Abstract
Water + elastin-like polypeptides (ELPs) exhibit a transition temperature below which the chains transform from collapsed to expanded states, reminiscent of the cold denaturation of proteins. This conformational change coincides with liquid-liquid phase separation. A statistical-thermodynamics theory is used to model the fluid-phase behavior of ELPs in aqueous solution and to extrapolate the behavior at ambient conditions over a range of pressures. At low pressures, closed-loop liquid-liquid equilibrium phase behavior is found, which is consistent with that of other hydrogen-bonding solvent + polymer mixtures. At pressures evocative of deep-sea conditions, liquid-liquid immiscibility bounded by two lower critical solution temperatures (LCSTs) is predicted. As pressure is increased further, the system exhibits two separate regions of closed-loop of liquid-liquid equilibrium (LLE). The observation of bimodal LCSTs and two re-entrant LLE regions herald a new type of binary global phase diagram: Type XII. At high-ELP concentrations the predicted phase diagram resembles a protein pressure denaturation diagram; possible "molten-globule"-like states are observed at low concentration.
Highlights
The persistence of life in high-pressure environments raises important questions about the behavior of proteins, the key functional molecules of organisms, at these extreme conditions
The liquid–liquid equilibrium (LLE) region associated with LCST2 is at first relatively small, it widens in composition with increasing pressure; this region is clearly seen at 70 and 150 MPa in Fig. 2(c) and (d), respectively
As the pressure is increased, the extent of the LLE region bounded by the LCST1 continues to decrease in both composition and temperature, whereas the T–wELP region associated with LCST2 expands
Summary
The persistence of life in high-pressure environments raises important questions about the behavior of proteins, the key functional molecules of organisms, at these extreme conditions. It is known that high pressures played a central role in the formation of life on Earth by stabilizing amino acids at high temperatures and enabling facile organic reaction pathways.[1] In the open sea, fish have been caught at depths exceeding 7000 m (70 MPa),[2] while invertebrate life has been found in the Mariana trench, where pressures exceed 100 MPa.[3] In an experimental setting, bacteria have been shown to be resilient to pressures of over 1 GPa.[4] Proteins can withstand pressures of hundreds of MPa before denaturing.[5,6]. We explore the high-pressure fluid-phase behavior of a model protein, an elastin-like polypeptide (ELP), which has a Department of Chemical Engineering, Centre for Process Systems Engineering and Institute for Molecular Science and Engineering, South Kensington Campus, Imperial College London, London SW7 2AZ, UK
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