Calorimetric reversibility transitions

There are a few polymers where reversible transitions have been discovered in the solid state. Furukawa, McCoskey and King (1952) and Marx and Dole (1955) studied calorimetrically the room temperature order-disorder transitions in polytetra-... 

Note that the initial state of this process is a real one (the pure liquid at pressure p°), but the final state (the gas behaving ideally at pressure p°) is hypothetical. The liquid and gas are not necessarily in equilibrium with one another at pressure p° and the temperature of interest, and we cannot evaluate Avap7/° from a calorimetric measurement with electrical work without further corrections. The same difificulty applies to the evaluation of Asub -In contrast, AvapT/ and Asub (without the ° symbol), as well as AfasH°, all refer to reversible transitions between two real phases coexisting in equilibrium. 

The DSC curve profiles suggest two kinds of binding sites for Al on pepsin. At each transition midpoints, T and T, and from ligand-induced shifts and calorimetric parameters for reversible transitions, an equilibrium binding affinity Kl were calculated... 

Moreover, in many cases, a shift of Tg to lower values of temperature has been detected, but in these cases the quality of adhesion between phases may be the main reason for the reversing of this attitude 11,14). If calorimetric measurements are executed in the neighbourhood of the glass transition zone, it is easy to show that jumps of energies appear in this neighbourhood. These jumps are very sensitive to the amount of filler added to the matrix polymer and they were used for the evaluation of the boundary layers developed around fillers. 

Figure 10 Oscillating differential scanning calorimetric (ODSC) curves showing the separation of the glass transition (reversible, i.e., thermodynamic component) and enthal-pic relaxation (irreversible, i.e., kinetic component) which overlap in the full DSC scan. (Reprinted with permission from Ref. 38.)... 

Hen egg-white lysozyme, lyophilized from aqueous solutions of different pH from pH 2.5 to 10.0 and then dissolved in water and in anhydrous glycerol, exhibits a cooperative conformational transition in both solvents occurring between 10 and 100°C (Burova, 2000). The thermal transition in glycerol is reversible and equilibrium follows the classical two-state mechanism. The transition enthalpies AHm in glycerol are substantially lower than in water, while transition temperatures Tm are similar to values in water, but follow similar pH dependences. The transition heat capacity increment ACp in glycerol does not depend on the pH and is 1.25 0.31 kj (mol K) 1 compared to 6.72 0.23 kj (mol K)-1 in water. Thermodynamic analysis of the calorimetric data reveals that the stability of the folded conformation of lysozyme in glycerol is similar to that in water at 20-80°C but exceeds it at lower and higher temperatures. 

Wilkes et al (22.23) coupled calorimetric, dynamic-mechanical and x-ray diffraction techniques to demonstrate crystallization of the lipids was completely reversible in neonatal rat stratum corneum, and only partially reversible in human stratum corneum. Melting regions near 40°C and from 70 to 90°C corresponded to the thermal transitions noted in the calorimetric studies for both species. The crystalline nature of the lipids did not appear to be dependent on the presence of water. X-ray diffraction and infrared spectroscopy studies (23.28-34) have also shown a to p conformational changes occurred in keratin and stratum corneum protein components with hydration or exposure to increased temperatures. Oertel (28) has reported pretreatment with dimethylsulfox-ide, hexylmethylsulfoxide and decylmethylsulfoxide resulted in the formation of p-sheet protein conformations in vitro in human... 

Foht PJ, Quynh MT, Lewis RNAH, McElhaney RN. Quantitation of the phase preferences of the major lipids of the Acholeplasma laidlawii B membrane. Biochemistry 1995 34 13811-13817. Lewis RNAH, McElhaney RN. Acholeplasma laidlawii B membranes contain a lipid (glycxerylphosphoryldiglucosyl diacylgly-cerol) which forms micellar rather than lamellar or reversed phases when dispersed in water. Biochemistry 1995 34 13818-13824. Steim JM, Tonrtellotte ME, Reinert JC, McElhaney RN, Rader RL. Calorimetric evidence for the liquid-crystalline state of lipids in a biomembrane. Proc. Natl. Acad. Sci. U.S.A. 1969 63 104-109. Reinert JC, Steim JM. Calorimetric detection of a membrane lipid phase transition in living cells. Science 1970 168 1580-1582. Melchior DL, Morowitz HJ, Sturtevant JM, Tsong TY. Characterization of the plasma membrane of Mycolplasma laidlawii. Vni. Phase transitions of membrane lipids. Biochim. Biophys. Acta 1970 219 114-122. 

Synthetic lipid A analogues were used in FTTR investigations to study possible intermolecular conformations of neighboring molecules [79]. There is good evidence that the bisphosphory-lated /3-(l—>6)-linked GlcN disaccharide backbone of lipid A is inclined (20—40° relative to the membrane normal), thus, the phosphate linked to 01 is reaching to the outside and that at 04 is buried in the membrane. This model is supported by data from transition temperature measurements and calorimetric experiments. However, it should be noted that other authors reported the reverse conformation with the 04 phosphate on the surface and the 01 phosphate in the membrane [80,81]. 

The universal character of calorimetric measurements also pays in the elucidation of the reversible transformations undergone by bilayer-forming phospholipids. The transitions of phosphatidylcholine and similar congeners between their vesicular and micellar states depending on their concentrations, the presence of simple detergents, and the temperature are quite sharp and accompanied by sensible heat effects that allow for their thermodynamic characterization. In a particularly illustrative example, the dissolution and reconstitution of lipid vesicles from Escherichia coli native polar lipid fraction by octyl-jS-o-glucoside as analyzed by ITC was reported (Figure 15). ... 

Heat capacity is the basic quantity derived from calorimetric measurements. For a full caloric description of a system, heat capacity information is combined with data on heats of transition, heats of reaction, etc., as outlined in Sect. 2.2.2. The basic descriptions of reversible and irreversible thermodynamics are given in Sects. 1.1.2, 2.1.1, and 2.1.2. In this section measurement and theory of heat capacity are discussed, leading to the /Advanced Tffermal y4nalysis System, ATHAS. This system was developed over the last 20 years to increase the precision of thermal analysis of linear macromolecules. It permits computation of the heat capacity from theoretical considerations or empirical addition schemes. Separating the heat capacity contribution fi-om the heat measured in a thermal analysis allows a more detailed interpretation of reversible and irreversible transitions and reactions. 

A second quiescent state in A. franciscana embryos that has been studied calorimetrically is anhydrobiosis, or life without water. These embryos are one of the most intensely studied anhydrobiotic systems (for reviews, see [119,120]), in part because their hydration state can be precisely controlled. Embryos enter a profound, yet reversible, state of metabolic arrest in response to cellular dehydration. While the metabolic transitions appear to be a function of water content, the controlling mechanisms involved are not fully understood. 

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