Linear and Nonlinear Dielectric Response of Intrinsically Disordered Proteins.

Linear and nonlinear dielectric responses of solutions of intrinsically disordered proteins (IDPs) were analyzed by combining molecular dynamics simulations with formal theories. A large increment of the linear dielectric function over that of the solvent is found and related to large dipole moments of IDPs. The nonlinear dielectric effect (NDE) of the IDP far exceeds that of the bulk electrolyte, offering a route to interrogate protein conformational and rotational statistics and dynamics. Conformational flexibility of the IDP makes the dipole moment statistics consistent with the gamma/log-normal distributions and contributes to the NDE through the dipole moment's non-Gaussian parameter. The intrinsic non-Gaussian parameter of the dipole moment combines with the protein osmotic compressibility in the nonlinear dielectric susceptibility when dipolar correlations are screened by the electrolyte. The NDE is dominated by dipolar correlations when electrolyte screening is reduced.

Model-Dependent Solvation of the K-18 Domain of the Intrinsically Disordered Protein Tau.

A known imbalance between intra-protein and protein-water interactions in many empirical force fields results in collapsed conformational ensembles of intrinsically disordered proteins in explicit solvent simulations that disagree with experiments. Multiple strategies have been introduced in the literature to modify protein-water interactions, which improve agreement between experiments and simulations. In this work, we combine simulations with standard and modified force fields with a spatially resolved analysis of solvation free energy contributions and compare the consequences of each strategy. We find that enhanced Lennard-Jones (LJ) interactions between protein atoms and water oxygens primarily improve the solvation of nonpolar functional groups of the protein. In contrast, modified electrostatics in the water model or strengthened LJ interactions between the protein and water hydrogens mainly affect the hydration of polar functional groups. Modified electrostatics further impact the average orientation of water molecules in the hydration shell. As a result, protein-water interactions with the first hydration layers are strengthened, while interactions with water molecules in higher hydration shells are weakened. Hence, distinct strategies to balance intra-protein and protein-water interactions in simulations have qualitatively different effects on protein solvation. These differences are not necessarily captured by comparisons to experiments that report on global parameters describing protein conformational ensembles, e.g., the radius of gyration, but will influence the tendency of a protein to form aggregates or phase-separated droplets.

Electric-field induced entropic effects in liquid water.

Externally applied electric fields in liquid water can induce a plethora of effects with wide implications in electrochemistry and hydrogen-based technologies. Although some effort has been made to elucidate the thermodynamics associated with the application of electric fields in aqueous systems, to the best of our knowledge, field-induced effects on the total and local entropy of bulk water have never been presented so far. Here, we report on classical TIP4P/2005 and ab initio molecular dynamics simulations measuring entropic contributions carried by diverse field intensities in liquid water at room temperature. We find that strong fields are capable of aligning large fractions of molecular dipoles. Nevertheless, the order-maker action of the field leads to quite modest entropy reductions in classical simulations. Albeit more significant variations are recorded during first-principles simulations, the associated entropy modifications are small compared to the entropy change involved in the freezing phenomenon, even at intense fields slightly beneath the molecular dissociation threshold. This finding further corroborates the idea that electrofreezing (i.e., the electric-field-induced crystallization) cannot take place in bulk water at room temperature. In addition, here, we propose a molecular-dynamics-based analysis (3D-2PT) that spatially resolves the local entropy and the number density of bulk water under an electric field, which enables us to map their field-induced changes in the environment of reference H2O molecules. By returning detailed spatial maps of the local order, the proposed approach is capable of establishing a link between entropic and structural modifications with atomistic resolution.

Two Keggin-Based Isostructural POMOF Hybrids: Synthesis, Crystal Structure, and Catalytic Properties.

In this work we synthesized two novel isostructural twin hybrids Comp1: [H(C10H10N2)Cu2][PMo12O40] & Comp2: [H(C10H10N2)Cu2][PW12O40], based on the Keggin ions (PMo12O40-3 & PW12O40-3), Cu(I) cation, and 4,4'-bipyridine, by in situ hydrothermal reduction of Cu, facilitated through extensive standardizations of synthetic pH conditions. Both compounds crystallized in monoclinic P21/ c space group with similar lattice parameters and crystal structures. The structural similarity prompted us to explore comparative catalytic properties of the hybrids, to understand the relative role of the POM species in the activity. While characterization techniques like powder X-ray diffraction (XRD), single-crystal XRD, IR, adsorption studies, etc. confirmed the identical structural hierarchy in the twin polyoxometalate-based metal organic frameworks (POMOFs), critical analyses through X-ray photoelectron spectroscopy, X-ray absorption near-edge structure spectroscopy, and magnetic property studies elucidated the electronic and local structural properties of the two. The hybrids were highly active for heterogeneous catalysis of small-molecule oxidation, with Comp 2 showing better activity than Comp1, particularly for oxidation of ethylbenzene and cyclooctene. Comp2 also outperformed Comp1 in photocatalytic degradation of methylene blue, with higher conversion efficiency of 83% and one order higher apparent rate constant of 0.0139 min-1, which is comparable to that of the well-known photocatalyst, P25. Electrochemical pseudocapacitance studies revealed that these POMOFs are having the potential to act as good charge storage and conducting devices if their electrochemical stability can be improved.