Folding of the polymer

The tertiary structure describes the shaping or folding of the polymer. Examples of this are given in Figures 2.16b, 2.17 left, and 2.17 right. 

Efficient and defect-free folding of the polymer will, at least, require a control of the regioselectivity of monomer incorporation and polymer tacticity. If, for example, the degree of tacticity control is poor, the stereochemical defects are irreversibly incorporated into the polymer, and the subsequent folding may fail or produce defective structures. By contrast, dynamic self-assembly may allow defects to be corrected and the hierarchical structure to be controlled or fine-tuned using external parameters (solvent, additives, temperature) prior to covalent fixation by polymerization. 

The first basic tenet of protein-structure prediction is that the amino acid sequence, the primary structure, contains all of the information required for the correct folding of the polymer chain. This is a first approximation which clearly ignores the role of environment on the induction of structure or the action of chaperone proteins which assist the in vivo folding process. The wide variety of structural motifs that have been observed for proteins is derived from only twenty different monomers (amino acids), many of which are structurally quite similar (i.e., isoleucine and leucine vary only in branching of the butyl side chain). However, there are many cases in which the substitution of amino acids with structurally similar residues (so-called conservative substitution) will lead to a protein that will not properly fold. Studies involving deletion of even small portions of the termini of the protein sequence provide similar results. On the other hand there are proteins related through evolution with as little as 20% sequence identity which adopt similar three-dimensional structures. Therefore the information encoded in the primary sequence is specific for one protein fold, however, there are numerous other sequences, only remotely related at first glance, which will produce the same fold. 

The evolution of cytochrome c and protein molecules in general has been the subject of many papers and reviews, and can be treated relatively briefly here. The starting points must be Anfinson s Molecular Basis of Evolution (57), based on the early amino acid sequence work, and the discovery that same year by Kendrew and Perutz of the identity of three-dimensional folding of the related proteins myoglobin and hemoglobin (98,99). These illustrate the two components of any study of mac-romolecular evolution the chemical sequence of the polymer and the folding of the polymer in three dimensions. 

An alternative possibility arises from considerations related to the development of crystalline structure in polyethylene [22], The main feature of this structure is the periodic folding of the polymer chains in the crystal. Theoretically this is explained within the context of the kinetics of crystal nucleation and growth from solution. According to Cormia, Price, and Turnbull [22], the fold-surface energy in polyethylene crystals is comparable to the end-interfacial energy of rodshaped nuclei. These surface free energies are of the order of 10 to... 

Structure. In Chapter 5, it was identified that small distortions of the lattice can be observed. Further, measurement of any crystalline polymer will indicate that the density is less that that from the predictions for the perfect crystalline material, indicating the presence of amorphous material. It is proposed that part of the amorphous content arises from disorder irregular folding of the polymer chains (Figure 6.3). 

Polymer metal complex formation of different polyvinylpyridines in solution, in hydrogels and at interfaces were investigated [83]. In aqueous solution linear or crosslinked polyvinylpyridines in the interaction with H2PtCl6 results in reduced viscosities and reduces swelling coefficients, respectively. Complexation leads to molecular bridges and folding of the polymer. Film formation was observed at the interface of poly(2-vinylpyridine) dissolved in benzene and metal salts dissolved in water. 

In the case of the chemical reactions of polyfunctionalized insoluble polymers, the situation is further complicated by the fact that the reactive groups on the polymer backbone are randomly distributed throughout the entire length of the chain and not confined to the ends of the polymer chain. Folding of the polymer chain and proximate groups are bound to affect its reactivity. 

---