Thermodynamical implications of a protein model with water interactions

Low-temperature-induced swelling of a hydrophobic polymer: A lattice approach

The authors investigate equilibrium properties of a simple model of hydrophobic polymer in aqueous solution by means of dynamic Monte Carlo simulations. The solvent is described by a simplified two-dimensional model, defined on a triangular lattice, which has been previously shown to account for most thermodynamic anomalies of pure water and of hydrophobic solvation for monomeric solutes. The polymer is modeled as a self-avoiding walk on the same lattice. In this framework, the degrees of freedom of water are taken into account explicitly, and in principle there is no need to introduce effective self-contact interactions for the polymer in order to mimic the hydrophobic effect. In certain conditions, the authors observe low-temperature-induced swelling, i.e., expansion of the polymer globule upon decreasing temperature. The authors discuss the relationship between this phenomenon and the anomalous properties of the solvent.

Localized heat urticaria in a patient is associated with a wealing response to heated autologous serum

We report a case of localized heat urticaria in a 71-year-old woman who developed weals and loss of consciousness after taking a bath. Exposing her skin to heat at 40 degrees C or immersing her hands in water at 40 degrees C produced urticarial lesions and increased her plasma histamine level. Desensitization with hot water improved her symptoms and normalized her plasma histamine level after heat challenge. An intracutaneous injection of her serum produced no reaction, while an injection of her serum that had been heated at 40 degrees C for 15 min induced a weal flare response. Further examination revealed that the weal-inducing activity of her heated serum remained for at least for 6 h and that treatment of her serum at 60 degrees C for 2 h did not abrogate its weal-inducing activity. These findings indicate that certain materials in her serum that are activated by heat are responsible for the development of her anaphylactic and urticarial reactions and that these reactions may be mediated by histamine.

Apolar and polar solvation thermodynamics related to the protein unfolding process

Thermodynamics related to hydrated water upon protein unfolding is studied over a broad temperature range (5-125 degrees C). The hydration effect arising from the apolar interior is modeled as an increased number of hydrogen bonds between water molecules compared with bulk water. The corresponding contribution from the polar interior is modeled as a two-step process. First, the polar interior breaks hydrogen bonds in bulk water upon unfolding. Second, due to strong bonds between the polar surface and the nearest water molecules, we assume quantization using a simplified two-state picture. The heat capacity change upon hydration is compared with model compound data evaluated previously for 20 different proteins. We obtain good correspondence with the data for both the apolar and the polar interior. We note that the effective coupling constants for both models have small variations among the proteins we have investigated.

Heat capacity of protein folding

We construct a Hamiltonian for a single domain protein where the contact enthalpy and the chain entropy decrease linearly with the number of native contacts. The hydration effect upon protein unfolding is included by modeling water as ideal dipoles that are ordered around the unfolded surfaces, where the influence of these surfaces, covered with an "ice-like" shell of water, is represented by an effective field that directs the water dipoles. An intermolecular pair interaction between water molecules is also introduced. The heat capacity of the model exhibits, the common feature of small globular proteins, two peaks corresponding to cold and warm unfolding, respectively. By introducing ad hoc vibrational modes, we obtain quantitatively good accordance with experiments on myoglobin.