Matrix effects on the multi-collector inductively coupled plasma mass spectrometric analysis of high-precision cadmium and zinc isotope ratios

Resin-derived contaminants added to samples during column chemistry are shown to cause matrix effects that lead to inaccuracy in multi-collector inductively coupled plasma mass spectrometry measurement of small natural variations in Cd and Zn isotopic compositions. These matrix effects were evaluated by comparing pure Cd and Zn standards and standards doped with bulk column blank from the anion exchange chromatography procedure. Doped standards exhibit signal enhancements (Cd, Ag, Zn and Cu), instrumental mass bias changes and inaccurate isotopic compositions relative to undoped standards, all of which are attributed to the combined presence of resin-derived organics and inorganics. The matrix effect associated with the inorganic component of the column blanks was evaluated separately by doping standards with metals at the trace levels detected in the column blanks. Mass bias effects introduced by the inorganic column blank matrix are smaller than for the bulk column blank matrix but can still lead to significant changes in ion signal intensity, instrumental mass bias and isotopic ratios. Chemical treatment with refluxed HNO₃ or HClO₄/HNO₃ removes resin-derived organic components resulting in matrix effects similar in magnitude to those associated with the inorganic component of the column blank.Mass bias correction using combined external normalization-SSB does not correct for these matrix effects because the instrumental mass biases experienced by Cd and Zn are decoupled from those of Ag and Cu, respectively. Our results demonstrate that ion exchange chromatography and associated resin-derived contaminants can be a source of error in MC-ICP-MS measurement of heavy stable element isotopic compositions.     

Stereoselective synthesis of acortatarins a and B

Acortatarins A and B have been synthesized via stereoselective spirocyclizations of glycals. Mercury-mediated spirocyclization of a pyrrole monoalcohol side chain leads to acortatarin A. Glycal epoxidation and reductive spirocyclization of a pyrrole dialdehyde side chain leads to acortatarin B. Acid equilibration and crystallographic analysis indicate that acortatarin B is a contrathermodynamic spiroketal with distinct ring conformations compared to acortatarin A.     

Optimization of Biocompatibility for a Hydrophilic Biological Molecule Encapsulation System

Despite considerable advances in recent years, challenges in delivery and storage of biological drugs persist and may delay or prohibit their clinical application. Though nanoparticle-based approaches for small molecule drug encapsulation are mature, encapsulation of proteins remains problematic due to destabilization of the protein. Reverse micelles composed of decylmonoacyl glycerol (10MAG) and lauryldimethylamino-N-oxide (LDAO) in low-viscosity alkanes have been shown to preserve the structure and stability of a wide range of biological macromolecules. Here, we present a first step on developing this system as a future platform for storage and delivery of biological drugs by replacing the non-biocompatible alkane solvent with solvents currently used in small molecule delivery systems. Using a novel screening approach, we performed a comprehensive evaluation of the 10MAG/LDAO system using two preparation methods across seven biocompatible solvents with analysis of toxicity and encapsulation efficiency for each solvent. By using an inexpensive hydrophilic small molecule to test a wide range of conditions, we identify optimal solvent properties for further development. We validate the predictions from this screen with preliminary protein encapsulation tests. The insight provided lays the foundation for further development of this system toward long-term room-temperature storage of biologics or toward water-in-oil-in-water biologic delivery systems.     

Tailoring peptide conformational space with organic gas modifiers in TIMS-MS

Recently, we showed the advantages of Trapped Ion Mobility Spectrometry for the study of kinetic intermediates of biomolecules as a function of the starting solvent composition (e.g., organic content and pH) and collisional induced activation. In the present work, we further characterize the influence of the bath composition (e.g., organic content) on the conformational space of an intrinsically disordered, DNA binding peptide: AT-hook 3 (Lys-Arg-Pro-Arg-Gly-Arg-Pro-Arg-Lys-Trp). Results show the dependence of the charge state distribution and mobility profiles by doping the solution and the bath gas with organic modifiers (e.g., methanol and acetone). The high resolving power of the TIMS analyzer allowed the separation of multiple IMS band per charge state, and their relative abundances are described as a function of the experimental conditions. The use of gas modifiers resulted in larger inverse  mobilities, with a direct correlation between the size of the modifier and the 1/K₀ differences. Conformational isomer inter-conversion rates were observed as a function of the trapping time. Different from solution experiments, a larger variety of organic gas modifiers can be used to tailor the peptide conformational space, since peptide precipitation is not a problem.     

Micellar affinity gradient focusing: a new method for electrokinetic focusing

This report describes a new method for the concentration and separation of neutral and/or hydrophobic analytes based on a combination of the analytes' electrophoretic mobility, and affinity for partitioning into a micellar phase. Micellar affinity gradient focusing (MAGF) works by creating a gradient in the micellar retention factor. An electric field is applied along the channel to cause the (negatively charged) micelles to move from the region of high retention to the region of low retention, and the mobile phase is forced to move from the region of low retention to the region of high retention. Consequently, the analyte moves into the gradient region from both directions where it is concentrated at a point where its total velocity is zero. Different analytes, which interact differently with the micelles, will have zero total velocity at different points along the gradient, and will thereby be simultaneously concentrated and separated.     

The homologous conjugate addition of thiols to electron-deficient cyclopropanes catalyzed by a calcium(II) complex

The synthesis of γ-sulfanyl malonates was achieved through the addition of thiols to electron deficient cyclopropanes. These reactions are catalyzed by calcium acetylacetonate, Ca(acac)₂. A variety of electron rich and electron deficient thiols were added without the need for prior activation or exogenous base. The thiol additions to donor–acceptor cyclopropanes bearing electron-rich and electron-deficient aromatic and heteroaromatic groups proceeded in good to excellent yields.     

Facile synthesis of phenyl-rich functional siloxanes from simple silanes

Phenyl-rich silicone polymers are used for their excellent thermal properties and high refractive indices. Traditional syntheses of these polymers utilize cationic or anionic equilibration, which limits the molecular weights that can be achieved due, in part, to the coproduction of cyclic monomers that must be removed. Kinetically controlled processes may reduce the impact of these limitations, but require high temperatures, alkyllithium initiators and an inert atmosphere; precise structures are difficult to access. The Piers-Rubinsztajn reaction, combined with hydrolysis, allows the synthesis of highly ordered, Si-H terminated, phenyl-rich silicone homo- and copolymers comprised of phenylmethyl, diphenyl and, dimethylsilicone monomers. The processes are mild and permit a high level of structural control, including alternating copolymers with different levels of phenyl content (Ph/Si = 0.3–1.5) with molecular weights up to ~100 kDa. Yet higher molecular weights could be achieved—M_n up to 300 kDa—when phenyl-rich siloxanes were incorporated into block copolymers with dimethylsilicones (Ph/Si = 0.4). Unlike kinetic processes in which cyclic byproducts are formed by redistribution or backbiting (particularly at high conversion), in this process cyclics form near the onset of the reaction and only with low molecular weight starting materials (< 4 siloxane units).     

Complete disassembly of carbon disulfide by a ditantalum complex

The side-on end-on dinitrogen complex [PhP(CH(2)SiMe(2)NPh)(2)Ta](μ-H)(2)(μ-η(2):η(1)-N(2)) reacts with CS(2) with complete cleavage of both C=S double bonds and the formation of [PhP(CH(2)SiMe(2)NPh)(2)Ta](μ-S)(2)(μ-CH(2)), which has two bridging sulfides and a bridging methylene unit. Further reaction with H(2) produces CH(4) and the disulfide complex.     

Membrane surface dynamics of DNA-threaded nanopores revealed by simultaneous single-molecule optical and ensemble electrical recording

We describe a method for simultaneous single-molecule optical and electrical characterization of membrane-based sensors that contain ion-channel nanopores. The technique is used to study the specific and nonspecific interactions of streptavidin-capped DNA polymers with lipid bilayers composed of diphytanoyl phosphatidylcholine and diphytanoyl phosphatidylglycerol. Biotinylated DNA that is bound to fluorescently labeled streptavidin is electrophoretically driven into, or away from, the lumen of alpha hemolysin (alphaHL) ion channels by an external electric field. Confocal microscopy simultaneously captures single-molecule fluorescence dynamics from the membrane interface at different applied potentials. Fluorescence correlation analysis is used to determine the surface number density and diffusion constant of membrane-associated complexes. The dual optical and electrical approach can detect membrane-associated species at a surface coverage below 10(-5) monolayers of streptavidin, a sensitivity that surpasses most other in vitro surface analysis techniques. By comparing the change in transmembrane current to the number of fluorescent molecules leaving the bilayer when the electrical potential is reversed, we demonstrate the general utility of the approach within the context of nanopore-based sensing and discuss a mechanism by which DNA-streptavidin complexes can be nonspecifically retained at the membrane interface.