Polypeptide copolymers lyotropic phases

In contrast to polypeptides that have many possible conformations, poly(hexyl isocynate) is known to have a stiff rodlike helical conformation in the solid state and in a wide range of solvents, which is responsible for the formation of a nematic liquid crystalline phase.45-47 The inherent chain stiffness of this polymer is primarily determined by chemical structure rather than by intramolecular hydrogen bonding. This results in a greater stability in the stiff rodlike characteristics in the solution as compared to polypeptides. The lyotropic liquid crystalline behavior in a number of different solvents was extensively studied by Aharoni et al.48-50 In contrast to homopolymers, interesting new supramolecular structures can be expected if a flexible block is connected to the rigid polyisocyanate block (rod—coil copolymers) because the molecule imparts both microphase separation characteristics of the blocks and a tendency of rod segments to form anisotropic order. 

This review covers the literature on the aggregation of (homo)polypeptide hybrid copolymers and copolypeptides in dilute solution, which was published up to June 2005 a recent review on amphiphiles consisting of peptide sequences is given elsewhere in [12]. It was a particular concern to give a comprehensive overview on secondary structure effects in the self-assembly of these copolymers. Briefly presented are also structures in concentrated solutions (lyotropic phases) and in heterophase systems (see also [14]). 

The phase behaviour of biomimetic polypeptide-based copolymers in solution was described and discussed with respect to the occurrence of secondary structure effects. Evidently, incorporation of crystallisable polypeptide segments inside the core of an aggregate has impact on the curvature of the corecorona interface and promotes the formation of fibrils or vesicles or other flat superstructures. Spherical micelles are usually not observed. Copolymers with soluble polypeptide segments, on the other hand, seem to behave like conventional block copolymers. A pH-induced change of the conformation of coronal polypeptide chains only affects the size of aggregates but not their shape. The lyotropic phases of polypeptide copolymers indicate the existence of hierarchical superstructures with ordering in the length-scale of microns. 

The lattice model, as put forth by Flory [84, 85], has been proved successful in the treatments of the liquid crystallinity in polymeric systems, despite its artificiality. In our series of work, the lattice model has been extended to the treatment of biopolypeptide systems. The relationship between the polypeptide ordering nature and the LC phase structure is well established. Recently, by taking advantage of the lattice model, we formulated a lattice theory of polypeptide-based diblock copolymer in solution [86]. The polypeptide-based diblock copolymer exhibits lyotropic phases with lamellar, cylindrical, and spherical structures when the copolymer concentration is above a critical value. The tendency of the rodlike block (polypeptide block) to form orientational order plays an important role in the formation of lyotropic phases. This theory is applicable for examining the ordering nature of polypeptide blocks in polypeptide block copolymer solutions. More work on polypeptide ordering and microstructure based on the Flory lattice model is expected. 

---