Pressure and temperature dependence of hydrophobic hydration: Volumetric, compressibility, and thermodynamic signatures

Abstract

The combined effect of pressure and temperature on hydrophobic hydration of a nonpolar methanelike solute is investigated by extensive simulations in the TIP4P model of water. Using test-particle insertion techniques, free energies of hydration under a range of pressures from 1 to 3000 atm are computed at eight temperatures ranging from 278.15 to 368.15 K. Corresponding enthalpy, entropy, and heat capacity accompanying the hydration process are estimated from the temperature dependence of the free energies. Partial molar and excess volumes calculated using pressure derivatives of the simulated free energies are consistent with those determined by direct volume simulations; but direct volume determination offers more reliable estimates for compressibility. At 298.15 K, partial molar and excess isothermal compressibilities of methane are negative at 1 atm. Partial molar and excess adiabatic (isentropic) compressibilities are estimated to be also negative under the same conditions. But partial molar and excess isothermal compressibilities are positive at high pressures, with a crossover from negative to positive compressibility at approximately 100-1000 atm. This trend is consistent with experiments on aliphatic amino acids and pressure-unfolded states of proteins. For the range of pressures simulated, hydration heat capacity exhibits little pressure dependence, also in apparent agreement with experiment. When pressure is raised at constant room temperature, hydration free energy increases while its entropic component remains essentially constant. Thus, the increasing unfavorability of hydration under raised pressure is seen as largely an enthalpic effect. Ramifications of the findings of the authors for biopolymer conformational transitions are discussed.

Pairwise association of neopentane as a function of hydrostatic pressure

“…They are negative, and increase slightly in magnitude with pressure, E a (in kJ mol −1 ) = −38.5 at 1 atm, −41.5 at 2000 atm, and −43.7 at 4000 atm, as a consequence of the increase, but do not overwhelm the G c increase. Thus, the obtained G • numbers are positive and increase markedly with hydrostatic pressure, in line with the computer simulation results obtained for methane [36]. Specifically, G • (in kJ mol −1 ) = 10.5 at 1 atm, 29.2 at 2000 atm and 47.3 at 4000 atm.…”

Origins of Pressure-Induced Protein Transitions

“…This result is in qualitative agreement with molecular dynamics simulations of the pressure dependence of the partial molar volume of methane in water. 48 Inspection of Fig. 2 reveals that while, at atmospheric pressure, the apparent volume occupied by a methylene group in a micelle is higher than that in a native protein, the difference rapidly diminishes with increasing pressure due to the higher intrinsic compressibility of a micelle.…”

Integral Equation Study of the Hydrophobic Interaction between Graphene Plates

“…A great deal of recent work has been applied to understanding the nature of hydrophobic effects at a fundamental level [11][12][13][14][15][16] . The nature of the shorter ranged attractions, less than a few molecular diameters, of hydrophobic species has been described with multiple mechanisms over the years [17][18][19][20][21][22][23] .…”