The aqueous proton displays an anomalously large diffusion coefficient that is up to 7 times that of similarly sized cations. There is general consensus that the proton achieves its high diffusion ...through the Grotthuss mechanism, whereby protons hop from one molecule to the next. A main assumption concerning the extraction of the timescale of the Grotthuss mechanism from experimental results has been that, on average, there is an equal probability for the proton to hop to any of its neighboring water molecules. Herein, we present
simulations that show this assumption is not generally valid. Specifically, we observe that there is an increased probability for the proton to revert back to its previous location. These correlations indicate that the interpretation of the experimental results need to be re-examined and suggest that the timescale of the Grotthuss mechanism is significantly shorter than was previously thought.
Bioprotonic devices show promise for medicinal/therapeutic treatments because of their ability to deliver a controlled flow of small molecules and ions to living organisms. These devices rely on ...biocompatible, conductive polymer membranes to facilitate molecular/ionic transport. Herein, ab initio molecular dynamics simulations are used to probe the effects structure and hydration level on water diffusion in chitosan‐based polymer membranes. The diffusion coefficient of water is shown to be highest in membranes with low overall densities and weak non‐covalent interactions. Insight from these results may aid in optimizing the molecular/ionic transport properties of polymer membranes for bioprotonic applications.
The effects of structure and hydration level on water diffusion in chitosan‐based membranes are explored using ab initio molecular dynamics simulations. Membranes with low densities and weak non‐covalent interactions exhibit high water diffusion coefficients. These two features can be used to optimize the transport properties of polymer membranes for applications in devices.
A randomized trial compared effects of a Family Critical Time Intervention (FCTI) to usual care for children in 200 newly homeless families in which mothers had diagnosable mental illness or ...substance problems. Adapted from an evidence-based practice to prevent chronic homelessness for adults with mental illnesses, FCTI combines housing and structured, time-limited case management to connect families leaving shelter with community services. Families were followed at five time points over 24 months. Data on 311 children—99 ages 1.5–5 years, 113 ages 6–10 years, and 99 ages 11–16 years—included mother-, teacher-, and child-reports of mental health, school experiences, and psychosocial well-being. Analyses used hierarchical linear modeling to investigate intervention effects and changes in child functioning over time. Referral to FCTI reduced internalizing and externalizing problems in preschool-aged children and externalizing for adolescents 11–16. The intervention led to declines in self-reported school troubles for children 6–10 and 11–16. Both experimental and control children in all age groups showed reductions in symptoms over time. Although experimental results were scattered, they suggest that FCTI has the potential to improve mental health and school outcomes for children experiencing homelessness.
Interfacial interactions between liquid crystal (LC) and two-dimensional (2D) materials provide a platform to facilitate novel optical and electronic material properties. These interactions are ...uniquely sensitive to the local energy landscape of the atomically thick 2D surface, which can be strongly influenced by defects that are introduced, either by design or as a byproduct of fabrication processes. Herein, we present density functional theory (DFT) calculations of the LC mesogen 4-cyan-4'-pentylbiphenyl (5CB) on graphene in the presence of a monovacancy (MV-G). We find that the monovacancy strengthens the binding of 5CB in the planar alignment and that the structure is lower in energy than the corresponding homeotropic structure. However, if the molecule is able to approach the monovacancy homeotropically, 5CB undergoes a chemical reaction, releasing 4.5 eV in the process. This reaction follows a step-by-step process gradually adding bonds, inserting the 5CB cyano group into MV-G. We conclude that this irreversible insertion reaction is likely spontaneous, potentially providing a new avenue for controlling both LC behavior and graphene properties.
Ice-nucleating organisms play important roles in the environment. With their ability to induce ice formation at temperatures just below the ice melting point, bacteria such as Pseudomonas syringae ...attack plants through frost damage using specialized ice-nucleating proteins. Besides the impact on agriculture and microbial ecology, airborne P. syringae can affect atmospheric glaciation processes, with consequences for cloud evolution, precipitation, and climate. Biogenic ice nucleation is also relevant for artificial snow production and for biomimetic materials for controlled interfacial freezing. We use interface-specific sum frequency generation (SFG) spectroscopy to show that hydrogen bonding at the water-bacteria contact imposes structural ordering on the adjacent water network. Experimental SFG data and molecular dynamics simulations demonstrate that ice-active sites within P. syringae feature unique hydrophilic-hydrophobic patterns to enhance ice nucleation. The freezing transition is further facilitated by the highly effective removal of latent heat from the nucleation site, as apparent from time-resolved SFG spectroscopy.
Trp-Cage Folding on Organic Surfaces Levine, Zachary A; Fischer, Sean A; Shea, Joan-Emma ...
The journal of physical chemistry. B,
08/2015, Volume:
119, Issue:
33
Journal Article
Peer reviewed
Trp-cage is an artificial miniprotein that is small, stable, and fast folding due to concerted hydrophobic shielding of a Trp residue by polyproline helices. Simulations have extensively ...characterized Trp-cage; however, the interactions of Trp-cage with organic surfaces (e.g., membranes) and their effect on protein conformation are largely unknown. To better understand these interactions we utilized a combination of replica-exchange molecular dynamics (REMD) and metadynamics (MetaD), to investigate Trp-cage folding on self-assembled monolayers (SAMs). We found that, with REMD and MetaD, Trp-cage strongly binds to neutral CH3 surfaces (-25kT) and moderately adsorbs to anionic COOH interfaces (-7.6kT), with hydrophobic interactions driving CH3 adhesion and electrostatic attractions driving COOH adhesion. Similar to solid-state surfaces, SAMs facilitate a number of intermediate Trp-cage conformations between folded and unfolded states. Regarding Trp-cage's aromatic groups in neutral CH3 systems, Tyr becomes oriented parallel to the surface in order to maximize hydrophobic interactions while Trp remains caged perpendicular to the surface; however, Trp can reorient itself parallel to the interface as the miniprotein more closely binds to the surface. In contrast, Tyr and Trp are both repelled from COOH surfaces, though the Trp-cage still adheres to the anionic interface via Lys and its N-terminated Asn residue.
ABSTRACT
Proton transport is essential in many areas of chemistry and biology and is especially important in the fields of proton exchange membrane fuel cells and biocompatible, protonic ...semiconductors. These devices make use of membranes to control the flow of protons for either the generation of energy or to more closely couple electronics and biology. In the present study, we make use of ab initio molecular dynamics simulations, including the effect of applied electric fields, to gain atomistic insight into the intrinsic conductivity of chitosan‐based polymers and demonstrate that chitosan does not act as a significant source of friction for the transport of protons while increasing the number of free ions. Published 2017.† J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1103–1109
Ab initio molecular dynamics simulations that include the effects of applied electric fields demonstrate that chitosan membranes can support high proton mobilities. The simulations also provide details on how acidic functional groups on the polymer enhance proton conduction while basic functional groups enhance hydroxide conduction.
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