Hydrophobic collapse and cold denaturation in the Jagla model of water

Abstract

The Jagla model is a coarse-grained model of water which describes interactions betweengroups of water molecules by a spherically symmetric potential characterized by a hardcore, a linear repulsive ramp and a long-range attractive ramp. The Jagla modelqualitatively reproduces the thermodynamics and dynamics of liquid water includingdensity and diffusion anomalies as well as certain chemical properties such the increase ofsolubility of small hydrophobic particles upon cooling. We examine, via molecular dynamicssimulation, the behavior of the bead-on-a-string polymers of various lengths in the Jaglamodel. We find that such polymers exhibit swelling upon cooling similar to colddenaturation of proteins in water. We show that while for short polymers the swelling isgradual, longer polymers exhibit a first-order-like phase transition between a globularphase at high temperatures to a random coil state at cold temperatures. Thistransition is associated with the formation of a liquid–polymer phase boundarysurrounding the globule and complete dewetting of the central parts of the globule athigh temperatures. We study thermodynamics of this transition and find thatthe entropy, volume, and potential energy of the solvent–random coil systemis lower than those of the globule–solvent system. Accordingly the slope of thecoil–globule transition line on a PT plane has positive slope. We present simplethermodynamic considerations similar to classical nucleation theory, which relatethe temperature of the cold swelling transition to polymer length and relate thedewetting of the globule to its diameter and to the Egelstaff–Widom length scale.