Sigmoidal baseline

The determination of heat of crystallization is not always simple, because the crystallization process in nonisothermal crystallization often lasts to the glass transition temperature. Also, since the absolute values of the melt and crystal heat capacities and their temperature dependences are different, application of the sigmoidal baseline is needed for generally accurate determination of the heat of crystallization (for the sigmoidal baseline, see Section 2.7.3). However, because of the frequent absence of a hnear baseline above Tg, but after completion of the crystilization process, the sigmoidal baseline cannot... 

Figure 2.48. Characteristic temperatures of a polymer melting, and determination of the sigmoidal basehne numbers 1 indicate spots where the cursor should be put down to determine the sigmoidal baseline. Unless the four points can be put on the same line, the sigmoidal baseline option should be used. If the four points are on the same hne, yon can stUl nse the sigmoidal baseline option, but then the basehne will be straight. (Ta = starting temperature of melting, = peak temperature of melting,...

The heat of fusion in joules per gram (J/g) calculated with the sigmoidal baseline. 

Cp n T) is the heat capacity function of the native state, ACp T) = Cp j) T)-Cp n(T) is the difference between the extrapolated heat capacity functions of the denatured and native states at temperature T, and the term ACpOiD describes the sigmoidal baseline under the transition peak. The term [A°/f°(T)] aDQ N characterises the heat absorption peak. The product Q n d = ctN(l Q n) is responsible for the appearance of the characteristic shape of the heat capacity peak. Typical curves are given in Figures 1 and 4. 

All regions of the function should be defined with real data. In cases of sigmoidal curves it is especially important to have data define the baseline, maximal asymptote, and mid-region of the curve. 

Fig. 3 Thermal denaturation transition of a DNA helix, (a) UV absorbance increases with temperature, following the unstacking of bases, following a sigmoidal shape. AD and Au are lower and upper baselines, also slightly dependent on temperature, (b) Fraction of single strands 6 extracted from data in (a), which defines the melting temperature corresponding to 9 = 0.5. Adapted with permission from [7]...

Data are recorded using e.g. TIDA software (HEKA Electronics, Lambrecht, Germany) and the results are typically expressed as fraction of baseline current. Concentration-response data can be fitted to an equation of the following form 1/10 = 1/(1 + flcom-poundl/ICso)) such that the IC50 can be calculated with a sigmoidal dose-response curve model. 

Fig. 2. Simulated DSC endotherm, (bold line), for a monomeric protein unfolding reversibly via a two-state transition (Jm = 60°, A/f° = 50 kcal/mol, A//cal/Aff = 1.0). The sigmoidal heat capacity baseline, (dashed line), shifts from the native to unfolded due to a ACp of 750 cal/...

A.nother approximation provides usually also satisfactory results. This is the use of a linear baseline (dotted line in Figure 3) instead of the sigmoidal curve. This simplification might introduce significant errors only for very broad or asymmetric peaks [16]. 

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