Differential scanning calorimetry reversible transition

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. 

Lepock, J.R., K.P. Ritchie, M.C. Kolios, A.M. Rodahl, K.A. Heinz, and J. Kruuv. 1992. Influence of transition rates and scan rate on kinetic simulations of differential scanning calorimetry profiles of reversible and irreversible protein denaturation. Biochemistry 31 12706-12712. 

Comparson of the transitions observed by differential scanning calorimetry in membranes of M. laidlawii and in water dispersions of the lipids from the membranes support the concept that most of the lipids exist as a smectic mesophase in the membranes. The evidence for a bilayer structure is straightforward in this case. Lipid transition temperatures are a function of fatty acid composition and correlate well with biological properties. The calorimeter possesses advantages over high resolution NMR for M. laidlawii, and perhaps in many other systems, because the data can be interpreted less ambiguously. In M. laidlawii membranes the bilayer appears to be compatible with the same physical properties observed in other membranes—a red-shifted ORD, lack of ft structure in the infrared, reversible dissociation by detergents, and poorly... 

Subsequently, refinement against sub-ambient PND data showed that the data could be fit to a doubled cubic unit cell in space group Fd3m with hydrogen fully occupying the 32 (x, x, x) site (i.e. an ordered distribution of hydrogen in the imide units Fig. 16.3b [34]. Moreover, a disordered antifluorite phase was not observed below 350 K and a reversible phase transition was identified by differential scanning calorimetry (DSC). In fact, this order-disorder transition corresponds to one found in differential thermal analysis... 

In the majority of cases the compatibility of the polymers is characterized by the glass-transition temperature Tg, determined by methods such as dilatometry, differential scanning calorimetry (DSC), reversed-phase gas chromatography (RGC), radiation thermal luminescence (RTL), dynamic mechanical spectroscopy (DMS), nuclear magnetic resonance (NMR), or dielectric loss. The existence of two... 

The latest development (1992) is the Modulated Differential Scanning Calorimetry (MDSC). Here the linear heating rate is superimposed by a sinoidal modulation. The following advantages of this mode are claimed direct measurement of heat capacity improved resolution of adjacent or superimposed effects improved sensitivity for weak transition effects separation of reversible from irreversible effects... 

Fig. 3. Differential scanning calorimetry scans of biomembrane transitions, all obtained with 50% ethylene glycol/water as solvent. (A) A. laidlawii membranes from cells grown in tryptose medium at 37 C (B) lysodeikticus membranes from cells grown in brain heart infusion at 37 C (C) JE. coli K12W945 whole cells grown in minimal salts with glucose at 20 C (D) the same cells as in (C), but scanned after thermal protein (E) rat liver microsomes (F) rat liver In all cases a lower temperature reversible lipid transition is followed by a higher temperature irreversible protein peak. The protein denaturation peaks are featureless in (A), (E), and (F), but show fine structure in (B), (C), and (D). Unlike other organisms, coli after heating shows two lipid transitions and residual reversible protein denaturation, as seen in (D).

Remarkably, Butler found the dissolution of oil-like groups to be exothermic. Therefore, the reverse reaction for loss of hydrophobic hydration must be endothermic. The reaction attending the inverse temperature transition to hydrophobic association is indeed endothermic as shown by differential scanning calorimetry data, specifically the middle curve in Figure 8.1 and the curves in Figure 7.1. 

Differential scanning calorimetry (DSC) is a valuable aid by which phase transition temperatures, transition heats, and transition entropies can be conveniently measured or calculated. This technique offers a direct and complimentary (to microscopy) evaluation of thermal behavior. Figure 4 shows the DSC curve for 4-octyloxybenzyli-dene-4 -chloroaniline in which can be seen Kj -K2, K2 Sg, Sg-S, and S -I transitions. All transitions are enantiotropic and all are reversible. The most extensive supercooling occurs for the meso-phase-solid transition, in this case the Sg-K2 transition. Optical microscopy and/or x-ray diffraction is required to assign the specific mesophase type. 

The TMDSC with Fourier analysis of the melting pentacontane and the calculations using saw-tooth analysis methods are given in the publication Wunderlich B, Boiler A, Okazaki I, Ishikiriyama K, Chen W, Pyda W, Pak J, Moon, 1, Androsch R (1999) Temperature-modulated Differential Scanning Calorimetry of Reversible and Irreversible First-order Transitions. Thermochim Acta 330 21-38. 

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