On the Formation of a Protic Ionic Liquid in Nature

The practical utility of ionic liquids (ILs) makes the absence (heretofore) of reported examples from nature quite puzzling, given the facility with which nature produces many other types of exotic but utilitarian substances. In that vein, we report here the identification and characterization of a naturally occurring protic IL. It can be formed during confrontations between the ants S. invicta and N. fulva. After being sprayed with alkaloid‐based S. invicta venom, N. fulva detoxifies by grooming with its own venom, formic acid. The mixture is a viscous liquid manifestly different from either of the constituents. Further, we find that the change results as a consequence of formic acid protonation of the N centers of the S. invicta venom alkaloids. The resulting mixed‐cation ammonium formate milieu has properties consistent with its classification as a protic IL.     

Great circle links and virtually fibered knots

We show that all two-bridge knot and link complements are virtually fibered. We also show that spherical Montesinos knot and link complements are virtually fibered. This is accomplished by showing that such manifolds are finitely covered by great circle link complements.     

Application of surface complexation modeling to the reactivity of iron(II) with nitroaromatic and oxime carbamate contaminants in aqueous TiO2 suspensions

This study reports on the application of surface complexation modeling to interpret observed kinetic trends for Fe(II) redox reactions with model nitroaromatic (4-chloronitrobenzene) and oxime carbamate (oxamyl) contaminants in aqueous TiO(2(s)) suspensions. Pseudo-first-order rate constants for reduction of the two probe contaminants (k(red), s(-1)) vary by several orders of magnitude with changing conditions (100-500 microM Fe(II), 0-15 g L(-1) TiO(2(s)), pH 2-9), but the relationship between reaction rates and Fe(II) speciation differs considerably for the two contaminants. For oxamyl, k(red) measurements are most strongly correlated with the volumetric total adsorbed Fe(II) concentration (moles Fe(II) adsorbed per liter of TiO(2(s)) suspension), whereas k(red) measurements for 4-chloronitrobenzene are proportional to the concentration of the hydrolyzed Fe(II) surface complex (equivalent TiOFe(II)OH(0)). The differing trends demonstrate that Fe(II) redox reactivity at the aqueous/TiO(2(s)) interface is influenced, in part, by specific molecular interactions with the target oxidant. Results are also geochemically relevant in that they demonstrate unambiguously that mononuclear Fe(II)-metal (hydr)oxide surface complexes are sufficiently reactive species to reduce nitroaromatic contaminants, an issue that remained open following earlier studies in Fe(III) (hydr)oxide suspensions because structural Fe(II) species are simultaneously present in such systems because of interfacial Fe(II)-to-Fe(III) electron transfer processes that occur on Fe(II) adsorption.     

Anomalous diffusion in living yeast cells

The viscoelastic properties of the cytoplasm of living yeast cells were investigated by studying the motion of lipid granules naturally occurring in the cytoplasm. A large frequency range of observation was obtained by a combination of video-based and laser-based tracking methods. At time scales from 10(-4) to 10(2) s, the granules typically perform subdiffusive motion with characteristics different from previous measurements in living cells. This subdiffusive behavior is thought to be due to the presence of polymer networks and membranous structures in the cytoplasm. Consistent with this hypothesis, we observe that the motion becomes less subdiffusive upon actin disruption.     

Two Distinct Structures of Membrane-Associated Homodimers of GTP- and GDP-Bound KRAS4B Revealed by Paramagnetic Relaxation Enhancement

KRAS homo-dimerization has been implicated in the activation of RAF kinases, however, the mechanism and structural basis remain elusive. We developed a system to study KRAS dimerization on nanodiscs using paramagnetic relaxation enhancement (PRE) NMR spectroscopy, and determined distinct structures of membrane-anchored KRAS dimers in the active GTP- and inactive GDP-loaded states. Both dimerize through an α4–α5 interface, but the relative orientation of the protomers and their contacts differ substantially. Dimerization of KRAS-GTP, stabilized by electrostatic interactions between R135 and E168, favors an orientation on the membrane that promotes accessibility of the effector-binding site. Remarkably, “cross”-dimerization between GTP- and GDP-bound KRAS molecules is unfavorable. These models provide a platform to elucidate the structural basis of RAF activation by RAS and to develop inhibitors that can disrupt the KRAS dimerization. The methodology is applicable to many other farnesylated small GTPases.     

A generalized variety problem

Semigroup Forum - Let $${\mathscr {C}}\! om $$ denote the variety of all commutative semigroups. For $$n\ge 1$$ let $${{\mathscr {N}}\! il }_n$$ (respectively $${\mathscr {N}}_n$$ ) denote the...     

Self assembly of Janus ellipsoids

We propose a primitive model of Janus ellipsoids that represents particles with an ellipsoidal core and two semisurfaces coded with dissimilar properties, for example, hydrophobicity and hydrophilicity, respectively. We investigate the effects of the aspect ratio on the self-assembly morphology and aggregation processes using Monte Carlo simulations. We also discuss certain differences between our results and those of earlier results for Janus spheres. In particular, we find that the size and structure of the aggregate can be controlled by the aspect ratio.     

Ultrasonic welding of CF/PPS composites with integrated triangular energy directors: melting,flow and weld strength development

This paper presents a fully experimental study on melting, flow and weld strength development during ultrasonic welding of CF/PPS composites with integrated triangular energy directors. The main goal of this research was assessing whether the heating time to achieve maximum weld strength could be significantly reduced as compared to ultrasonic welding with flat energy directors. The main conclusion is that, in the specific case under study, the triangular energy directors did heat up, melt and collapse approximately two times faster than the time it took for the flat energy directors to melt and significantly flow. However the heating time needed to achieve maximum weld strength for the integrated triangular energy directors did not differ drastically from that for flat energy directors. This was caused by the fact that a fully welded overlap was not directly achieved right after the collapsing of the triangular energy directors. Instead a solidified resin-rich interface was created which needed to be re-melted as a whole in order to achieve a fully welded overlap and hence maximum weld strength.     

Two Distinct Structures of Membrane‐Associated Homodimers of GTP‐ and GDP‐Bound KRAS4B Revealed by Paramagnetic Relaxation Enhancement

KRAS homo‐dimerization has been implicated in the activation of RAF kinases, however, the mechanism and structural basis remain elusive. We developed a system to study KRAS dimerization on nanodiscs using paramagnetic relaxation enhancement (PRE) NMR spectroscopy, and determined distinct structures of membrane‐anchored KRAS dimers in the active GTP‐ and inactive GDP‐loaded states. Both dimerize through an α4–α5 interface, but the relative orientation of the protomers and their contacts differ substantially. Dimerization of KRAS‐GTP, stabilized by electrostatic interactions between R135 and E168, favors an orientation on the membrane that promotes accessibility of the effector‐binding site. Remarkably, “cross”‐dimerization between GTP‐ and GDP‐bound KRAS molecules is unfavorable. These models provide a platform to elucidate the structural basis of RAF activation by RAS and to develop inhibitors that can disrupt the KRAS dimerization. The methodology is applicable to many other farnesylated small GTPases.     

Composition analysis of an ethylene–propylene block copolymer by fractionation and turbidity measurements at the lower critical solubility gap

The composition of an ethylene-propylene block copolymer with a nominal 15% content of ethylene by weight (EP?B15%) has been investigated through fractionation near the lower critical solution temperature (LCST). Observation of the solution heated above the boiling point of the solvent indicated that some polymer was phase separating, apparently continuously, between the LCSTs of polyethylene (PE) an polypropylene (PP) of similar molecular weight. IR and DSC analysis of three fractions obtained by twice separating the concentrated phase from the dilute phase gave the following result: EP?B15% consists of an ethylene-rich block copolymer (93% E), an EP copolymer of intermediate composition, and a propylene-rich copolymer (94% P). The three fractions constitute respectively 12%, 12%, and 76% of the total weights. In order to choose a suitable temperature for fractionation, a turbidity analysis of the solution of the initial polymer is made continuously during phase separation. The trace of turbidity against temperature shows three peaks of turbidity at temperatures T₀, T₁ and T₂, which can be associated with the above fractions. A mixture of PE, PP, and a 33% E random copolymer gives a turbidity trace with characteristic temperatures very similar to that of EP-B15% in the same solvent. Fractionation from several solvents or mixtures of solvents indicated that the composition of the fractions did not depend significantly on the nature of the solvent. Conditions for obtaining a quantitative analysis of a mixture from a thermogram are discussed. Turbidity analysis during phase separation and fractionation at the LCST can be a useful tool in analyzing and separating complex mixtures before use of powerful analytical techniques such as NMR or IR spectroscopy.