Conformational forms, macromolecules

Glassy materials can be deformed without fracture up to hundreds of percents. Under this so-called forced deformation, the conformations of macromolecules change but the structure of the dense network does not change. After the stresses have been removed, the sample is still in a deformed state, which is a metastable equilibrium state. The macromolecules in such a sample are frozen in a deformed state, so that on heating, when the crosslink number density decreases, forces are exerted and the sample can return to its initial form (Laius and Kuvshinskii 1963 Shishkin et al. 1963). 

At interfaces proteins usually adsorb in a globular form in some cases changes in the conformation of macromolecules in the adsorption layer may take place. Protein adsorption is, to a significant extent, irreversible, which makes it difficult to describe using the Gibbs equation [30]. 

Macromolecules exist in a variety of conformational forms that range from randomly coiled chains to more spatially ordered structures. Of particular interest are the polymers that adopt helical symmetry. Helical conformation is a result of an orderly repeated unit with internal rotational angles along the polymer backbone. In the crystalline state, polyoxyethylene exists in a helical conformation that contains seven chemical units (-CH2CH2-O-) and two turns in a backbone identity period of 19.3 A (7-9). 

Figure 4-7. Important conformational forms of macromolecules (after S.-L. Mizushima and...

The conformation of helices can be elucidated from the observed number and spacing of layer lines in helix-forming macromolecules. In a 3i helix every... 

The densities of amorphous and crystalline polymers can differ by up to 15% (Table 5-3). Polymers with unsubstituted monomeric units, such as poly(ethylene) and nylon 6,6, for example, show the greatest difference in density. These chains crystallize in an dAVtrans conformation with particularly close packing of molecular chains. In helix-forming macromolecules with large substituents, such as it-poly(styrene), for example, the packing is, by contrast, less efficient. 

The phenomenon by which various monomer units can replace each other in the lattice is termed isomorphism. Isomorphism is possible in copolymers if the corresponding unipolymers show analogous crystal modifications, similar lattice constants, and the same helix type. For example, according to Table 5-5, the y form of it-poly(propylene) and modification 1 of it-poly (butene-1) possess triclinic crystal form, similar lattice constants for the c dimension, and the same helix type. The copolymers of propylene and butene-1 therefore show isomorphism. Isomorphism occurs particularly readily in helix-forming macromolecules, since the helix conformations lead to channels in the crystal lattice, which can easily accommodate different substituents. 

The conformation of helices can be elucidated from the observed number and spacing of layer lines in helix-forming macromolecules. In a 31 helix every fourth, seventh, etc., chain link is in the same position as the first. For this case, there should be three layer lines, as is shown in Figure 5-6 for a drawn film of it-poly(propylene). 

Birshtein, T. M. and O. B. Ptitsyn, Conformations of Macromolecules , Interscience, New York, 1966. A presentation of the principal concepts and mathematical methods which form the basis of macromolecular conformations High Polym, vol. 22). 

The CIF file format was quickly and widely adopted by the scientific community for at least two reasons [165J it was, and still is, endorsed by the lUCr and submission of data to the journal Acta Ciystallographka, Section C in a form conforming to CI F assures faster processing and hence faster publication of accepted papers. The current CIF file dictionary defines about 1200 data names, but it is still unable to represent all the details of the crystallographic measurements of macromolecules. Thus, yet another STAR-based data format is needed. 

Within the class of polymer crystals having, ideally, long-range positional order for all the atoms, different crystalline forms (polymorphs) may arise as a result of having different almost isoenergetic macromolecular conformations (of the main chain, in most known cases) or as a result of different, almost isoenergetic modes of packing of macromolecules with identical conformations [1-3]. 

The sedimentation coefficient s°, or its normalized form 5 0 w function of the conformation and flexibility of a macromolecule (via its translational frictional property) and its mass. So if we are going to obtain conformation and flexibility information we need to know the molecular weight (molar mass)... 

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