Differential scanning calorimetry endothermic transition

Structured proteins have also been investigated by thermal analysis [40,41], denaturing resulting in an endotherm which is readily detected by differential scanning calorimetry (DSC). DSC of recombinant resilin in the swollen state showed no transitions over a wide temperature range (25°C-140°C), further evidence of the absence of any strucmre. This is in contrast to the strucmred proteins wool and bovine serum albumin, which show denamration endotherms at 145°C and 62°C, respectively (Figure 9.6). 

Although there are other ways, one of the most convenient and rapid ways to measure AH is by differential scanning calorimetry. When the temperature is reached at which a phase transition occurs, heat is absorbed, so more heat must flow to the sample in order to keep the temperature equal to that of the reference. This produces a peak in the endothermic direction. If the transition is readily reversible, cooling the sample will result in heat being liberated as the sample is transformed into the original phase, and a peak in the exothermic direction will be observed. The area of the peak is proportional to the enthalpy change for transformation of the sample into the new phase. Before the sample is completely transformed into the new phase, the fraction transformed at a specific temperature can be determined by comparing the partial peak area up to that temperature to the total area. That fraction, a, determined as a function of temperature can be used as the variable for kinetic analysis of the transformation. 

The results of differential scanning calorimetry(DSC) indicate the change in aggregation state. The trans micelle showed a main endothermic peak at 14 2°C(A H =1.0 kcal/mol), corresponding to a gel-liquid crystal phase transition, whereas the transition temperature for the cis micelle appeared at 11.9°C( AH = 0.8 kcal/mol). This is unequivocal evidence that the trans-cis photoisomerization is a sufficient perturbation to alter the state of molecular aggregation. 

The thermal unfolding of proteins is best measured by differential scanning calorimetry, which measures the heat absorbed by a protein as it is slowly heated through its melting transition (Figure 17.1). A solution of about 1 mg of protein in 1 mL of buffer and a separate reference sample of buffer alone are heated electrically.6 The additional current required to heat the protein solution is recorded. As the protein denatures, there is a large uptake of heat because the process is highly endothermic. The temperature at the maximum of the peak is... 

Differential Scanning Calorimetry. Figures 1 and 2 show 320—520 K DSC scans of the as-reacted polymer and the DMF solution cast samples at heating rate of 20°C/min. Two broad endothermic transition regions are observed—a very broad and weak transition near 350°C and a narrower, stronger transition below 500°C. 

Differential scanning calorimetry (DSC) can be used to determine experimentally the glass transition temperature. The glass transition process is illustrated in Fig. 1.5b for a glassy polymer which does not crystallize and is being slowly heated from a temperature below Tg. Here, the drop which is marked Tg at its midpoint, represents the increase in energy which is supplied to the sample to maintain it at the same temperature as the reference material. This is necessary due to the relatively rapid increase in the heat capacity of the sample as its temperature is increases pass Tg. The addition of heat energy corresponds to the endothermal direction. 

Figure 24.1 Differential scanning calorimetry (DSC) traces for the different N,N -dimethylpyr-rolidinium (Pn) species. The y-axis has been adjusted to allow comparison between species. Thermal transitions are recorded as peaks (endothermic) or troughs (exothermic) and indicate changes of phase. The structurally similar dicyanamide (DCA) and thiocyanate species both show broad phase transitions in comparison to the TFSA species.

Figure 5.1. Schematic representation of typical differential scanning calorimetry curves showing a glass transition and possible exothermal and endothermal relaxations.

Cramer WA, Whitmarsh J, Low PS. Differential scanning calorimetry of chloroplast membranes identification of an endothermic transition association with the water-splitting complex of photosystem II. Biochemistry 1981 20 157-162. 

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