Polypeptides with low molecular weights usually adopt the β-sheet conformation.
When polypeptides are dissolved in denaturant solvents, the random coil conformation can be observed [28, 29].
The ordered structures assembled from polypeptide homopolymers and
copolymers in both bulk and solution have attracted a great deal of attention over
the past few decades and could continue to be an active theme in the future
[30–38]. In solution, both the α-helix and β-sheet conformations of polypeptides
facilitate the formation of ordered structures in liquid crystals (LCs), gels, and
micelles during the assembly process. Usually, LC structures are assembled from
polypeptide homopolymers in concentrated solutions [39–41]. In solutions with
moderate concentrations, gels are usually formed by both polypeptide homopolymers
and copolymers [42–44]. In dilute solutions, polypeptide copolymers are able to selfassemble into diverse micelle structures with a solvophilic shell, while the
solvophobic polypeptide chains are packed in an ordered manner in the micelle
core [45–47].
The ordering of polypeptide chains, typically for α-helix conformation, is similar
in the structures of LCs, gels, and micelles. In LCs, polypeptide homopolymers are
usually packed in a side-by-side manner to form a highly ordered arrangement in the
form of nematic and cholesteric phases [39, 40]. For polypeptide homopolymers with
identical chain length (polydispersity index, PDI ¼ 1.0) or polydisperse polypeptides
end-capped with a bulky group, a smectic LC phase can be observed [48, 49]. In
homopolymer gel structures, larger dislocation of side-by-side packing of polypeptide helices is essential to form fiber-like structures as well as physical crosslinkers
[42, 50–52]. For block copolymer gels, because of the existence of flexible chains, a
smectic-like packing manner of polypeptide helices is adopted, in which the long axis
of polypeptide helices is perpendicular to the long axis of gel fiber, and the flexible
chains are spread out into the surroundings [44, 48, 53, 54]. Polypeptide-based
copolymer micelles self-assembled in dilute solutions can preserve the ordered
packing of polypeptides in the aggregates of cylinders, vesicles, and complex hierarchical structures. The ordering nature within the domain endows the polypeptide
micelles with unique properties compared with conventional coil–coil type copolymer micelles, e.g., high stability and diverse morphologies [55–59]. For the
polypeptides with β-sheet conformation, ordered structures can be achieved by
parallel or antiparallel packing of polypeptide chains, which can be found in polypeptide gels and micelles [36, 54, 60]. When the α-helix or β-sheet conformation
transforms to a random coil conformation, the ordering feature of polypeptides in
these structures is destroyed and a subsequent variation in structure and morphology
can be observed [61–63].
The research on polypeptides and their assembly behaviors is important and
beneficial for several areas. First, polypeptides can be used as a model polymer with
various chain rigidities. Polypeptides can adopt conformations of α-helix, β-sheet
and random coil, which can transform into each other under controlled conditions.
The α-helix to random coil transition in solutions is especially interesting. Thus,
polypeptides can serve as an ideal model for investigating the influence of polymer
rigidity on the assembly behavior of polymers. Second, the synthetic polypeptides
162
C. Cai et al.
When polypeptides are dissolved in denaturant solvents, the random coil conformation can be observed [28, 29].
The ordered structures assembled from polypeptide homopolymers and
copolymers in both bulk and solution have attracted a great deal of attention over
the past few decades and could continue to be an active theme in the future
[30–38]. In solution, both the α-helix and β-sheet conformations of polypeptides
facilitate the formation of ordered structures in liquid crystals (LCs), gels, and
micelles during the assembly process. Usually, LC structures are assembled from
polypeptide homopolymers in concentrated solutions [39–41]. In solutions with
moderate concentrations, gels are usually formed by both polypeptide homopolymers
and copolymers [42–44]. In dilute solutions, polypeptide copolymers are able to selfassemble into diverse micelle structures with a solvophilic shell, while the
solvophobic polypeptide chains are packed in an ordered manner in the micelle
core [45–47].
The ordering of polypeptide chains, typically for α-helix conformation, is similar
in the structures of LCs, gels, and micelles. In LCs, polypeptide homopolymers are
usually packed in a side-by-side manner to form a highly ordered arrangement in the
form of nematic and cholesteric phases [39, 40]. For polypeptide homopolymers with
identical chain length (polydispersity index, PDI ¼ 1.0) or polydisperse polypeptides
end-capped with a bulky group, a smectic LC phase can be observed [48, 49]. In
homopolymer gel structures, larger dislocation of side-by-side packing of polypeptide helices is essential to form fiber-like structures as well as physical crosslinkers
[42, 50–52]. For block copolymer gels, because of the existence of flexible chains, a
smectic-like packing manner of polypeptide helices is adopted, in which the long axis
of polypeptide helices is perpendicular to the long axis of gel fiber, and the flexible
chains are spread out into the surroundings [44, 48, 53, 54]. Polypeptide-based
copolymer micelles self-assembled in dilute solutions can preserve the ordered
packing of polypeptides in the aggregates of cylinders, vesicles, and complex hierarchical structures. The ordering nature within the domain endows the polypeptide
micelles with unique properties compared with conventional coil–coil type copolymer micelles, e.g., high stability and diverse morphologies [55–59]. For the
polypeptides with β-sheet conformation, ordered structures can be achieved by
parallel or antiparallel packing of polypeptide chains, which can be found in polypeptide gels and micelles [36, 54, 60]. When the α-helix or β-sheet conformation
transforms to a random coil conformation, the ordering feature of polypeptides in
these structures is destroyed and a subsequent variation in structure and morphology
can be observed [61–63].
The research on polypeptides and their assembly behaviors is important and
beneficial for several areas. First, polypeptides can be used as a model polymer with
various chain rigidities. Polypeptides can adopt conformations of α-helix, β-sheet
and random coil, which can transform into each other under controlled conditions.
The α-helix to random coil transition in solutions is especially interesting. Thus,
polypeptides can serve as an ideal model for investigating the influence of polymer
rigidity on the assembly behavior of polymers. Second, the synthetic polypeptides
162
C. Cai et al.