in dilute solution due to
the conformational ordering effect.
Gels have attracted considerable attention for a long time. In polypeptide
homopolymer gels, polypeptides assemble into fiber structures with dislocated
side-by-side packing of rods. Thus, they possess a similar structure to LC
structures; however, the order parameter is relatively lower. For polypeptide
block copolymers, gels are formed with packing of polypeptide chains, while the
other flexible chains are spread out into the surrounding to stabilize the gels in
solution. Both the α-helix and β-sheet conformation of polypeptides support the
formation of gels through inter- and intramolecular attractions. The gels formed
from β-sheet polypeptides are found to have higher strength than those formed by
α-helix polypeptides. However, most of the gels, especially the hydrogels, are still
not strong enough for practical applications. Modified hydrophilic polypeptides are
promising for the preparation of strong hydrogels. It is fundamentally important to
construct gels with controlled structures and morphologies for diverse applications
in the fields of template synthesis, functional materials, and tissue engineering
scaffolds.
The ordered packing of polypeptides can be also found in copolymer micelles
self-assembled in dilute solutions. The ordering tendency of polypeptide segments
is favorable for the formation of cylindrical micelles, large vesicles, and hierarchical structures. The formed structures show higher stabilities due to the ordering
within domains in the assemblies. Introducing a second component, including
polymers and nanoparticles, is an effective way to adjust the self-assembly behavior
of parent block and graft copolymers. The conformation transition of the polypeptide
chains is also an important factor affecting the assembly behavior of polypeptide
copolymers. The study of hierarchical structures of polypeptides is an especially
attractive and promising topic because polypeptides are the fundamental building
blocks for fabricating hierarchical structures in living organisms. Such hierarchical
structures usually show chirality. Further work eliciting the relationship between
handedness of the polypeptides and the chirality of the formed hierarchical
192
C. Cai et al.