Nonequilibrium Adsorption and Reorientation Dynamics of Molecules at Electrode/Electrolyte Interfaces Probed via Real-Time Second Harmonic Generation

Nonequilibrium adsorption and subsequent reorientation of organic molecules at electrode/electrolyte interfaces are important steps in electrochemical reactions and other interfacial processes, yet real-time quantitative characterization and monitoring of these processes, particularly for the reorientation step, are still challenging experimentally. Herein, we investigated the nonequilibrium adsorption process of 4-(4-(diethylamino)­styryl)-<i>N</i>-methyl-pyridinium iodide (D289) molecules from acetonitrile solution onto a polycrystalline platinum electrode surface using real-time second harmonic generation (SHG) in combination with the potential step method. The time-dependent SHG curves exhibit two distinct regimes, which were interpreted with a two-step adsorption model consisting of a fast adsorption and a slow reorientation step for D289 on the surface. D289 was assumed to initially adsorb in an orientation perpendicular to the surface and then reorient to a parallel orientation. We derived a quantitative mathematical expression containing a biexponential function to fit the temporal SHG curves and obtain the rate constants for the two steps. The rate constants for fast adsorption and the slower reorientation processes show similar potential-dependent behavior: the rate decreases with an increase in the negative potential. We further proposed a molecular mechanism involving the displacement of D289 and CH₃CN molecules adsorbed on the electrode interface to explain this potential-dependent behavior. On the basis of such analysis, we obtained a detailed picture of the adsorption of D289 molecules on the Pt electrode/CH₃CN electrolyte, which consists of three consecutive steps: diffusion, adsorption, and reorientation. The results of this study may shed light on adsorption mechanisms at the electrode/electrolyte interface as well as at biological and other functional material interfaces

Statistical Pull Off of Nanoparticles Adhering to Compliant Substrates

It is widely known in adhesive contact mechanics that a spherical particle will not detach from an elastic half-space unless a critical level of pulling force is reached, as already revealed by JKR- or DMT-type deterministic models. This article focuses on the scenario of particle–substrate adhesion where the size of particles is down to the nanometer scale. A consequence of particle size reduction to this range is that the energy scale confining the state of system equilibrium becomes comparable to the unit of thermal energy, leading to statistical particle detachment even below the critical pull-off force. We describe the process by Kramers’ theory as a thermally activated escape from an energy well and develop a Smoluchowski partial differential equation that governs the spatial–temporal evolution of the adhesion state in probabilistic terms. These results show that the forced or spontaneous separation of nanometer-sized particles from compliant substrates occurs diffusively and statistically rather than ballistically and deterministically as assumed in existing models

Cell Morphogenesis Proteins Are Translationally Controlled through UTRs by the Ndr/LATS Target Ssd1

<div><p>Eukaryotic cells control their growth and morphogenesis to maintain integrity and viability. Free-living cells are further challenged by their direct interaction with the environment and in many cases maintain a resilient cell wall to stay alive under widely varying conditions. For these organisms, stringent and highly localized control of the cell wall's remodeling and expansion is crucial for cell growth and reproduction. In the budding yeast <i>Saccharomyces cerevisiae</i> the RNA binding protein Ssd1 helps control cell wall remodeling by repressing translation of proteins involved in wall expansion. Ssd1 is itself negatively regulated by the highly conserved Ndr/LATS protein kinase Cbk1. We sought to identify mRNA regions that confer Ssd1-mediated translational control. After validating a GFP reporter system as a readout of Ssd1 activity we found that 3′ untranslated regions of the known Ssd1 targets <i>CTS1, SIM1</i> and <i>UTH1</i> are sufficient for Cbk1-regulated translational control. The 5′ untranslated region of <i>UTH1</i> also facilitated Ssd1-mediated translational control in a heterologous context. The <i>CTS1</i> and <i>SIM1</i> 3′ untranslated regions confer Ssd1 binding, and the <i>SIM1</i> 3′ untranslated region improves Ssd1 immunoprecipitation of the endogenous <i>SIM1</i> transcript. However, <i>SIM1</i>'s 3′ untranslated region is not essential for Ssd1-regulated control of the message's translation. We propose that Ssd1 regulates translation of its target message primarily through UTRs and the <i>SIM1</i> message through multiple potential points of interaction, permitting fine translational control in various contexts.</p></div

Destabilized GFP reporters show Cbk1-phosphoregulation of Ssd1-dependent changes in expression.

<p><b>(A)</b> Destabilized GFP-Cln2^PEST bearing the <i>SIM1</i> 3′UTR shows moderate shifts in population fluorescence depending on the phosphorylation state of Ssd1, but a destabilized GFP bearing the <i>CYC1</i> 3′UTR remains unaffected. Ssd1 phosphorylation state was modulated by the introduction of the hypomorphic <i>cbk1-as</i> allele and treatment of these cells with DMSO or 1NA-PP1. <b>(B)</b> Destabilized GFP-Cln2^PEST harboring the Ssd1-regulated <i>CTS1</i> 3′UTR responds to Ssd1 hyperactivation through Cbk1 inhibition. A significant difference in GFP levels was observed by flow cytometry between <i>cbk1-as SSD1</i> cells treated with DMSO or 1NA-PP1 through the fraction of the population above baseline fluorescence or. The fraction of cells expressing destabilized GFP was significantly dependent on the presence of Ssd1 (compare <i>CBK1 SSD1</i> to <i>CBK1 ssd1Δ</i>). <b>(C)</b> Prolonged Cbk1 inhibition results in complete depletion of GFP fluorescence in cells expressing an Ssd1-regulated reporter. Flow cytometry was performed on cells fixed at one hour intervals as described in Materials in Methods. We report the relative %GFP positive at each time point t>1 h as a fold change relative to the %GFP positive population at t = 0 h. Additional controls shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0085212#pone.0085212.s004" target="_blank">Figure S4C</a>. (A) and (C) are representative trials of replicated experiments. Data in (B) represent three independent trials. Error bars represent ± SEM, ** indicates P-value of 0.001 to 0.01 and * indicates P-value 0.01 to 0.05 at 95% confidence intervals as calculated by unpaired two-tailed Student's t-test.</p

Inhibition of hMSC proliferation after polybrene exposure.

<p>Plot of cell number over 21 days as measured by the CyQUANT assay. Human MSCs were seeded at the same density in 96-well plates, exposed to different concentrations of polybrene for different lengths of time, and cultured in 50 µl of medium per well without (A) or with FGF-2 (B). The untreated controls are the same for the different time points. The graphs are all on the same scale. Values are mean ± SD.</p

Phase contrast images of hMSCs cultured in 50 µl or 200 µl of medium per well.

<p>Phase contrast images of hMSCs from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0023891#pone-0023891-g002" target="_blank">Fig. 2</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0023891#pone-0023891-g002" target="_blank">3</a> that were either untreated or treated with to 8 µg/mL of polybrene for 24 hr. Cells were cultured for 14 days in either normal culture conditions (A) or under simulated hypoxic conditions (B). The medium was removed prior to image capture. All images are at the same magnification. The insert shows an increased magnification of the cells. White bar  = 500 µm (100 µm in insert).</p

Visual representation of Ssd1-mediated translational control of GFP reporters and multivalent interaction with <i>SIM1</i>.

<p>(A) Depiction of our observed results in this study: left, a reporter mRNA bearing an Ssd1-bound UTR produces GFP in an unregulated manner; center, the presence of Ssd1 subjects this reporter mRNA to Ssd1 binding and translational control, resulting in reduced expression; right, Cbk1 inhibition hyperactivates the UTR-bound Ssd1 and represses the translation of the protein encoded by the Ssd1-associated mRNA. (B) Ssd1 may interact both directly with the <i>SIM1</i> transcript's 3′UTR and its 5′UTR in a ‘closed-loop’ mRNA configuration. This is a depiction of our observation that ablating the 3′UTR of the Ssd1-bound message <i>SIM1</i> reduces immunoprecipitation efficiency but does not disrupt the ability of Ssd1 to confer translational repression. Left, a reporter expressing only the 3′UTR of an Ssd1-bound messages confers direct binding, permitting translational repression; center, the endogenous <i>SIM1</i> transcript containing both native 5′ and 3′UTRs confers Ssd1 binding through interaction with a 5′UTR-bound RNA binding protein and through direct RNA binding at the 3′UTR; right, the endogenous <i>SIM1</i> transcript lacking its native 3′UTR is no longer receptive to direct Ssd1-mRNA interaction at its 3′UTR but still permits Ssd1 binding and translational repression at its 5′UTR. We emphasize that this panel describes our results for the <i>SIM1</i> transcript and may not be a general mode of Ssd1-mRNA interaction. Note that all depicted Ssd1-mRNA interaction may not be direct.</p

Disparate Strain Dependent Thermal Conductivity of Two-dimensional Penta-Structures

Two-dimensional (2D) carbon allotrope called penta-graphene was recently proposed from first-principles calculations and various similar penta-structures emerged. Despite significant effort having been dedicated to electronic structures and mechanical properties, little research has been focused on thermal transport in penta-structures. Motivated by this, we performed a comparative study of thermal transport properties of three representative pentagonal structures, namely penta-graphene, penta-SiC₂, and penta-SiN₂, by solving the phonon Boltzmann transport equation with interatomic force constants extracted from first-principles calculations. Unexpectedly, the thermal conductivity of the three penta-structures exhibits diverse strain dependence, despite their very similar geometry structures. While the thermal conductivity of penta-graphene exhibits standard monotonic reduction by stretching, penta-SiC₂ possesses an unusual nonmonotonic up-and-down behavior. More interestingly, the thermal conductivity of penta-SiN₂ has 1 order of magnitude enhancement due to the strain induced buckled to planar structure transition. The mechanism governing the diverse strain dependence is identified as the competition between the change of phonon group velocity and phonon lifetime of acoustic phonon modes with combined effect from the unique structure transition for penta-SiN₂. The disparate thermal transport behavior is further correlated to the fundamentally different bonding nature in the atomic structures with solid evidence from the distribution of deformation charge density and more in-depth molecular orbital analysis. The reported giant and robust tunability of thermal conductivity may inspire intensive research on other derivatives of penta-structures as potential materials for emerging nanoelectronic devices. The fundamental physics understood from this study also solidifies the strategy to engineer thermal transport properties of broad 2D materials by simple mechanical strain

Lentiviral transduction of hMSCs with polybrene.

<p>Human MSCs were cultured with lentivirus containing a dual reporter gene that encodes a fusion protein of luciferase and mRFP. (A) Positive correlation of transduction efficiency with polybrene concentration as determined by mRFP flow analysis. (Spearman corr. coeff.  = 0.95, p<0.001) Values are mean ± SD. (B) Image of the bioluminescence signal from the transduced cells. (C) Chart of the number of photons detected in each well as a function of polybrene concentration. (Spearman corr. coeff.  = 0.97, p<0.001) Values are mean ± SD.</p

Multilayered Core–Shell Structure in an Impact Polypropylene Copolymer Investigated by Atomic Force Microscopy–Infrared

The balanced mechanical properties of impact polypropylene copolymer (IPC) are largely attributed to the core–shell structure of its dispersed rubber particles, yet experimental observation of the outer shell interface between the rubber phase and the polypropylene (PP) matrix is challenging. In this article, atomic force microscopy-infrared (AFM-IR) was employed to study a commercial IPC to determine its phase structure. Quantitative analysis of the nanodomain composition in situ by AFM-IR in combination with the chain structure of the copolymers obtained ex situ by fractionation and NMR revealed a core surrounded by a rubber layer, comprising the ethylene–propylene segmented copolymer (EsP) and ethylene–propylene random copolymer (EPR), respectively, which suggests the existence of an outer shell for the particle composed of the ethylene–propylene block copolymer (EbP). The EbP fraction in the IPC was then replaced by an ethylene-deuterated propylene diblock copolymer (EbDP), which was then melt-blended with all other fractions to reconstruct the IPC. Both AFM-IR spectroscopic analysis and imaging of the nanodomains in the reconstructed IPC showed that the EbDP molecules are located at the interface between the rubber phase and the PP matrix, forming an outer shell for the particle. The results provide direct and unambiguous experimental evidence for the multilayered particle structure in the IPC. Mechanical test results further demonstrated that the outer shell for the rubber particle was beneficial to the tensile and impact properties of the alloy