Protein-like polymers/copolymers

When discussing various methods for the synthesis of protein-like HP-copolymers from the monomeric precursors (Sect. 2.1), we pointed to the possibility of implementation of both polymerization and polycondensation processes. The studies of the potentials of the latter approach in the creation of protein-like macromolecular systems have already been started. The first published results show that using true selected reactions of the polycondensation type and appropriate synthetic conditions (structure and reactivity of comonomers, solvent, temperature, reagent concentration and comonomer ratio, the order of the reagents introduction into the feed, etc.) one has a chance to produce the polymer chains with a desirable set of monomer sequences. 

Fig. 24 The side-chain models of amphiphilic polymers a amphiphilic homopolymer (poly-A), b regular alternating HA copolymer, c regular multiblock HA copolymer, and d protein-like HA copolymer. Each hydrophobic monomer unit (H) is considered as a single interaction site (bead) each amphiphilic group (A) is modeled by a dumbbell consisting of hydrophobic (H) and hydrophilic (P) beads...

A large portion of the research up to this point has been carried out on materials composed of a synthetic block and a peptide block. These peptide-polymer conjugates combine the advantages of synthetic polymers (such as solubility, processability, and rubber elasticity) with those of polypeptides or protein-like polymers (such as secondary structure, functionality, and biocompatibihty). While synthetic polymers generally present a coil structure, peptide sequences can adopt ordered conformations such as a-helices or jS-strands (Figure 20.1). The former case produces block copolymers with a rod-coil character such rod-coil block copolymers have generated unconventional nanoscale structures not observed for purely amorphous block copolymers. Copolymers containing peptide sequences with a jS-strand... 

The transition enthalpies of the s- and p-fractions obtained from the feed with a comonomer molar ratio of 85 15 were equal to 6 and 7 J/g, respectively, i.e. the values are very close. This, therefore, can be indicative of almost the same average length of oligoNVCl blocks. Moreover, as we have already stressed, the fractions also had virtually the same final comonomer composition. However, since the solution properties of these fractions are drastically different, one can draw the conclusion that this is apparently due to a specific distribution of hydrophobic and hydrophilic residues along the polymer chains. In turn, because of all the properties that are exhibited by the s-fraction, this fraction can be considered to be a protein-like copolymer [27]. 

Apart from the data of thermonephelometry and HS-DSC,1H NMR studies have also revealed [27] some properties that allowed the attribution of such s-type copolymers to the protein-like ones. A marked broadening of the water proton signal was observed caused by the decreased mobility of bound water just in the vicinity of the temperature of HS-DSC peak. These data indicated the heat-induced compaction of the interior of the polymer coils, as would occur with protein-like macromolecules. Figure 5 demonstrates the experimental data, viz., the temperature dependences of signal width at half-height for the peaks of water protons recorded in D2 O-solutions of p- and s-fractions of the copolymer synthesized from the feed with an initial comonomer ratio of 85 15 (mole/mole). 

Another (quite different) implication of the issue of information capacity of protein-like copolymer is the possibility of recording a small region in conformation space near the native state instead of recording several essentially different conformations in the polymer sequence. Moreover, this can be done so that the transitions between the conformations within the above-mentioned region occur fast, without barriers, and possibly in an ordered... 

Fig. 29 Characteristic wave number q as a function of polymer volume fraction for the systems of protein-like copolymers with L = 63 and random-block copolymers with different block lengths. The domain spacing is defined as r = lir/q. Adapted from [153]...

What is most important for our discussion is the fact that the spatial scale r of the segregated structure for protein-like copolymers is appreciably larger than that for random-block copolymers with the same composition and the same average block length. Also, MIST in the protein-like copolymer system occurs at a temperature higher than that of the random-block system, which is in agreement with the prediction of the polymer RISM theory [153]. 

The comonomer distribution can be alternated by controlling the synthesis conditions, such as the copolymerization at different reaction temperatures at which the thermally sensitive chain backbone has different conformations (extended coil or collapsed globule). In this way, hydrophilic comonomers can be incorporated into the thermally sensitive chain backbone in a more random or more segmented (protein-like) fashion. On the other hand, short segments made of hydrophobic comonomers can be inserted into a hydrophilic chain backbone by micelle polymerization. One of the most convenient ways to control and alternate the degree of amphiphilicity of a copolymer chain, i.e., the solubility difference of different comonomers in a selective solvent, is to use a thermally sensitive polymer as the chain backbone, such as poly(N-isopropylacrylamidc) (PNIPAM) and Poly(N,N-diethylacrylamide) (PDEA). In this way, the incorporation of a hydrophilic or hydrophobic comonomer into a thermally sensitive chain backbone allows us to adjust the degree of amphiphilicity by a temperature variation. 

SEC, which is mainly used for high-molecular weight compounds like polymers and proteins, has been coupled to FTIR. Using SEC-FTIR, liu and Dwyer [100] examined the types of branching in styrene-butadiene copolymers and Jordan and Taylor [101] measured the additives in several commercial polymers. 

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