Crystalline Assemblies and Densest Packings of a Family of Truncated Tetrahedra and the Role of Directional Entropic Forces

Polyhedra and their arrangements have intrigued humankind since the ancient Greeks and are today important motifs in condensed matter, with application to many classes of liquids and solids. Yet, little is known about the thermodynamically stable phases of polyhedrally-shaped building blocks, such as faceted nanoparticles and colloids. Although hard particles are known to organize due to entropy alone, and some unusual phases are reported in the literature, the role of entropic forces in connection with polyhedral shape is not well understood. Here, we study thermodynamic self-assembly of a family of truncated tetrahedra and report several atomic crystal isostructures, including diamond, {\beta}-tin, and high- pressure lithium, as the polyhedron shape varies from tetrahedral to octahedral. We compare our findings with the densest packings of the truncated tetrahedron family obtained by numerical compression and report a new space filling polyhedron, which has been overlooked in previous searches. Interestingly, the self-assembled structures differ from the densest packings. We show that the self-assembled crystal structures can be understood as a tendency for polyhedra to maximize face-to-face alignment, which can be generalized as directional entropic forces.Comment: Article + supplementary information. 23 pages, 10 figures, 2 table

Вариации метана в атмосфере Антарктиды в 2009–2017 гг. по данным наземных и спутниковых измерений

The article presents the results of systematic measurements (2009−2017) of the total column abundances of methane (TC_CH4 ) and the column-averaged concentration (X_CH4 ) at the Antarctic station Novolazarevskaya. Solar radiation is recorded in the range 2990 – 3006 cm–1 using a diffraction spectrometer with 0.2 cm–1 resolution. The inverse task CH4 total column determining is achieved using the SFIT4 v0.9.4.4 code. The analysis shows that during the measurement period the average TC_CH4 was (3.4±0.8).1019 molecules/cm2 during the measurement period, and the average X_CH4 is (1663±34) ppbv. TC_CH4 trend is (4.5±2.2).1015 molecules/cm2 /month, and X_CH4 trend is (0.28 ± 0.11) ppbv/month. The average TC_CH4 values and trend at Novolazarevskaya are in good agreement with the measurements by the Brucker120HR instrument at the Arrival Heights station. Seasonal variations of atmospheric methane have the maximum in October–November and the minimum in May–July. The trends of surface methane concentrations at Sywa, the Halley station and the Amundsen-Scott South Pole station are 0.59 – 0.61 ppbv/month and exceed the trend of the column-averaged concentration at Novolazarevskaya and AIRS trends for the troposphere (0.24 – 0.32) ppbv/month, due to a decrease in the maximal values of TC_CH4 in the period after 2014. The closest agreement of X_CH4 variations at Novolazarevskaya with AIRS data is observed at the levels of 150–200 hPa. Significant semiannual harmonics varied with height are characteristic of CH4 variations according to the AIRS data. The interference of annual and semiannual harmonics leads to the appearance of two maxima in the seasonal variations of methane with relative position to each other varies with height. The statistical model is developed for all the series considered. It approximates the trend, annual and semi-annual components of CH4 oscillations.The authors have no competing interests.Представлены результаты систематических измерений (2009–2017 гг.) общего содержания и средней по высоте концентрации метана на ст. Новолазаревская. Рассчитанные значения линейных трендов и параметров внутригодовых колебаний атмосферного метана сопоставлены с данными анализа вариаций приземных концентраций метана на станциях Сёва (Sywa), Халли (Halley Station) и Амундсен-Скотт (Amundsen-Scott South Pole Station), общего содержания метана на станции Арривал-Хайтс (ArrivalHeights), а также спутниковыми данными AIRS. Средние значения и тренд общего содержания метана на ст. Новолазаревская и ст. Арривал-Хайтс хорошо согласуются. Для периода измерений 2009–2014 гг. тренд средней по высоте объемной концентрации метана на ст. Новолазаревская совпадает в пределах погрешности с трендом приземных концентраций метана на станциях Сёва, Халли и Амундсен-Скотт, однако в 2015–2016 гг. согласно данным ст. Новолазаревская, Арривал-Хайтс и спутниковым данным наблюдалось замедление роста концентраций метана. Для вариаций средней по высоте объемной концентрации на ст. Новолазаревская и концентрации метана по данным AIRS, наряду с годовыми колебаниями, характерны значительные полугодовые вариации. Для всех рассмотренных рядов построена статистическая модель, которая аппроксимирует трендовую, годовую и полугодовую составляющие колебаний СН4.Авторы заявляют об отсутствии конфликта интересов

Adult Neural Stem Cells: Basic Research and Production Strategies for Neurorestorative Therapy

Over many decades, constructing genetically and phenotypically stable lines of neural stem cells (NSC) for clinical purposes with the aim of restoring irreversibly lost functions of nervous tissue has been one of the major goals for multiple research groups. The unique ability of stem cells to maintain their own pluripotent state even in the adult body has made them into the choice object of study. With the development of the technology for induced pluripotent stem cells (iPSCs) and direct transdifferentiation of somatic cells into the desired cell type, the initial research approaches based on the use of allogeneic NSCs from embryonic or fetal nervous tissue are gradually becoming a thing of the past. This review deals with basic molecular mechanisms for maintaining the pluripotent state of embryonic/induced stem and reprogrammed somatic cells, as well as with currently existing reprogramming strategies. The focus is on performing direct reprogramming while bypassing the stage of iPSCs which is known for genetic instability and an increased risk of tumorigenesis. A detailed description of various protocols for obtaining reprogrammed neural cells used in the therapy of the nervous system pathology is also provided

Precipitation of Oppositely Charged Nanoparticles by Dilution and/or Temperature Increase

Mixtures of oppositely charged nanoparticles (NPs) exhibit anomalous solubility behavior and precipitate either upon dilution or upon temperature increase. Precipitation is reversible and can be explained by a thermodynamic model that accounts for changes in the electrostatic interactions due to the adsorption/desorption of counterions from the surface of the NPs. Specifically, decreasing the salt concentration via dilution Or increasing the temperature causes dissociation of counterions from the NP surfaces, increasing the magnitude of electrostatic interactions between NPs and resulting in their precipitation. Model predictions of NP solubility are in quantitative agreement with the experimental observations. Such predictions are of practical importance for the preparation of "patchy" electrostatic coatings and ionic-like NP supracrystals

Tissue Engineered Neural Constructs Composed of Neural Precursor Cells, Recombinant Spidroin and PRP for Neural Tissue Regeneration

Abstract We have designed a novel two-component matrix (SPRPix) for the encapsulation of directly reprogrammed human neural precursor cells (drNPC). The matrix is comprised of 1) a solid anisotropic complex scaffold prepared by electrospinning a mixture of recombinant analogues of the spider dragline silk proteins – spidroin 1 (rS1/9) and spidroin 2 (rS2/12) - and polycaprolactone (PCL) (rSS-PCL), and 2) a “liquid matrix” based on platelet-rich plasma (PRP). The combination of PRP and spidroin promoted drNPC proliferation with the formation of neural tissue organoids and dramatically activated neurogenesis. Differentiation of drNPCs generated large numbers of βIII-tubulin and MAP2 positive neurons as well as some GFAP-positive astrocytes, which likely had a neuronal supporting function. Interestingly the SPRPix microfibrils appeared to provide strong guidance cues as the differentiating neurons oriented their processes parallel to them. Implantation of the SPRPix matrix containing human drNPC into the brain and spinal cord of two healthy Rhesus macaque monkeys showed good biocompatibility: no astroglial and microglial reaction was present around the implanted construct. Importantly, the human drNPCs survived for the 3 month study period and differentiated into MAP2 positive neurons. Tissue engineered constructs based on SPRPix exhibits important attributes that warrant further examination in spinal cord injury treatment

Mechanism of the Cooperative Adsorption of Oppositely Charged Nanoparticles

Quartz crystal microbalance experiments were performed to study the kinetics of surface adsorption from solutions containing oppositely charged nanoparticles. A theoretical model was developed according to which formation of dense nanoparticle (NP) monolayers is driven by a cooperative process, in which the already-adsorbed NPs facilitate adsorption of NPs from solution. The kinetic rate constants change with the NP solution concentration and can be used to backtrack adsorption free energies. These energies agree with the predictions of a simple DLVO model

Ruthenium p-cymene iminophosphonamide complexes: Activation under basic conditions and transfer hydrogenation catalysis

International audienceComplex [(η6-Cym)RuCl(NPN)] Cym = p-cymene; NPN = (pTolN)2PPh2 (1) yields a thermally sensitive hydride derivative [(η6-Cym)RuH(NPN)] (2) by reaction with iPrOH in the presence of a strong base, via an observable isopropoxide intermediate [(η6-Cym)Ru(OiPr)(NPN)] (3), or with NaBHEt3 in toluene. Partial conversion also occurs in iPrOH in the absence of base. 2 is stabilized by dihydrogen bonding with isopropyl alcohol, but attempts to isolate it induce isomerization by hydride migration to a ring CH position to yield a 16-electron cyclohexadienyl derivative [η5-p-C6H5(Me)(iPr)Ru(NPN)], which has been crystallographically characterized as a disordered mixture of two regioisomers (4/4′). Complex 2 is able to release H2 upon treatment with medium strength proton donors (fluorinated alcohols), but also slowly with iPrOH. 2 is an active catalyst for the transfer hydrogenation of acetophenone to phenylethanol in isopropyl alcohol. The catalytic transformation is first order in acetophenone and first order in catalyst, with k = 117 ± 10 m–1 h–1 at 40 °C. The temperature dependence of the rate constant (25–80 °C) gave the activation parameters ΔH‡ = 9.6 ± 1.3 kcal mol–1 and ΔS‡ = –31 ± 4 cal mol–1 K–1. DFT calculations have validated the slow isomerization of 2 to 4/4′ (high energy TS), the preference of the cyclohexadienyl system for 4/4′ relative to the other isomers 4Me and 4iPr, where the hydride has migrated to the CMe or CiPr position, and suggest that the hydrogen transfer mechanism involves outer sphere hydride transfer to the ketone substrate with H-bonding assistance of isopropyl alcohol to yield a σ complex intermediate [(η6-Cym)Ru+(NPN)H-C(Me)(Ph)O–]