Order - disorder conformational change

From this point of view, polypeptides containing photochromic units in the side chains are quite special polymers. They can exist in ordered or disordered conformations, and photoisomerization of their photochromic side chains can produce order = disorder conformational changes. These photostimulated structural variations, such as random coil a-helix, take place as highly cooperative transitions therefore photochromic polypeptides actually work as amplifiers and transducers of the primary photochemical events occurring in the photosensitive side chains. 

Several research groups are currently investigating the inherent thermochromism of various polymers. Certain polysiloxanes exhibit reversible thermochromic activity [59, 60]. The thermochromic behavior of these macromolecules is due to order-disorder conformational changes that accompany a particular temperature change. This transition perturbs the electron delocalization of the silicone backbone and results in a shift in the absorption maxima in the UV-visible range. 

Order-disorder conformational transitions very often occur on changing physical and/or chemical conditions of polysaccharide solutions. DMSO, for example, is the solvent, which is commonly used as co-solvent for stabilizing or destabilizing ordered solution conformations. Schizophyllan, a triple helical polysaccharide with a [P-D-(l-3)-glc] backbone exhibits a highly cooperative order-disorder transition in aqueous solution. When small quantities of DMSO are added to aqueous solutions the ordered state is remarkably stabilized, as has been observed in the heat capacity curves by means of the DSC technique. ... 

This is, for instance, the case of PTFE, which at atmospheric pressure presents two reversible first-order transitions at 19 °C and 30 °C [67], In the transition at 19 °C the molecular conformation changes slightly, from a 13/6 to a 15/7 helix and the molecular packing changes from an ordered structure with a triclinic unit cell (corresponding to a positioning of the chain axes nearly hexagonal) toward a partially disordered structure (partial intermolecular rotational disorder) with a... 

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... 

Tapia and Eklund (1986) carried out a Monte Carlo simulation of the substrate channel of liver alcohol dehydrogenase, based on the X-ray diffraction structure for this enzyme. The addition of substrate and the associated conformation change induce an order—disorder transition for the solvent in the channel. A solvent network, connecting the active-site zinc ion and the protein surface, may provide the basis for a proton relay system. A molecular dynamics simulation of carbonic anhydrase showed two proton relay networks connecting the active-site zinc atom to the surrounding solvent (Vedani et ai, 1989). They remain intact when the substrate, HCOf, is bound. 

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