岡本 隆一

Polyethylene glycol (PEG) is a widely used precipitant to concentrate proteins. The effect of PEG is generally understood to be an entropic attraction between proteins due to the depletion effect of PEG around proteins. However, measurements by Bloustine et al. [Phys. Rev. Lett. 96, 087803 (2006)] of the liquid–liquid phase separation (LLPS) temperature have shown that a lysozyme solution is stabilized and destabilized by the addition of low and high molecular-weight PEG, respectively. They also presented a theoretical model of the LLPS temperature as a virial expansion of the free energy and concluded that, in addition to the depletion effect, the attractive interaction between protein and PEG is necessary to explain the experiments. In the present study, theoretical calculations based on liquid-state density functional theory utilizing coarse-grained models are conducted to demonstrate that the protein–PEG effective attraction is responsible for the suppression and promotion of LLPS upon the addition of low- and high-weight PEG, respectively. In contrast, if the interactions between the protein and the PEG are solely due to the excluded volume effect, PEG of any molecular weight destabilizes the solution. These results suggest the necessity to reconsider the conventional understanding of the effects of polymer addition, which have been historically attributed to solely the depletion force.

The three important contributions to the solvation free energy of alcohols in water are quantified as functions of temperature and pressure based the perturbation combining method and other step-wise methods.

To gain quantitative insight into how the overall strength of the hydrophobic interaction varies with the molecular size, we calculate osmotic second virial coefficients B for hydrophobic spherical molecules of different diameters σ in water based on molecular simulation with corrections to the finite-size and finite-concentration effects. It is shown that B (<0) changes by two orders of magnitude greater as σ increases twofold and its solute-size dependence is best fit by a power law B ∝ σ ^α with the exponent α ≃ 6, which contrasts with the cubic power law that the second virial coefficients of gases obey. It is also found that values of B for the solutes in a nonpolar solvent are positive but they obey the same power law as in water. A thermodynamic identity for B derived earlier [K. Koga, V. Holten, and B. Widom, J. Phys. Chem. B 119, 13391 (2015)] indicates that if B is asymptotically proportional to a power of σ, the exponent α must be equal to or greater than 6.

We investigate bridging and aggregation of two colloidal particles in a near-critical binary mixture when the fluid far from the particles is outside the coexistence (CX) curve and is rich in the component disfavored by the colloid surfaces. In such situations, the adsorption-induced interaction is enhanced, leading to bridging and aggregation of the particles. We realize bridging firstly by changing the temperature with a fixed interparticle separation and secondly by letting the two particles aggregate. The interparticle attractive force dramatically increases upon bridging. The dynamics is governed by hydrodynamic flow around the colloid surfaces. In aggregation, the adsorption layers move with the particles and squeezing occurs at narrow separation. These results suggest relevance of bridging in the reversible colloid aggregation observed so far. We use the local functional theory [J. Chem. Phys., 2012, 136, 114704] to take into account the renormalization effect and the simulation method [Phys. Rev. Lett., 2000, 85, 1338] to calculate the hydrodynamic flow around the colloidal particles.

We study a simple model for the interstitialcy mechanism—one of the main mechanisms of the self-diffusion in crystalline solids. The diffusion coefficient depends on the interstitialcy concentration as well as the hopping rates of particles. We obtain an approximate expressions of the diffusion coefficient for various kinds of lattices by noting that the dynamics of an interstitialcy is identical with that of a particle in the vacancy assisted mechanism. These expressions are found to be consistent with our Monte Carlo simulation results for all the lattices examined as far as the interstitialcy-concentration is not too high.

Lower vacancy-density in a crystalline solid slows down the tracer diffusion via the vacancy-assisted mechanism, which can be modeled by means of particles hopping to their respective nearest-neighbor lattice-sites stochastically with double occupancy prohibited. The explicit expressions of the diffusion coefficient were previously obtained for various lattices in terms of Nakazato and Kitahara's method [Prog. Theor. Phys. 64 (1980) 2261]. This method yields a set of linear simultaneous algebraic equations as many as the number of lattice sites, which is reduced to a simple equation with respect to the diffusion coefficient in the final step of the method. We here give a systematic way of the reduction in terms of the group theory.