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Nanoplastics, small plastic particles smaller than microplastics, have been suggested to have a wide-range of unique interactions when they encounter lipid membranes. Recent studies have demonstrated that the smaller size of nanoplastic particles may allow them to penetrate and dissolve in lipid membranes. Following this penetration, however, there is not yet a clear picture of how such particles impact the local lipid environment. A recent study by the present authors found that when lipid vesicles that included laurdan, a fluorescent dye molecule typically thought to report on the membrane phase, were exposed to polystyrene nanoparticles, they exhibited a concentration-dependent blue shift consistent with a fluid-to-gel phase transition. However, coarse-grained simulations suggested that no such transition was taking place; instead, the simulations observed that polymer chains from the polystyrene nanoparticles penetrated into the liposome membrane. In the present work, we use all-atom molecular dynamics simulations to demonstrate that the inclusion of polystyrene within a lipid membrane causes significant changes to the local hydration and structure of that membrane while maintaining the membrane phase. Specifically, through the explicit incorporation of laurdan within the present simulations, we demonstrate that the local hydration environment of the dye molecule changes significantly but continuously as membranes are exposed to polystyrene, thus suggesting a possible explanation for the previously reported experimental observation. The present results provide a picture of the complex heterogeneity generated within polymer-containing membranes. 

The computational spectroscopy of water has proven to be a powerful tool for probing the structure and dynamics of chemical systems and for providing atomistic insight into experimental vibrational spectroscopic results. However, such calculations have been limited for biochemical systems due to the lack of empirical vibrational frequency maps for the TIP3P water model, which is used in many popular biomolecular force fields. Here, we develop an empirical map for the TIP3P model and evaluate its efficacy for reproducing the experimental vibrational spectroscopy of water. We observe that the calculated infrared and Raman spectra are blueshifted and narrowed compared to the experimental spectra. Further analysis finds that the blueshift originates from a shifted distribution of frequencies, rather than other dynamical effects, suggesting that the TIP3P model forms a significantly different electrostatic environment than other three-point water models. This is explored further by examining the two-dimensional infrared spectra, which demonstrates that the blueshift is significant for the first two vibrational transitions. Similarly, spectral diffusion timescales, evaluated through both the center line slope and the frequency–frequency time correlation function demonstrate that TIP3P exhibits significantly faster spectral dynamics than other three-point models. Finally, sum-frequency generation spectroscopy calculations suggest that despite these challenges, the TIP3P empirical map can provide phenomenological, qualitative, insight into the behavior of water at the air–water and lipid–water interfaces. As these interfaces are models for hydrophobic and hydrophilic environments observed in biochemical systems, the presently developed empirical map will be useful for future studies of biochemical systems. 

It has long been understood that the structural features of water are determined by hydrogen bonding (H-bonding) and that the exchange of, or "jumps" between, H-bond partners underlies many of the dynamical processes in water. Despite the importance of H-bond exchanges there is, as yet, no direct method for experimentally measuring the timescale of the process or its associated activation energy. Here, we identify and exploit relationships between water's structural and dynamical properties that provide an indirect route for determining the H-bond exchange activation energy from experimental data. Specifically, we show that the enthalpy and entropy determining the radial distribution function in liquid water are linearly correlated with the activation energies for H-bond jumps, OH reorientation, and diffusion. Using temperature-dependent measurements of the radial distribution function from the literature, we demonstrate how these correlations allow us to infer the value of the jump activation energy, Ea,0, from experimental results. This analysis gives Ea,0 = 3.43 kcal/mol, which is in good agreement with that predicted by the TIP4P/2005 water model. We also illustrate other approaches for estimating this activation energy consistent with these estimates.. 

A Maxwell relation for a reaction rate constant (or other dynamical timescale) obtained under constant pressure, p , and temperature, T , is introduced and discussed. Examination of this relationship in the context of fluctuation theory provides insight into the p and T dependence of the timescale and the underlying molecular origins. This Maxwell relation motivates a suggestion for the general form of the timescale as a function of pressure and temperature. This is illustrated by accurately fitting simulation results and existing experimental data on the self-diffusion coefficient and shear viscosity of liquid water. A key advantage of this approach is that each fitting parameter is physically meaningful.