Chain topologies

To understand the global mechanical and statistical properties of polymeric systems as well as studying the conformational relaxation of melts and amorphous systems, it is important to go beyond the atomistic level. One of the central questions of the physics of polymer melts and networks throughout the last 20 years or so dealt with the role of chain topology for melt dynamics and the elastic modulus of polymer networks. The fact that the different polymer strands cannot cut through each other in the... 

The term star-block copolymer is used for a star architecture in which each arm is a diblock. The influence of chain topology on mechanical and morphological properties was investigated for copolymers composed of PS and PB with a constant styrene content of = 0.74 by Michler s group (Fig. 32) [101,102], While hexagonally packed cylinders of PB in a PS matrix were observed in a symmetric PS-fo-PB-fr-PS triblock copolymer, an L phase... 

In the solid state the molecules pack in a way that zigzag chains -S-I- -S-I- (angular L S—I, approximately linear S- -I-S) are formed.60 The kind of chain topology is similar as that of the Se-I- -Se-L Se-I chains of dureneselenenyl iodide solid (2,3,5,6-Me4C6HSeI). Channels of weak soft-soft interactions... 

Bacillus ferredoxins, cysteine residues, 38 246 Bacillus thermoproteolyticus ferredoxins, chain topology, 38 244, 246 Back donation and bonding, 16 92, 93 thallium(III) cyano complexes, 43 17-18 (d-d)n Back donation, 33 335 Bacteria... 

The combination of oligo- or polysaccharides with non-natural polymeric structures opens up a novel class of materials. By varying the chain topology of the individual blocks as well as of the whole copolymer, the type of blocks, the composition etc., a complete set with tailor-made properties can be designed. 

Other things that are particularly controlled in condensation polymerization are chain topology and cascade reactivity of monomers. In general condensation polymerization, not only linear polymers but also cyclic polymers can be produced because both ends of the polymer are reactive during polymerization. When cascade reactions selectively occur on a monomer, condensation polymerization proceeds in a way different from general polymerization behavior based on the principle of Carothers and Flory. Thus, in condensation polymerization of AA and BB monomers, if AA monomers successively react with both the functional groups on a BB monomer, a poly-... 

These ratios are the most important means of quantitatively characterizing the effects of heterointeractions in copolymers, since their analytical formulas depend only on the chain composition and the cross excluded volume parameter uAB. For a particular miktoarm chain the ratio oG increases from a common good to selective and then to a common theta solvent since the intensity of heterointeractions increases. In Fig. 3 is illustrated the dependence of the chain topology as function of the length fraction of the B branch B=NB/(NA+NB). Similarly, the ratios y have higher values in a common theta than in a common good... 

Interestingly, C-family DNA polymerases (bacterial replicases) have a different chain topology in the palm, and the location of the Asp residues are also unique (13, 14). The palm architecture is shared by the X-family of DNA polymerases that include eukaryotic Pol 3 and certain nucleotidal transferases (Table 1). Presumably X-family polymerases share a common ancestor with DNA polymerases of the C-family. 

Figure 2 Structure of DNA polymerases, (a) The structure of Pol I represents the general right-hand shape of all DNA polymerases (PDB ID IQSY). Subdomains and the 3 -5 exonuclease domain are indicated, (b) The chain topology in the palm domain of DNA polymerases indicates their evolutionary heritage. Left The A, B, and Y family have four antiparallel sheets supported by two a helices. The three aspartic residues that chelate the metal ions are indicated by dots. Right The C-famiiy and X-family DNA polymerases contain a set of paraiiei strands on which the acidic residues that chelate metal are oriented differently from the A-, B-, and Y-family DNA polymerases, (c) During misincorporation of an incorrect nucleotide, the DNA must leave the polymerase active site (left) and enter the 3 -5 exonuclease active site (right). This action requires the melting of three to four base pairs.

Fig. 64e shows a [1102(804)2] chain that can be considered as based upon the cl/lb chain topology with each black vertex having a double link to the additional white vertex. Replacement of the latter by a grey vertex results in a chain shown in Fig. 64f Here grey vertices correspond to the NOs triangular groups that share edges with the UO7 pentagonal bipyramids.

Fersht A (2000) Transition-state structure as a unifying basis in protein-folding mechanisms Contact order, chain topology, stability, and the extended nucleus mechanism. Proc Natl Acad Sci USA 97 1525-1929... 

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