Steric incompatibility

Similarly, the random introduction by copolymerization of sterically incompatible repeating unit B into chains of crystalline A reduces the crystalline melting point and degree of crystallinity. If Tm is reduced to T, crystals cannot form. Isotactic polypropylene and linear polyethylene homopolymers are each highly crystalline plastics. However, a random 65% ethylene—35% propylene copolymer of the two, poly(ethylene-L -prop iene) is a completely amorphous ethylene—propylene mbber (EPR). On the other hand, block copolymers of the two, poly (ethylene- -propylene) of the same overall composition, are highly crystalline. X-ray studies of these materials reveal both the polyethylene lattice and the isotactic polypropylene lattice, as the different blocks crystallize in their own lattices. 

To test this first question, all conformations of an alanyl hexapeptide with blocked termini were generated, with three residues locked into a helix (C3-C2-C1) and the remaining three (Ccap-C -C") varied exhaustively, as above. The working set contained 12,726 sterically allowed conformations. Of these, 1638 had positive values of < ) for the C residue smd 888 had distances that were consistent with an interaction between the a-carbons of C3 and C, as observed in the motif (i.e. < 5.4 A,), but no single conformer satisfied both constraints simultaneously. Thus, the presence of a Cp atom at C is sterically incompatible with the motif. 

The process of filling connected f-graphs with Cg-bonds optimizes the cancellation due to steric incompatibility. For the short-ranged interaction considered, all s-mer graphs for s > 2 vanish because of steric incompatibility, because they contain either a triangle of two f-bonds and one Cg-bond, or a triangle of f-bonds. It is also possible to construct potentials that allow trimers and not tetramers, or potentials that have a cutoff at some higher s-mer. 

The extension of these ideas to fluids with multiple attraction sites has been given by Wertheim and used by him to develop a theory of equilibrium polymerization. The difficulty with extension of the single-attraction site theory to multiple attraction sites is in incorporating the increasingly complex steric incompatibility (SI). In particular, the difficulty commonly encountered is SI3, which physically corresponds to the absence of self-hindrance, where the rigidity of an s-mer prevents two of its component molecules from encountering each other. In another paper, Wertheim has considered fluids of dimerizing hard spheres and fluid mixtures of hard spheres and diatomics. 

The most interesting examples for PAH metabolite-DNA steric incompatibility are the enantiomers 19 and 20 of the carcinogenic metabolites 8 and 9 of benzo[a]pyrene, respectively. All of these four structures are electronically highly stabilised [16] and form stable complexes with the DNA fragment, but with 19 and 20 the fragment... 

The cause of noninterpenetration in Vollmert s experiments was left up in the air because of the possibility of thermodynamic (and even steric) incompatibility. Braun (1976) endeavoured to circumvent this ambiguity by carrying out crosslinking reactions on identical polymer molecules. This was based upon the observation that 1,1-diphenylethylene in the presence of sodium metal forms radical anions that rapidly dimerize in quantitative yield. This reaction was applied to poly(l-phenyl-l-(4-vinylphenyl) ethylene-costyrene), which can in principle undergo both intramolecular and intermolecular reactions. Intramolecular reactions cause coil contraction, which is manifest by a decrease in the intrinsic viscosity of the polymer solutions. Intermolecular reactions stiffen the polymer chains, their structure... 

Multiple block copolymers form a domain-matrix morphology due to the chemical and steric incompatibilities of the two chemically different blocks. The surface molecular and morphological structures of a series of block copolyether-urethane-ureas have been studied in detail via Electron Spectroscopy for Chemical Analysis (ESCA) and Fourier Transform Infrared Spectroscopy (FTIR) coupled with internal reflectance techniques. ESCA provides elemental information concerning the very surface, while FTIR provides the molecular and secondary bonding Information of the surface and into the bulk. Bulk and surface chemical and morphological structures are shown to be quite different, and are affected by synthetic and fabrication variables. 

In the case of the ER, when a region of 20 amino acids between 250 and 270 is missing, the receptor is located strictly in the cytoplasm. Domains of similar size and function have also been located in the receptors of glucocorticoids and of progesterone. The zone of nuclear location overlaps with one of the sequences for interacting with hsp90, which at the same time is next to the DBD. The coincidence of the three functions in a space so restricted implies that they are totally or partly incompatible sterically (Evans 1989 Gruber et al. 2002). 

For polycatenar hydrogen bonded complexes with fluorinated chains at both ends (e.g., 138,139, see Fig. 36) formation of columnar phases was observed [246]. However, compound 137, having a branched Rp-chain at one end and three RH-chains at the other has a sequence of three distinct phases in the unusual sequence Cub-Col-SmA-Iso. For the SmA phase of compound 137 a structure with intercalated aromatic cores and RF-chains and separated layers of the hydrocarbon chains was proposed. At lower temperature, when incompatibility rises and the aromatics and Rp-chains disintegrate, all three components form their own layers. However, this produces interface curvature and a columnar phase with square lattice is formed. On further cooling a transition to a cubic phase with Im3m lattice takes place which is most likely of the bicontinuous type [262]. This leads to the unusual phase sequence Cubv-Col-SmA where the positions of the Cubv and Col phases are exchanged with respect to the usually observed phase sequences. The Col-Cub transition at lower temperature could be the result of the decreased conformational disorder of the terminal chains which reduces the steric frustration and hence reduces the interface curvature. 

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