Attraction between like-charged monovalent ions

Ions with like-charges repel each other with a magnitude given by the Coulomb law. The repulsion is also known to persist in aqueous solutions albeit factored by the medium's dielectric constant. In this paper, we report results from molecular dynamics simulations of alkali halides salt solutions indicating an effective attraction between some of the like-charged monovalent ions. The attraction is observed between anions, as well as between cations, leading to the formation of dimers with lifetimes on the order of few picoseconds. Two mechanisms have been identified to drive this counterintuitive attraction. The first is exhibited by high-charge density ions, such as fluoride, at low salt concentrations, yielding effective attractions with magnitude up to the order of 1-2 kT. In this case, the stronger local electric field generated when the two ions are in contact augments the alignment of neighboring waters toward the ions. This results in a gain of substantial favorable ion-water interaction energy. For fluorides, this interaction constitutes the major change among the different energy components compensating for the anion-anion repulsion, and therefore, rendering like-charge association possible. The second mechanism involves mediation by counterions, the attractions increase with salt concentration and are characterized by small magnitudes. In particular, clusters of ion triplets, in which a counterion is either bridging the two like-charged ions or is paired to only one of them, are formed. Although these two mechanisms may not yield net attractions in many cases, they might still be operational and significant, explaining effective repulsions between like-charged ions with magnitudes much smaller than expected based on continuum electrostatics.     

On the Lifetime of the Transients (NP)?(CH3)n (NP=Ag0, Au0, TiO2 Nanoparticles) Formed in the Reactions Between Methyl Radicals and Nanoparticles Suspended in Aqueous Solutions

Methyl radicals react in fast reactions, with rate constants k>1×10⁸ M ^?1 s^?1, with Au⁰, Ag⁰ and TiO₂ nanoparticles (NPs) dispersed in aqueous solutions to form intermediates, (NP)? (CH₃)_n, in which the methyl groups are covalently bound to the NPs. These intermediates decompose to form ethane. As n≥2 is required for the formation of C₂H₆, the minimal lifetime (τ) of the methyls bound to the NPs, (NP)? CH₃, can be estimated from the rate of production of the CH₃^. radicals and the NPs concentration. The results obtained in this study, using a very low dose rate γ‐source for NP=Ag⁰, Au⁰, and TiO₂ point out that τ of these intermediates is surprisingly long, for example, ≥8 and ≥188 sec for silver and gold, respectively. These data point out that the NP? C bond dissociation energies are ≥70 kJ mol^?1. Under low rates of production of CH₃^., that is, when the rate of formation of ethane is very low, other reactions may occur, consequently the mechanism proposed is “broken”. This is observed in the present study only for TiO₂ NPs. These results have to be considered whenever alkyl radicals are formed near surfaces. Furthermore, the results point out that the rate of reaction of methyl radicals with (NP)? (CH₃)_n depends on n, that is, the number of methyl radicals bound to the NPs affect the properties of the NPs.     

Carbon nanotubes as nanocarriers in medicine

Carbon nanotubes (CNTs) possess outstanding properties and a unique physicochemical architecture, which may serve as an alternative platform for the delivery of various therapeutic molecules. This review focuses on recent progress in the field of CNTs for biomedical applications. After a short, general physico-chemical introduction to CNTs, we introduce different methods for CNT surface modification, facilitating their dispersions in physiological solutions, on the one hand, and binding a wide range of molecules or drug-loaded liposomes, on the other. We summarize imaging evidences on the structure of CNT-drug conjugates and their relevant uptake mechanisms by the cell. Lastly, we review current repots on CNT toxicity and new developments in CNT-based medical applications: photo-thermal therapy, drug delivery and gene therapy.     

Chitosan/anionic surfactant microparticles synthesized by high pressure spraying method for removal of phenolic pollutants

Biopolymeric microparticles were prepared by rapid expansion of high pressure CO₂-chitosan (Chi) solution in sodium bis-(2-ethyl hexyl) sulfosuccinate (AOT) solution. At pressures higher than 2 MPa, ultrafine particles were formed while under this value, wires were obtained. The formation of Chi/AOT complex was confirmed by Fourier Transform Infrared (FTIR) Spectroscopy, whereas scanning electron microscopy was used to characterize the morphology, size and shape of the particles. The FTIR spectrum proved the interaction between the sulfonate groups of AOT and the amino groups of Chi. Microparticles are quasi-spherical in wet conditions and irregular after freeze drying, presenting a rough surface with many pores. Lyophilized hydrophobic microparticles were used to remove phenol and o-cresol from aqueous solution, and the adsorption process showed a maximum efficiency in the 7–8 pH range. The uptake of phenol and o-cresol increased with the amount of particles and decreased with increasing the pollutant concentration. The adsorption occurred rapidly in the first 60–120 minutes, and leveled off thereafter.      

Convex hulls in the hyperbolic space

We show that the convex hull of any N points in the hyperbolic space ${\mathbb{H}^{n}}$ is of volume smaller than ${\frac{2 (2 \sqrt \pi)^n}{\Gamma(\frac n 2)} N}$ , and that for any dimension n there exists a constant C _n > 0 such that for any set ${A \subset \mathbb{H}^{n}}$ , $$Vol(Conv(A_1)) \leq C_n Vol(A_1)$$ where A ₁ is the set of points of hyperbolic distance to A smaller than 1.     

Single-shot subpixel response measurement with an aperture array pixel mask

We first point out that the subpixel response function is another kernel function in digital imaging. Then we show that the subpixel response function of CMOS imaging sensor pixels can be measured with an aperture array pixel mask in a single-shot image capture. Our technique permits high-resolution subpixel response function measurement of imaging pixels for superresolution imaging applications.     

Covalent flexible peptide docking in Rosetta

Electrophilic peptides that form an irreversible covalent bond with their target have great potential for binding targets that have been previously considered undruggable. However, the discovery of such peptides remains a challenge. Here, we present Rosetta CovPepDock, a computational pipeline for peptide docking that incorporates covalent binding between the peptide and a receptor cysteine. We applied CovPepDock retrospectively to a dataset of 115 disulfide-bound peptides and a dataset of 54 electrophilic peptides. It produced a top-five scoring, near-native model, in 89% and 100% of the cases when docking from the native conformation, and 20% and 90% when docking from an extended peptide conformation, respectively. In addition, we developed a protocol for designing electrophilic peptide binders based on known non-covalent binders or protein–protein interfaces. We identified 7154 peptide candidates in the PDB for application of this protocol. As a proof-of-concept we validated the protocol on the non-covalent complex of 14-3-3σ and YAP1 phosphopeptide. The protocol identified seven highly potent and selective irreversible peptide binders. The predicted binding mode of one of the peptides was validated using X-ray crystallography. This case-study demonstrates the utility and impact of CovPepDock. It suggests that many new electrophilic peptide binders can be rapidly discovered, with significant potential as therapeutic molecules and chemical probes.

We developed Rosetta CovPepDock, a computational pipeline for covalent peptide docking. We showed it is highly accurate in retrospective benchmarks, and applied it prospectively to design potent and selective covalent binders of 14-3-3σ.     

The underaddressed optical multiple-input, multiple-output channel: capacity and outage

We study an optical space-division multiplexed system where the number of modes that are addressed by the transmitter and receiver is allowed to be smaller than the total number of optical modes supported by the fiber. This situation will be of relevance if, for instance, fibers supporting more modes than can be processed with current multiple-input, multiple-output technology are deployed with the purpose of future-proof installation. We calculate the ergodic capacity and the outage probability of the link and study their dependence on the number of addressed modes at the transmitter and receiver.