Macro-conformations

It is now generally accepted that the morphology of a polymer depends on the contributions of three different macro-conformations (a) the random coil or irregularly folded molecule as found in the glassy state, (b) the folded chain, as found in lamellar structures and (c) the extended chain. The fringed micelle (d) may be seen as mixture of (a), (b) and (c) (see Fig. 2.12) with paracrystallinity as an extreme. 

FIG. 2.12 Schematic drawing of the different macro-conformations possible in solid linear macromolecules, (a) Random, glassy (b) folded chain, lamellar (c) extended chain equilibrium (d) fringed micelle, mixture of (a) to (c) (after Wunderlich, 1970). 

Rg. 5.11 A schematic representation of the macro-conformations of polymer chains. The vertices indicate the limiting cases. A, amorphous B, chain-folded C, chain-extended. The area indicates intermediate structures D, fringed micelle. (Reproduced by permission of Academic Press.)... 

Macropolycyclic ligands, 2,942 classification, 2,917 metal complexes binding sites, 2, 922 cavity size, 2,924 chirality, 2, 924 conformation, 2,923 dimensionality, 2, 924 electronic effects, 2, 922 shaping groups, 2,923 structural effects, 2,922 molecular cation complexes, 2,947 molecular neutral complexes, 2,952 multidentate, 2,915-953 nomenclature, 2,920 Macro tetrolide actins metal complexes, 2,973 Macrotricycles anionic complexes, 2,951 cylindrical... 

The conformation of macro- or polyions has been defined and discussed briefly in Section 4.1.1. The conformation of a polyion is determined by a balance between contractile forces, which depend on conformation free energy, and extension forces, which arise from electrical free energy. The extent of conformational change is determined by several factors. Changes are facilitated by the degree of flexibility of the polyion, and conformational change is greatest at low concentration of polyions. 

Finally, as in macro-Raman experiments, orientation-insensitive spectra can also be calculated for spectromicroscopy. A method has been developed recently for uniaxially oriented systems and successfully tested on high-density PE rods stretched to a draw ratio of 13 and on Bombyx mori cocoon silk fibers [65]. This method has been theoretically expanded to biaxial samples using the K2 Raman invariant and has proved to be useful to determine the molecular conformation in various polymer thin films [58]. 

Prior to the synthesis of the epothilones, the use of RCM for the preparation of macrolactones had received little attention [5]. In part, this could be attributed to the generally held assumption that only conformational biased precursors would undergo cyclization. Although several examples of macro-RCM had been reported, notably by Pandit [9],Hoveyda [10] andFurstner [11], preparation of the densely functionalized 16-membered lactone core of the epothilones was not a trivial undertaking. Preliminary studies focused on the preparation and cyclization of model substrates in order to assess the viability of the RCM approach and to provide precedent for subsequent, more ambitious, synthetic endeavors. 

The reactions of intramolecular cross-linking is a rather poorly investigated area in the field of macro-molecular reactions. However, the problems of regularities of such processes are related to such important problems of polymer chemistry as chemical modification of polymers, networks formation, sorption of low molecular reagents by polymers, intramolecular catalysis, conformational transitions and so on. In spite of the great importance of the study of regularities of cross-linking reactions, the experimental and theoretical analysis of such processes is complicated by many difficulties. ... 

The spatial macrostructure of the native protein (the equilibrium location of the polypeptide main chain backbone and bulky side groups) is strictly determined. Individual protein molecules having the same sequence of amino acid residues do not differ in their three-dimensional structure, which is the equilibrium one and averaged in time. The activation energy of conformational transitions may be as high as several hundreds of kilojoules per mole. Therefore, the extended fluctuations which are associated with the unfolding of the native macro structure and transitions between conformations occur rather rarely. 

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