(231b) Thermodynamics of Protein Folding Near the Low-Temperature Liquid-Liquid Transition in Water: A Case Study with the Trp-Cage Miniprotein

It has now been well established that several models of water exhibit a low-temperature liquid-liquid phase transition (LLPT). As water is critical to biology, the existence of the LLPT begs a fundamental question: how does the LLPT affect the thermodynamics of important biological functions like protein folding? Because experiments are hindered by the homogeneous nucleation of water, we explore this question with molecular dynamics simulation. To properly sample the thermodynamics of folding for our system at low temperatures, we employ advanced sampling in the form of Parallel Tempering in the Well-Tempered Ensemble (PT-WTE). Our results show not only the expected cold denaturation at low temperatures, but a more surprising “re-folding” of the the Trp-cage miniprotein at super-cooled temperatures below the LLPT. We present a simple thermodynamic function that utilizes a variable heat capacity of unfolding to explain this observation. These finding reveal how the fundamental nature of water’s structure impacts the thermodynamic stability of complex biological molecules.