Protein Denaturation, Zero Entropy Temperature, and the Structure of Water around Hydrophobic and Amphiphilic Solutes

Comment on "Aggregation Interface and Rigid Spots Sustain the Stable Framework of a Thermophilic N-Demethylase"

The thermal properties of proteins are very important in industrial, agricultural, and food chemistry. A recent article (Li, B., et al. J. Agric. Food Chem. 2023, 71, 5614-5629) examines the thermal denaturation of enzymes TrSOX and BSOX by measuring the enthalpy change and melting temperature in the denaturation. In this work, we report the numerical values of entropy in the denaturation of proteins and show that both proteins TrSOX and BSOX exhibit enthalpy-entropy compensation in thermal denaturation, which results in a limited variation of melting temperature in both proteins. Our analysis may serve to improve our understanding of thermal properties in proteins in food chemistry.

Enthalpy-entropy compensation in the denaturation of proteins of bovine masseter and cutaneous trunci

Thermal properties of muscles are of great interest in meat science. A recent article (Vaskoska et al., 2021) examines the thermal denaturation of proteins in bovine muscles, masseter and cutaneous trunci, by measuring the enthalpy and melting temperature of the denaturation. In the present article, we report the numerical values of entropy in the denaturation of proteins. Furthermore, we describe a characteristic phenomenon, enthalpy-entropy compensation in the denaturation of bovine proteins. To our knowledge, this is the first observation of the compensation in situ condition. The analysis shown in this paper may develop a new approach in meat science by introducing a new parameter, entropy, that is rarely reported in relation to differential scanning calorimetry results in this field of research. Therefore, it may enhance our understanding of the thermal properties of meat from a physical chemistry perspective.

Bridging Gaussian Density Fluctuations from Microscopic to Macroscopic Volumes: Applications to Non-Polar Solute Hydration Thermodynamics

The hydration of hydrophobic solutes is intimately related to the spontaneous formation of cavities in water through ambient density fluctuations. Information theory-based modeling and simulations have shown that water density fluctuations in small volumes are approximately Gaussian. For limiting cases of microscopic and macroscopic volumes, water density fluctuations are known exactly and are rigorously related to the density and isothermal compressibility of water. Here, we develop a theory—interpolated gaussian fluctuation theory (IGFT)—that builds an analytical bridge to describe water density fluctuations from microscopic to molecular scales. This theory requires no detailed information about the water structure beyond the effective size of a water molecule and quantities that are readily obtained from water’s equation-of-state—namely, the density and compressibility. Using simulations, we show that IGFT provides a good description of density fluctuations near the mean, that is, it characterizes the variance of occupancy fluctuations over all solute sizes. Moreover, when combined with the information theory, IGFT reproduces the well-known signatures of hydrophobic hydration, such as entropy convergence and solubility minima, for atomic-scale solutes smaller than the crossover length scale beyond which the Gaussian assumption breaks down. We further show that near hydrophobic and hydrophilic self-assembled monolayer surfaces in contact with water, the normalized solvent density fluctuations within observation volumes depend similarly on size as observed in the bulk, suggesting the feasibility of a modified version of IGFT for interfacial systems. Our work highlights the utility of a density fluctuation-based approach toward understanding and quantifying the solvation of non-polar solutes in water and the forces that drive them toward surfaces with different hydrophobicities.

Temperature, Pressure, and Length-Scale Dependence of Solvation in Water-like Solvents. II. Large Solvophovic Solutes

We use molecular simulation to determine solvation free energies, isochoric solvation energies and entropies, isobaric solvation enthalpies and entropies, partial molecular volumes, and isothermal density derivatives of the solvation free energy as a function of temperature and pressure for hard-sphere solutes with diameters ranging from 4 to 36 Å in TIP4P/2005 and Jagla water-like solvents exhibiting unusual thermodynamics. An important piece of our discussion focuses on the nanometer-sized solutes, for which simulation results are found to be accounted for by the most basic classical thermodynamic treatment contemplating bulk and interfacial contributions to the solvation free energy. Thus, since water's liquid-vapor surface tension is only special inasmuch as it takes unusually large values, solvent's water-like unusual thermodynamics manifests through a term proportional to the pressure in the solvation free energy. As a result, such solvent's unusual thermodynamics is found to be relevant to the temperature and pressure dependence of the isochoric solvation energy and entropy as well as to the isothermal density derivative of the solvation free energy. This sharply contrasts with the findings of the first part of this series indicating that the solvation free energy of small hard spheres responds to temperature and pressure changes as solvent's density does, with such a contrasting picture embodying a "pressure-density dichotomy." As for the length-scale dependence, we find the zero nominal pressure and the solvent's temperature of the maximum density as singular conditions for cavity surface-area size scaling of large solutes to occur for all solvation quantities. We finally argue that the overall study undertaken in this series suggests that water's unusual thermodynamics may be relevant to the thermodynamic stability of clusters of solvophobic units in the temperature-pressure plane. Some comments on the role of solute-solvent attractive interactions are also depicted.