Liquid-Vapor Interface of Aqueous Ethylene Glycol Solutions: A Molecular Dynamics Study.

While the organic constituent in an aqueous binary solution enriches its liquid-vapor (l-v) interface, the extent of enrichment can depend nonlinearly on its mole fraction. A microscopic quantification and rationalization of this behavior are crucial to understand the dependence of properties such as surface tension and evaporation rate of the solution on its composition. Extensive all-atom molecular dynamics simulations of aqueous ethylene glycol (EG) solutions show that the composition of the solution at the l-v interface deviates the most from that in the bulk solution at an EG mole fraction of 0.3. The population of EG molecules with their central C-C dihedral in the gauche conformation was found to be higher at the l-v interface than that in the bulk solution to facilitate the orientation of its hydrophobic methyl groups toward the vapor phase. Free energy calculations reveal that in dilute EG solutions, an EG molecule is most stable at the l-v interface. The behavior of vapor pressure in aqueous EG solutions is ideal and follows Raoult's law, while in contrast, the aqueous solution of dimethyl sulfoxide does not. A rationale for the same is provided through the orientational distribution of interfacial water molecules in the respective solutions.

Endobronchial lesion in an adolescent with hemoptysis biopsy or not to biopsy?

An 11-year-old girl was brought with complaints of recurrent massive hemoptysis. A computerized tomography (CT) of the chest showed ground glass opacities on the right lower lobe, and a CT angiography showed hypertrophied right pulmonary artery. Flexible bronchoscopy revealed a sessile friable lesion in the right lower lobe, raising suspicion of either a tumor or a vascular malformation. An endobronchial ultrasound (EBUS) revealed a cystic lesion in the submucous plane, with vascularity noted on Doppler mode. This confirmed the diagnosis of bronchial Dieulafoy disease. A bronchial angiography revealed a vascular malformation overlying the lesion with a bronchopulmonary shunt, which was ligated. This case demonstrates the importance of EBUS in endobronchial lesions, to avoid biopsy of a vascular malformation.

Conformer Selection Upon Dilution with Water: The Fascinating Case of Liquid Ethylene Glycol Studied via Molecular Dynamics Simulations.

The aqueous solution of ethylene glycol (EG) is a binary liquid mixture that displays rich conformational and structural behaviour, which has not yet been adequately explored through atomistic molecular dynamics simulations. Herein, employing an accurate force field for EG, several physical properties of this solution are calculated to be in quantitative agreement with experimental data. While 79 % of molecules in neat liquid EG exist with their central OCCO dihedral in the gauche state, this fraction increases to 89 % in the dilute aqueous solution, largely in response to the increase in the static dielectric constant of the solution from that of neat liquid EG. The increase in gauche conformers increases the mean dipole moment of EG molecules in the solution which is additionally contributed by specific conformational states of the two terminal HOCC dihedral angles.

Gate Opening without Volume Change Triggers Cooperative Gas Interactions, Underpins an Isotherm Step in Metal-Organic Frameworks.

Three halogenated metal-organic frameworks (MOFs) reported recently exhibited a second step in their CO2 gas adsorption isotherms. The emergence of halogen-bonding interactions beyond a threshold gas pressure between the framework halogen and the CO2 guest was conjectured to be the underlying reason for the additional step in the isotherm. Our investigation employing periodic density functional theory calculations did not show significant interactions between the halogen and CO2 molecules. Further, using a combination of DFT-based ab initio molecular dynamics and grand canonical Monte Carlo simulations, we find that the increased separation of framework nitrate pairs facing each other across the pore channel enables the accommodation of an additional CO2 molecule which is further stabilized by cooperative interactions─an observation that facilely explains the second isotherm step. The increased separation between the nitrate groups can occur without any lattice expansion, consistent with experiments. The results point to a structural feature to achieve this isotherm step in MOFs that neither possess large pores nor exhibit large-scale structural changes such as breathing.

Liquid ethylene glycol: prediction of physical properties, conformer population and interfacial enrichment with a refined non-polarizable force field.

Periodic density functional theory based molecular dynamics simulations confirm the fraction of molecules in neat liquid ethylene glycol with their central OCCO dihedral in the trans conformation to be 21% at ambient conditions, while the rest are gauche conformers. Using this result as a benchmark, two non-polarizable force fields are developed herein to reproduce the conformer populations in the liquid, an important aspect inadequately addressed in several generic force fields. The mean dipole moment of a molecule in the liquid is estimated to be about 40% enhanced over its value in the gas phase, a feature discerned via AIMD simulations and fairly reproduced by our force fields. They are also shown to quantitatively predict all the physical properties of the liquid. Molecules present at the liquid-vapor interface of ethylene glycol are oriented with their methylene groups pointing towards the vapor phase, a requirement that enriches the interface with gauche conformers, in line with polarized sum frequency generation spectroscopy results.

Ileal Perforation and Enteric Fever: Implications for Burden of Disease Estimation.

BACKGROUND: Ileal perforation occurs in about 1% of enteric fevers as a complication, with a case fatality risk (CFR) of 20%-30% in the early 1990s that decreased to 15.4% in 2011 in South East Asia. We report nontraumatic ileal perforations and its associated CFR from a 2-year prospective enteric fever surveillance across India. METHODS: The Surveillance for Enteric Fever in India (SEFI) project established a multitiered surveillance system for enteric fever between December 2017 and March 2020. Nontraumatic ileal perforations were surveilled at 8 tertiary care and 6 secondary care hospitals and classified according to etiology. RESULTS: Of the 158 nontraumatic ileal perforation cases identified,126 were consented and enrolled. Enteric fever (34.7%), tuberculosis (19.0%), malignancy (5.8%), and perforation of Meckel diverticulum (4.9%) were the common etiology. In those with enteric fever ileal perforation, the CFR was 7.1%. CONCLUSIONS: Enteric fever remains the most common cause of nontraumatic ileal perforation in India, followed by tuberculosis. Better modalities of establishing etiology are required to classify the illness, and frame management guidelines and preventive measures. CFR data are critical for comprehensive disease burden estimation and policymaking.

Orientational Switch of the Lipase A Enzyme at the Oil-Water Interface: An Order of Magnitude Increase in Turnover Rate with a Single Surfactant Tag Explained.

Interfacially active lipases can be immobilized at a biphasic interface to enhance turnover recyclability and to facilitate product separation. Extensive coarse-grained molecular dynamics simulations of lipase A (LipA) from Bacillus subtilis show a bimodal orientational distribution of the enzyme at an oil-water interface, arising from its ellipsoidal Janus particle-like character. The relative orientational preference can be tuned by pH. The simulations rationalize a rare experimental observation of an order of magnitude increase in the turnover rate of this lipase upon its noncovalent tagging by a single surfactant molecule at the interface, compared to its rate in bulk water. The adsorption free energy, the interfacial activation, a decrease in the number of orientational fluctuations, and an increased rate of translational diffusion, to all of which the Janus character of LipA contributes, are the factors responsible for this enhancement. This study can spur further investigations of the Janus behavior of enzymes to enhance their activity as well as to stabilize the biphasic emulsion needed for interfacial catalysis.

Bioinspired, ATP-driven co-operative supramolecular polymerization and its pathway dependence.

A bio-inspired, ATP-driven nucleation growth assembly is demonstrated using an amphiphilic naphthalene diimide (NDI) derivative appended with guanidinium receptors to promote specific salt-bridge type interaction with nucleotide phosphates. Detailed spectroscopic and microscopic probing revealed a pathway-dependent co-operative self-assembly to yield two-dimensional and scrolled nano-tubular bilayer assemblies under kinetic and thermodynamic conditions, respectively.

Biomimetic temporal self-assembly via fuel-driven controlled supramolecular polymerization.

Temporal control of supramolecular assemblies to modulate the structural and transient characteristics of synthetic nanostructures is an active field of research within supramolecular chemistry. Molecular designs to attain temporal control have often taken inspiration from biological assemblies. One such assembly in Nature which has been studied extensively, for its well-defined structure and programmable self-assembly, is the ATP-driven seeded self-assembly of actin. Here we show, in a synthetic manifestation of actin self-assembly, an ATP-selective and ATP-fuelled, controlled supramolecular polymerization of a phosphate receptor functionalised monomer. It undergoes fuel-driven nucleation and seeded growth that provide length control and narrow dispersity of the resultant assemblies. Furthermore, coupling via ATP-hydrolysing enzymes yielded its transient characteristics. These results will usher investigations into synthetic analogues of important biological self-assembly motifs and will prove to be a significant advancement toward biomimetic temporally programmed materials.

Separation/purification of ethylene from an acetylene/ethylene mixture in a pillared-layer porous metal-organic framework.

Here we report the synthesis, structure and porous properties of a 3D pillared-layer porous framework of Mn(ii)-Mn(iii), {[Mn3(bipy)3(H2O)4][Mn(CN)6]2·2(bipy)·4H2O}n (1). The guest-removed framework (1a) shows significant uptake of C2H2, whereas it excludes the other two C2 hydrocarbons (C2H4 and C2H6). Furthermore, excellent separation proficiency for C2H2 from a mixture of C2H2 and C2H4 (1 : 99, v/v) is realized in a breakthrough column experiment under ambient conditions.

Understanding the self-assembly of amino ester-based benzene-1,3,5-tricarboxamides using molecular dynamics simulations.

Amino ester-based benzene-1,3,5-tricarboxamides (BTAs) are widely studied experimentally for their facile self-assembly, which leads to strong three-fold hydrogen bonded supramolecular polymers. Understanding the supramolecular assembly of these BTAs is complicated by the presence of two types of dimers, based on the nature of the intermolecular hydrogen bonding pattern: amide-amide (AA) and amide-carboxylate (AC). AA dimers form three hydrogen bonds between the two molecules, are typical of BTA stacks, and act as a basic building block of assembly. In contrast, AC hydrogen bonding results in six hydrogen bonds between two molecules, and this face-to-face orientation results in a dimer that is more stable than the AA one, however, unfavorable for further assembly. We perform atomistic molecular dynamics (MD) simulations of three derivatives of BTA in order to rationalize the large body of experimental data for these systems, specifically the relative stabilities of AA and AC dimers and oligomers. We find that at zero Kelvin, the AC dimer is more stable than the AA dimer by roughly 20 kcal mol-1. MD simulations of three BTA derivatives (BTA-Met, BTA-Nle, and BTA-Phe) under realistic conditions show that BTA-Met and BTA-Phe can aggregate to form longer assemblies via additional stabilization offered by weak CHS and CHπ hydrogen bonds, respectively. However, the aggregation of BTA-Nle, which is devoid of such functionalities, is limited to that of a dimer. We then employ umbrella sampling to show that oligomers of BTA-Met and BTA-Phe are stable over those of dimers and demonstrate that this results from such weak interactions.

Supramolecular polymerization of benzene-1,3,5-tricarboxamide: a molecular dynamics simulation study.

Supramolecular polymerization in the family of benzene-1,3,5-tricarboxamide (BTA) has been investigated using atomistic molecular dynamics (MD) simulations. Gas phase calculations using a nonpolarizable force field reproduce the cooperativity in binding energy and intermolecular structure seen in quantum chemical calculations. Both quantum chemical and force field based calculations suggest that the ground state structure of the BTA dimer contains two donor hydrogen bonds and one acceptor hydrogen bond rather than the conjectured three-donor and zero-acceptor hydrogen-bonded state. MD simulations of BTA molecules in a realistic solvent, n-nonane, demonstrate the self-assembly process. The free energy (FE) of dimerization and of solvation has been determined. The solvated dimer of BTA with hexyl tails is more stable than two monomers by about 13 kcal/mol. Furthermore, the FE of association of a BTA molecule to an oligomer exhibits a dependence on the oligomer size, which is a robust signature of cooperative self-assembly.