Random coiled Copolymer

Fig. 1 Vesicle construct formed from poly(L-lysine)-i)-poly(L-leucme) polypeptides where the poly(L-leucine) block corresponds to the a-helical hydrophobic segments and the poly (L-lysine) block corresponds to the random coil hydrophilic segments. Note that this is one specific example and not all vesicle constructs have a-helical and random coil blocks. Moreover, the amphiphilic copolymer can be comprised of either a pure block copolypeptide or a macromolecule consisting of a polypeptide and another type of polymer. Adapted from [20] with permission. Copyright 2010 American Chemical Society...

Thermal and charge induced random coil to a-helix transitions of polyll-glutamic acid) (PGA) are measured by optical rotatory dispersion in various solvents. The data of PGA in 0.1 M NaCI are analyzed by the Zimm-Rice theory. The initiation parameter, o, of the Zimm-Rice theory is given by a value of 5 ( 1) x 10 3. Random copolymers of L-glutamic acid and L-alanine containing 10, 30, and 40 molar percents of alanyl residue are analyzed as well. 

Hoffman (37) has offered a variety of circumstantial evidence supporting the random coil model. In A-B block copolymers of styrene and butadiene, for instance, the characteristic dimension of the dispersed phase particles depends on the molecular weight of blocks in the dispersed phase according to ... 

By varying the distance between nearest adsorption sites, rs, one can control the composition variation period of the synthesized copolymer. From the chemical correlators defined by Eq. 16, it is easy to find the average number of segments in the repeating chain sections, N, for different rs values. It is instructive to analyze the relation between N and rs. As expected, a power law N oc is observed. It is clear that exponent //. in this dependence should be between //, = 1 (for a completely stretched chain) and //. = v 1 with v 0.6 (for a random coil with excluded volume [75]). The calculation [95] yields yu 1.33 for N > 15. This supports the aforementioned assumption that the repeating chain sections are strongly stretched between the adsorption sites. The same conclusion can be drawn from the visual analysis of typical snapshots similar to that presented in Fig. 22. 

In this review, hydrophilically and hydrophobically modified poly(N-iso-propylacrylamide) (PNIPAM) copolymers are mainly used to illustrate how amphiphilic copolymer chains can fold from an extended random coil to a collapsed globule in extremely dilute solutions and associate to form a stable mesoglobular phase which exists between single-chain globules and macroscopic precipitation. The copolymers used can be prepared by free-radical reaction. 

The lower transition temperature also indicates that the folding of the copolymer chains prepared at higher temperatures is much easier, or in a sense, these chains could memorize the parent collapsed globular state in which they were formed. As we discussed earlier, the conformational change can be better viewed in terms of the ratio of Rg)/ Rh). For a random coil and a uniform nondraining sphere, (Rg)/(Rh) 1.5 and 0.774, respec-... 

The second question is related to the existence of an additional transition at 29.4 °C. As expected, hydrophobic styrene stickers tend to associate in water. At lower temperatures, water is such a good solvent for PNIPAM that the copolymer chain adopts a random coil conformation. The movements of the stickers are also random and not correlated to each other because the PNIPAM segments randomly fluctuate in solution. As the temperature increases in the range 25-30.6 °C, the solvency of water for PNIPAM gradually decreases and the hydrophobic stickers tend to gather towards the chain center and to move in a more correlated fashion. Since water is not a poor solvent yet, the PNIPAM chain backbone is still in its swollen and coiled state, reflected in the fact that the decrease of (R) in Fig. 24 is only 30%. In order to distinguish such a chain conformation from a normal random coil, it was named an ordered coil . 

Fig. 25 Schematic of transitions of chain conformation of a segmented copolymer chain with stickers in dilute solution from a random coil to an ordered coil and then from an ordered coil to a collapsed core-shell globule as the solvency of water for the PNIPAM chain backbone decreases [94]...

Calculations of end-to-end distances of random coil conformations have also been carried out for polypeptide copolymers (Miller et al., 1967). The values of (rzyQlnplz were found to vary markedly with composition and amino acid sequence in the copolymers. For example, the introduction of glycine residues randomly into poly-L-alanine led to a monotonic decrease 6 ... 

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