Structural cooperative transitions in thermosensitive nonionic hydrogels of poly(N-vinylcaprolactam) (PVCa) were studied using high-sensitivity differential scanning calorimetry (HS-DSC). The thermoshrinking of the PVCa gel is endothermic, much like first-order phase transitions. According to the HS-DSC data, the gel system undergoes two successive cooperative transitions between 25 and 50 °C. The low-temperature transition (31.5 °C) is proposed to be associated with the microsegregation resulting in formation of hydrophobic domains (micromicelles) in the gel whereas the high-temperature transition (37.6 °C) is due to the gel volume collapse. The transition temperatures decrease in the presence of NaCl and increase with increasing sodium dodecyl sulfate (SDS) concentration. Dependences of the transition enthalpies and entropies on NaCl and SDS concentrations display a more complex character. The influence of sodium chloride and SDS on the transition temperatures is in agreement with the general expectations taking into account their effect on hydrophobic interactions and the ability to form micromicelles.

To gain insight into the thermodynamics of protein denaturation, the complex heat capacity, C p * () C p ′ -iC p ′′) of lysozyme-water system has been measured at pH 2.5 in the 293-368 K range by using temperaturemodulated scanning calorimetry (TMSC), a technique in which the thermally reversible enthalpy changes are measured separately and simultaneously with the thermally irreversible enthalpy changes. The plot of C p ′ against the temperature T shows a broad peak, which is similar to that observed in C p,DSC , measured here and elsewhere by differential scanning calorimetry (DSC), a technique which gives the sum of both the reversible and irreversible contributions in the apparent heat capacity value. This peak in C p,DSC has been generally attributed to endothermic heat absorption on reversible and irreversible unfolding processes and irreversible thermal denaturation. It is shown that the observed C p ′ peak results from heat absorption when the equilibrium constant between the native lysozyme state and a conformationally different intermediate state increases with T. The plot of C p ′ versus T is subdivided into four regions, corresponding to the dominance of a certain process. Thermal denaturation of lysozyme was found to occur according to a scheme, native state T unfolded (intermediate) state f denatured state. This conclusion is consistent with the general view that the first step of denaturation of small one-domain globular protein like lysozyme is a reversible conformational (unfolding) transition, and the second step is irreversible denaturation. It is shown that when kept isothermally at T > 341 K, i.e., within the transition temperature range, C p ′ of lysozyme decreases. This decrease is exponential in time and corresponds to a rate constant, which varies according to the Arrhenius-type equation, with a preexponential factor of 5 × 10 20 s -1 and energy of 167 kJ/mol. The overall kinetics of the denaturation reaction is of the first order.

High-sensitivity differential scanning calorimetry was applied for the first time to study the micellization of bovine β-casein in aqueous solutions, complemented by analytical ultracentrifugation data. The micellization was described as a single endothermic heat capacity peak located in the temperature range of 0−50 °C. The transition originated at temperatures very close to 0 °C. The position of the heat capacity peak was strongly dependent on the protein concentration and the solvent composition. Thermodynamic parameters of the micellization, the transition temperature and enthalpy, were determined over a wide range of protein concentration. The observed thermal behavior of β-casein was well fitted by the thermodynamic shell model of micellization proposed by Kegeles (J. Phys. Chem. 1979, 83, 1728). The effects of different cosolutes (inorganic salts, urea, Tris, and ethanol) on the thermodynamic parameters of micellization of β-casein underline the hydrophobic character of the transition.