4 Ordering of Polypeptide Chains in Micelles
In dilute solutions, block copolymers can self-assemble into micelles with a
core–shell structure, which have great potential applications in drug delivery
systems, coatings, cosmetics, and nanoreactors [120–127]. In selective solvents,
block copolymers consisting of solvophobic polypeptide segments, such as PBLG
and PZLys, can self-assemble into diverse structures through ordering of polypeptide chains with the α-helix conformation. The helical polypeptide–solvophilic
polymer conjugates are typical rod–coil copolymers. As compared with intensively
investigated coil–coil block copolymers, the self-assembly behavior of rod–coil
block copolymers is far from well studied. Polypeptide-based rod–coil block
copolymers can be used as model copolymers for investigating the effect of ordered
packing of the rods on the self-assembly behavior of block copolymers. Besides the
α-helix conformation, ordered packing of β-sheet polypeptides is also observed in
micelles. Graft copolymers are another important category of copolymers capable
of forming aggregates. This section describes the ordering of polypeptide chains of
block and graft copolymers as well as copolymer mixtures in assembly structures of
cylindrical micelles, vesicles, and other complex structures.
4.1 Cylindrical Micelles Self-Assembled from Polypeptide
Copolymers
The fiber-like structure of polypeptide block copolymers is essential in organogels
that are formed by the side-by-side packing of polypeptide rods. Similarly, such
side-by-side packing of polypeptide rods can also be found in cylindrical micelles
self-assembled from polypeptide block copolymers. In both the fiber-like gels and
cylindrical micelles, polypeptide rods assemble into ordered structures, while the
flexible chains are spread out into the surroundings to stabilize the structures.
In a recent work, Lin et al. reported the self-assembly behavior of PBLG-b-PEG
block copolymer in CHCl 3 /ethanol/TFA mixed solution. In solution, the micelles
are formed with PEG as the shell and PBLG as the core [61]. In the absence of TFA,
PBLG adopts a rigid α-helix conformation and the block copolymers self-assemble
into cylindrical micelles (Fig. 17a). With the introduction of TFA into the solution,
the conformation of PBLG transforms from α-helix to random coil and the aggregate morphology transforms to spherical micelles (Fig. 17b). Concomitantly with
the changes in morphology, the micelle size tends to become smaller. Further
understanding of such a micelle morphology transition induced by polypeptide
conformation variation is assisted by a Brownian dynamics (BD) simulation. Based
on the experimental and simulation results, a well-founded mechanism regarding
the effect of PBLG conformation on the self-assembly behavior was proposed. As
shown in Fig. 17c, polypeptide blocks within the cores are interdigitated and favor
ordered parallel packing, with their long axis aligning in an orientation vector.
Ordering of Polypeptides in Liquid Crystals, Gels and Micelles
181
In dilute solutions, block copolymers can self-assemble into micelles with a
core–shell structure, which have great potential applications in drug delivery
systems, coatings, cosmetics, and nanoreactors [120–127]. In selective solvents,
block copolymers consisting of solvophobic polypeptide segments, such as PBLG
and PZLys, can self-assemble into diverse structures through ordering of polypeptide chains with the α-helix conformation. The helical polypeptide–solvophilic
polymer conjugates are typical rod–coil copolymers. As compared with intensively
investigated coil–coil block copolymers, the self-assembly behavior of rod–coil
block copolymers is far from well studied. Polypeptide-based rod–coil block
copolymers can be used as model copolymers for investigating the effect of ordered
packing of the rods on the self-assembly behavior of block copolymers. Besides the
α-helix conformation, ordered packing of β-sheet polypeptides is also observed in
micelles. Graft copolymers are another important category of copolymers capable
of forming aggregates. This section describes the ordering of polypeptide chains of
block and graft copolymers as well as copolymer mixtures in assembly structures of
cylindrical micelles, vesicles, and other complex structures.
4.1 Cylindrical Micelles Self-Assembled from Polypeptide
Copolymers
The fiber-like structure of polypeptide block copolymers is essential in organogels
that are formed by the side-by-side packing of polypeptide rods. Similarly, such
side-by-side packing of polypeptide rods can also be found in cylindrical micelles
self-assembled from polypeptide block copolymers. In both the fiber-like gels and
cylindrical micelles, polypeptide rods assemble into ordered structures, while the
flexible chains are spread out into the surroundings to stabilize the structures.
In a recent work, Lin et al. reported the self-assembly behavior of PBLG-b-PEG
block copolymer in CHCl 3 /ethanol/TFA mixed solution. In solution, the micelles
are formed with PEG as the shell and PBLG as the core [61]. In the absence of TFA,
PBLG adopts a rigid α-helix conformation and the block copolymers self-assemble
into cylindrical micelles (Fig. 17a). With the introduction of TFA into the solution,
the conformation of PBLG transforms from α-helix to random coil and the aggregate morphology transforms to spherical micelles (Fig. 17b). Concomitantly with
the changes in morphology, the micelle size tends to become smaller. Further
understanding of such a micelle morphology transition induced by polypeptide
conformation variation is assisted by a Brownian dynamics (BD) simulation. Based
on the experimental and simulation results, a well-founded mechanism regarding
the effect of PBLG conformation on the self-assembly behavior was proposed. As
shown in Fig. 17c, polypeptide blocks within the cores are interdigitated and favor
ordered parallel packing, with their long axis aligning in an orientation vector.
Ordering of Polypeptides in Liquid Crystals, Gels and Micelles
181