Hair Damage 380 of protein stability and function is rate-limited initially by the denaturation step, which may then be followed by coagulation, aggregation, and/or gelation. The transition of protein from a native to a denatured conformation is accompanied by the rupture of inter- and intra- molecular bonds, and the process needs to occur in a cooperative manner to be discerned by DSC. Analysis of a DSC thermogram enables the determination of two important parameters: transition peak temperature peak (Tp) (sometimes termed maximum temperature (Tmax), or denaturation temperature (Td)), and enthalpy of denaturation (ΔH). The denaturation temperature is a measure of the thermal stability of a protein, and is influenced by the protein concentration, as well by the biochemical environment (especially pH). While pH can affect the activation energy (or activation enthalpy) and the frequency factor (or activation entropy) of the denaturation kinetics, many other chemical and mechanical modifiers affect only the frequency factor. Often, modifications affecting the activation entropy appear to be linked to the hydration of the protein. The ΔH value, calculated from the area under the transition peak, represents the heat uptake that induces the unfolding transition and is independent of any denaturation model assumption. Assuming a two-state transition model for protein denaturation, the fractional heat uptake is correlated with the content of ordered secondary structure of a protein. The ΔH value is actually a net value from a combination of endothermic reactions, such as the disruption of hydrogen bonds (determined as 1.7 kcal per mole of hydrogen bond), and exothermic processes, including protein aggregation and the breakup of hydrophobic interactions. Thermodynamically speaking, denaturation is the condition when sufficient energy is transferred to a native protein such that an alteration in its molecular conformation can take place. At low temperatures the heat capacity of the protein increases monotonically with temperature in a manner typical of organic solids. As the protein begins to unfold at higher temperatures the DSC trace becomes more positive, showing
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