Disordered conformation

Xanthan is reported to undergo a chiroptically detected temperature or salt-driven conformational change from an ordered conformation at high salt and low temperature to a disordered conformation either associated with lowering the salt concentration, or with increasing the temperature (2-5). The primary structure of xanthan has been known for about a decade (6,7), but different structures have been suggested both for the ordered and disordered conformation. Some workers (8-13) conclude that the ordered conformation is double-stranded or double-helix, whereas others (14-17) claim that a single stranded description can account for the observed data under... 

In this study we use electron microscopy (EM) to study xanthan strandedness and topology both in the ordered and disordered conformation. Correlation of data obtained from electron micrographs to physical properties of dilute aqueous solution on the same sample will be used to provide a working hypothesis of the solution configuration of xanthan. Electron micrographs obtained from xanthan of different origins will be compared to assess similarities and differences in secondary structure at the level of resolution in the used EM technique. 

Disordered conformation. Figure 5 shows electron micrographs of xanthan D and F obtained from xanthan vacuum-dried from solutions yielding the disordered conformation. The various molecular assemblies are assigned as follow I = single -stranded, II = perfectly matched double stranded, III = branched from double - to single stranded. This assignment will be discussed below. 

Polymers showing crystalline mesomorphic modifications, characterized by chains in disordered conformations, are exceptions to the equivalence principle. Examples of these cases are shown in Section 2.6. 

In the case of cisPI (A = A = H and B = CH3 and B = H), the two conformations corresponding to the two nonequivalent energy minima in the map of Figure 2.16c,68 (A+dsA T) [s(2/l) symmetry] and (A d.vA TA cis A T) (tc symmetry), correspond to the conformations of the chains in the crystal structure of cisPI proposed by Nyburg76 and Bunn,77 respectively. The X-ray diffraction data are, however, better accounted for by a disordered conformation, proposed by Corradini and co-workers,72,78 characterized by a statistical succession of monomeric units which assume these two energy minimum conformations. 

Crystalline polymers characterized by disordered conformations of the chains are, for instance, polytetrafluoroethylene (PTFE), /ra .s-1,4-poly (1,3-butadiene), and cis-1,4-poly(isoprcnc). 

Moreover, we have already shown in the Section 2.6 that chains characterized by disordered conformation may crystallize. The presence of disorder in polymer crystals is a typical feature, so that the real crystalline forms are generally different from the ideal models. These disordered modifications can,... 

Examples of mesomorphic forms characterized by disorder in the conformation of the chains have already been described in Section 2.6. For instance, a mesomorphic form is present in the high-temperature form I of polytetrafluoro-ethylene.106,107 In this phase the chains are in disordered conformation due to the presence of helix reversals along the chains.108-110 Moreover, intermolec-ular disorder is also present due to the random rotations of the chains around the chain axes.109 A long-range three-dimensional order is present only in the pseudohexagonal placement of the chain axes.107,109... 

The Lewis basicity of the solvents was found to be more important for the solvatochromism than the solvent polarity, since such Si... solvent and Si... F interactions are competitive in the presence of solvents with donor atoms. In contrast, the analogous non-fluorinated polysilane, poly(methyl-/z-propylsilylene), revealed a disordered conformation in both coordinating and non-coordinating solvents. Additionally, the UV spectra of various molecular weight fractions of 87 showed an unusual molecular weight dependency an isosbestic point is apparent, suggesting an equilibrium between globule- and rod-like conformations at room temperature, which was also evident from the... 

Figure 16. Model of a segment of syndiotactic polypropylene in a disordered conformation. From icf. (179) Copyright Hiithig and Wepf, Basel.

---