We study the dynamics and conformation of polymers composed by active monomers. By means of Brownian dynamics simulations we show that when the direction of the self-propulsion of each monomer is aligned with the backbone, the polymer undergoes a coil-to-globule-like transition, highlighted by a marked change of the scaling exponent of the gyration radius. Concurrently, the diffusion coefficient of the center of mass of the polymer becomes essentially independent of the polymer size for sufficiently long polymers or large magnitudes of the self-propulsion. These effects are reduced when the self-propulsion of the monomers is not bound to be tangent to the backbone of the polymer. Our results, rationalized by a minimal stochastic model, open new routes for activity-controlled polymer and, possibly, for a new generation of polymer-based drug carriers.

The mechanisms of cold and pressure denaturation of proteins are matter of debate and are commonly understood as due to water-mediated interactions. Here, we study several cases of proteins, with or without a unique native state, with or without hydrophilic residues, by means of a coarse-grain protein model in explicit solvent. We show, using Monte Carlo simulations, that taking into account how water at the protein interface changes its hydrogen bond properties and its density fluctuations is enough to predict protein stability regions with elliptic shapes in the temperature-pressure plane, consistent with previous theories. Our results clearly identify the different mechanisms with which water participates to denaturation and open the perspective to develop advanced computational design tools for protein engineering. DOI: 10.1103/PhysRevLett.115.108101 PACS numbers: 87.15.Cc, 87.15.A-, 87.15.kr Water plays an essential role in driving the folding of a protein and in stabilizing the tertiary protein structure in its native state [1,2]. Proteins can denaturate-unfolding their structure and losing their activity-as a consequence of changes in the environmental conditions. Experimental data show that for many proteins the native folded state is stable in a limited range of temperatures T and pressures P [3][4][5][6][7][8] and that partial folding is T modulated also in "intrinsically disordered proteins" [9]. By hypothesizing that proteins have only two different states, folded (f) and unfolded (u), and that the f⟷u process is reversible at any moment, Hawley proposed a theory [10] that predicts a close stability region (SR) with an elliptic shape in the T-P plane, consistent with the experimental data [11].Cold and P denaturation of proteins have been related to the equilibrium properties of the hydration water [12][13][14][15][16][17][18][19][20][21][22][23]. However, the interpretations of the mechanism are still controversial [8,[24][25][26][27][28][29][30][31][32][33][34][35][36][37]. High-T denaturation is easily understood in terms of thermal fluctuations that disrupt the compact protein conformation: the open protein structure increases the entropy S minimizing the global Gibbs free energy G ≡ H − TS, where H is the total enthalpy. High-P unfolding can be explained by the loss of internal cavities in the folded states of proteins [36], while denaturation at negative P has been experimentally observed [38] and simulated [38,39] recently. Cold and P unfolding can be thermodynamically justified assuming an enthalpic gain of the solvent upon the denaturation process, without specifying the origin of this gain from molecular interactions [40]. Here, we propose a molecular-interactions model for proteins solvated by explicit water, based on the "many-body" water model [32,[41][42][43][44][45]. We demonstrate how the cold-and P-denaturation mechanisms can emerge as a competition between different free energy contributions coming from water, one from hydration water and another from bulk water. Moreover, we sh...

Proteins that are functional at ambient conditions do not necessarily work at extreme conditions of temperature T and pressure P. Furthermore, there are limits of T and P above which no protein has a stable functional state. Here, we show that these limits and the selection mechanisms for working proteins depend on how the properties of the surrounding water change with T and P. We find that proteins selected at high T are superstable and are characterized by a nonextreme segregation of a hydrophilic surface and a hydrophobic core. Surprisingly, a larger segregation reduces the stability range in T and P. Our computer simulations, based on a new protein design protocol, explain the hydropathy profile of proteins as a consequence of a selection process influenced by water. Our results, potentially useful for engineering proteins and drugs working far from ambient conditions, offer an alternative rationale to the evolutionary action exerted by the environment in extreme conditions.

The properties of water can have a strong dependence on the confinement. Here, we consider a water monolayer nanoconfined between hydrophobic parallel walls under conditions that prevent its crystallization. We investigate, by simulations of a many-body coarse-grained water model, how the properties of the liquid are affected by the confinement. We show, by studying the response functions and the correlation length and by performing finite-size scaling of the appropriate order parameter, that at low temperature the monolayer undergoes a liquid-liquid phase transition ending in a critical point in the universality class of the two-dimensional (2D) Ising model. Surprisingly, by reducing the linear size L of the walls, keeping the walls separation h constant, we find a 2D-3D crossover for the universality class of the liquid-liquid critical point for , i.e. for a monolayer thickness that is small compared to its extension. This result is drastically different from what is reported for simple liquids, where the crossover occurs for , and is consistent with experimental results and atomistic simulations. We shed light on these findings showing that they are a consequence of the strong cooperativity and the low coordination number of the hydrogen bond network that characterizes water.