Recent advances in the calculation and interpretation of the activation energy for a dynamical process are described. Specifically, new approaches that apply the fluctuation theory of statistical mechanics to dynamics enable the direct determination of the activation energy for an arbitrary dynamical time scale from simulations at a single temperature. This opens up significant new possibilities for understanding activated processes in cases where a traditional Arrhenius analysis is not possible. The methods also enable a rigorous decomposition of the activation energy into contributions associated with the different interactions and motions present in the system. These components can be understood in the context of Tolman’s interpretation of the activation energy. Specifically, they provide insight into how energy can be most effectively deposited to accelerate the dynamics of interest, promising important new mechanistic information for a broad range of chemical processes. The general approach can be extended beyond activation energies to the examination of non-Arrhenius behavior as well as the changes in dynamical time scales with respect to other thermodynamic variables such as pressure.

Mechanisms of corona formation around nanomaterials remain enigmatic. Here, we provide evidence for spontaneous lipid corona formation that engenders new particle properties without the need for active mixing upon attachment to stationary and suspended lipid bilayer membranes. The mechanism of lipid corona formation can be used to improve control over nano-bio interactions and to help understand why some nanomaterial-ligand combinations are detrimental to organisms but others are not. SUMMARYAlthough mixing nanoparticles with certain biological molecules can result in coronas that afford some control over how engineered nanomaterials interact with living systems, corona formation mechanisms remain enigmatic. Here, we report results from experiments and computer simulations that provide concrete lines of evidence for spontaneous lipid corona formation without active mixing upon attachment to stationary and suspended lipid bilayer membranes. Experiments show that polycation-wrapped particles disrupt the tails of zwitterionic lipids, increase bilayer fluidity, and leave the membrane with reduced z potentials. Computer simulations suggest that the contact ion pairing between the lipid head groups and the polycations' ammonium groups leads to the formation of stable, albeit fragmented, lipid bilayer coronas. The mechanistic insight regarding lipid corona formation can be used to improve control over nanobio interactions and to help understand why some nanomaterial-ligand combinations are detrimental to organisms but others are not.

General approaches for directly calculating the temperature dependence of dynamical quantities from simulations at a single temperature are presented. The method is demonstrated for self-diffusion and OH reorientation in liquid water. For quantities which possess an activation energy, e.g., the diffusion coefficient and the reorientation time, the results from the direct calculation are in excellent agreement with those obtained from an Arrhenius plot. However, additional information is obtained, including the decomposition of the contributions to the activation energy. These results are discussed along with prospects for additional applications of the direct approach.

We compare atomistic and two popular coarse-grained (POL- and BMW-MARTINI) models by studying the interaction between a cationic gold nanoparticle functionalized with primary alkane amines and a lipid bilayer that consists of either zwitterionic lipids or a mixture of zwitterionic and anionic lipids. In the atomistic simulations, the nanoparticle does not exhibit any notable affinity to the zwitterionic bilayer but readily binds to the 9:1 zwitterionic:anionic bilayer, and nanoparticle adsorption leads to local segregation of anionic lipids and slowing down of their diffusion. At the coarse-grained level, the POL-MARTINI model does not lead to nanoparticle–membrane binding for either bilayer system, while the BMW-MARTINI model leads to nanoparticle binding to both bilayers; with the BMW-MARTINI model, nanoparticle binding leads to much less demixing of zwitterionic and anionic lipids and moderately higher rates of lipid diffusion. Analysis of nanoparticle properties in solution reveals notable discrepancies in the interfacial charge and water distributions at the coarse-grained level that are likely relevant to their limitations in describing binding interactions with other (bio)­molecules.

Approaches for directly calculating the activation energy for a chemical reaction from a simulation at a single temperature are explored with applications to both classical and quantum systems. The activation energy is obtained from a time correlation function that can be evaluated from the same molecular dynamics trajectories or quantum dynamics used to evaluate the rate constant itself and thus requires essentially no extra computational work. Published by AIP Publishing. [http://dx