Irreversible transitions

Diphenyl-l,3,4-oxadiazole is polymorphic (centrosymmetric monoclinic structure with space group P21/c and monoclinic non-centrosymmetric structure with space group P21/c). DSC investigations showed an irreversible transition from the first to the second form at 97 °C <2003JST219>. 

J. J. Tyson and B. Novak, Regulation of the eukaryotic cell cycle Molecular antagonism, hysteresis, and irreversible transitions. J. Theor. Biol. 210, 249-263 (2001). 

Fig. 14. DSC-traces of glasses of a macromolecule with a mesogen in the side-chain. Amorphous glass (top curve) and the corresponding LC-glass (bottom curve) drawn after data from Ref.21). Heating rates 50 K/min. The exotherm indicates the irreversible transition to the mesophase...

Conditions required to generate A- and B-type crystal conformations are reasonably well-understood. Under cool, wet conditions (such as in a potato tuber), B-type starch crystals form, while in warmer, drier conditions (e.g. in a cereal grain), the A allomorph is preferred. Chain length also affects the selection of crystal form, chains of DP <10 do not crystallize, chains with a DP from 10 to 12 tend to form A-type crystals, and chains with a DP >12 tend to yield the B form.37,38 This is likely the result of differences in loss of entropy on crystallization experienced by chains of different lengths.39 An irreversible transition from B-type starch to A-type starch can be accomplished under conditions of low humidity and high temperature. This so-called... 

A viscoelastic fluid has the appearance of a solid body it deforms and wholly recovers below t0, and only partly recovers above t0. From 0 to t0, the fluid undergoes an elastic conformational transition above t0, the fluid undergoes an irreversible transition, whence the mass begins to flow toward a new equilibrium position. Carrageenan-water-polyol systems have been suggested to be industrially useful in consideration of their significant t0 followed by shear-thinning (Tye, 1988). 

The reversible character of the Al transitions was recently confirmed by 27A1-MAS-NMR and Al K-edge XANES studies by van Bokhoven et al.. 2 Following heating and steaming up to 450°C the Al reverts completely to its tetrahedral lattice position upon ammonia treatment. At 550°C irreversible transitions of aluminum come to the fore. 

For all positronium forming solids and liquids investigated so far epithermal positronium and its slowing-down could be observed, indicating that no special sites are required for positronium formation (cf. Sect. 3). Evidence for the existence of two kinds of o-Ps in liquid rare gases and for irreversible transitions between them is given in Sect. 4. 

Sage and Singer (1958, 1962) showed that ribonuclease could not only be recovered from neutral ethylene glycol into aqueous solution with essentially full retention of enzymatic activity, but that this was so even after all six of its tyrosine residues had been converted to the phenoxide ion form in ethylene glycol. This is in contrast to the situation in water solutions of this protein, in which the titration of more than three of the six tyrosines results in an essentially instantaneous irreversible loss of enzymatic activity (Sela and Anfinsen, 1957). This suggests the interesting possibility that the irreversible transition that occurs in aqueous solutions... 

The term monotropy applies in the case of an irreversible transition from one form to another. Monotropy is bound to the existence of metastable thermodynamic forms. The liquid-gas curve crosses the solid-gas curves for the two forms before their point of intersection. 

The stable crystalline form is corundum (a-alumlna). Kappa-alumina is one of several structurally related, metastable forms. These occur In "active alumina" and are much studied (5) due to their Importance in adsorbents and catalysts. Llppens and Steggerda (5) summarized the classifications of metastable forms, their crystal structures and conditions of formation by dehydration. Okuralya et al. (6) recently published an extensive study on the formation and structural relations of 1C-AI2O3 and its precursors le -AlgOg and tohdlte. Kappa-alumina belongs to the "nearly anhydrous", high-temperature classification (5) based on its formation temperture of 900 to 1000 C (somewhat lower in vacuo). The Irreversible transition x+o is thermally activated at 1200 C (2, 5). Thermochemical studies cited In this table presume that the samples of x-Al Og were essentially anhydrous. 

The best documented case of imderlayer alloy formation is the Cu 100)-c(2x2)-Pd surface alloy which undergoes an irreversible transition to a structure consisting of a mixed c(2x2) CuPd second layer capped by a Cu monolayer illustrated in figure 18. 

The idea that polysaccharide-polysaccharidase interaction could be treated simply as the sum of the interactions of various subsites is intuitively attractive. Hiromi first tackled the algebra, but made the assumption, now known to be implausible, that the microscopic rate constant for bond cleavage in the ES complex was independent of subsite occupancy. This is very unlikely for enzymes in general terms, since it is now accepted that they exploit interactions with the substrate remote from the site of bond cleavage to lower the free energy for the catalysed reaction (the Circe effect, Section 5.4.5.3). The Hiromi assumption was shown to be incorrect by direct experiments with an endoxylanase. A better fit to experimental data was obtained if the subsite affinities were calculated for the first irreversible transition state i.e. on kcat/A)n and cleavage patterns of oligosaccharides), but careful analysis in some systems indicated that even this approximation failed. ... 

The industrial quest for new, desirable chemical products is now accomplished by the combinatorial chemistry of the MCRs and their libraries, including those of the isocyanides [20-22]. The reactions of the isocyanides proceed especially well, as all correspond to irreversible transitions of divalent carbon atoms, C", into their CIV atoms. If no byproducts are formed by some competing reactions, then quantitative yields of MCR products are formed. 

It was recognized early on that the functional isocyano group differs profoundly from any other functional class of chemical compounds. In the 1890s, Nef [44] had mentioned the fact that the functional group -NC of the isocyanides contains a divalent carbon atom C11, and therefore their chemistry differs very much from other stable organic chemical compounds that contain only tetravalent carbon atoms CIV. Any synthesis of the isocyanides corresponds to a conversion of Clv into C",and all characteristic chemical reactions of the isocyanides correspond to exothermic irreversible transitions of the carbon atoms C11 into CIV. Under the correct conditions, when no competing formation of by-products takes place, such reactions yield the products quantitatively. 

Vibrational relaxation opens an additional decay channel due to irreversible transitions from the /> and w> levels to lower-lying vibronic states of the /)) manifold. The overall set of rate equation describing the system after the initial dephasing is... 

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