Buffer Standards for the Physiological pH of the Zwitterionic Compound, HEPPSO from 5 to 55 °C

The values of the thermodynamic second dissociation constant, pK ₂, and related thermodynamic quantities of N-(2-hydroxyethyl)piperazine-N′-2-hydroxypropanesulfonic acid (HEPPSO) have already been reported from 5 to 55?°C, including 37?°C, by the emf method. This paper reports the results for the pH of one chloride-free buffer solution containing the composition: (a) HEPPSO (0.08 mol?kg^?1)+NaHEPPSO (0.04 mol?kg^?1). The remaining seventeen buffer solutions contain a saline medium of ionic strength I=0.16 mol?kg^?1, matching closely that of physiological fluids. Conventional pH values, denoted as pa _H, for all eighteen buffer solutions from 5 to 55?°C have been calculated. The operational pH values, designated as pH, with residual liquid-junction corrections for five buffer solutions, one without NaCl, and four with buffer solutions in saline media of I=0.16 mol?kg^?1 are recommended as pH standards in the range of physiological application. These are based on the NBS/NIST standard scale for pH measurements.     

Quantification of Active Sites and Elucidation of the Reaction Mechanism of the Electrochemical Nitrogen Reduction Reaction on Vanadium Nitride

Despite recent intense interest in the development of catalysts for the electrochemical nitrogen reduction reaction (ENRR), mechanistic understanding and catalyst design principles remain lacking. In this work, we develop a strategy to determine the density of initial and steady‐state active sites on ENRR catalysts that follow the Mars–van Krevelen mechanism via quantitative isotope‐exchange experiments. This method allows the comparison of intrinsic activities of active sites and facilitates the identification and improvement of active‐site structures for ENRR. Combined with detailed density functional theory calculations, we show that the rate‐limiting step in the ENRR is likely the initial N≡N bond activation via the addition of a proton and an electron to the adsorbed N₂ on the N vacancies to form N₂H. The methodology developed and mechanistic insights gained in this work could guide the rational catalyst design in the ENRR.     

Understanding the pH Dependence of Underpotential Deposited Hydrogen on Platinum

Understanding the pH dependent shift of the oxidation peak of the underpotential deposited hydrogen (H_upd) in cyclic voltammograms on the Pt surface is of significance in terms of both the fundamentals of electrochemistry and the rational design of catalysts for the hydrogen oxidation/evolution reactions (HOR/HER). In this work, we provide compelling evidence that the pH dependent shift in the H_upd peak on Pt surfaces is driven by the structure of interfacial water rather than the specific adsorption of cations on the electrode surface. Combined cyclic voltammetric and surface enhanced spectroscopic investigations using an organic cation and crown‐ether chelated alkali metal cations show that specific adsorption of metal and organic cations on the Pt surface at the conditions relevant to the HOR/HER is unlikely. The vibrational band corresponding to strongly bound water is monitored when the electrode potential is varied in the H_upd range in both acid and base.     

Fourier transform ion cyclotron resonance mass spectrometry in a high homogeneity 25 tesla resistive magnet

Many performance parameters of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry improve dramatically with increasing magnetic field. Our prior results from a 20 tesla resistive magnet showed that performance was limited by the large spatial inhomogeneity in spite of the high field. In this paper, we compare matrix-assisted laser desorption/ionization (MALDI) mass spectra at the same magnetic field for two resistive magnets with different field spatial homogeneity. In addition, we report MALDI spectra at 25 tesla—the highest magnetic field for FT-ICR to date. The first broadband FT-ICR mass spectrum [poly(ethylene glycol) 2000] from a resistive magnet is accurately fitted by the standard ICR mass calibration function.     

Review: Veratrum californicum Alkaloids

Veratrum spp. grow throughout the world and are especially prevalent in high mountain meadows of North America. All parts of Veratrum plants have been used for the treatment of ailments including injuries, hypertension, and rheumatic pain since as far back as the 1600s. Of the 17–45 Veratrum spp., Veratrum californicum alkaloids have been proven to possess favorable medicinal properties associated with inhibition of hedgehog (Hh) pathway signaling. Aberrant Hh signaling leads to proliferation of over 20 cancers, including basal cell carcinoma, prostate and colon among others. Six of the most well-studied V. californicum alkaloids are cyclopamine (1), veratramine (2), isorubijervine (3), muldamine (4), cycloposine (5), and veratrosine (6). Recent inspection of the ethanolic extract from V. californicum root and rhizome via liquid chromatography–mass spectrometry has detected up to five additional alkaloids that are proposed to be verazine (7), etioline (8), tetrahydrojervine (9), dihydrojervine (10), 22-keto-26-aminocholesterol (11). For each alkaloid identified or proposed in V. californicum, this review surveys literature precedents for extraction methods, isolation, identification, characterization and bioactivity to guide natural product drug discovery associated with this medicinal plant.     

Vinylazaarenes as dienophiles in Lewis acid-promoted Diels–Alder reactions

Described are the first examples of Lewis acid-promoted Diels–Alder reactions of vinylpyridines and other vinylazaarenes with unactivated dienes. Cyclohexyl-appended azaarenes constitute a class of substructures of rising prominence in drug discovery. Despite this, thermal variants of the vinylazaarene Diels–Alder reaction are rare and have not been adopted for synthesis, and Lewis acid-promoted variants are virtually unexplored. The presented work addresses this gap and in the process furnishes increased scope, dramatically higher yields, improved regioselectivity, and high levels of diastereoselectivity compared to prior thermal examples. These reactions provide scalable access to druglike scaffolds not readily available through other methods. More broadly, these studies establish a useful new class of dienophiles that, based on preliminary mechanistic studies, should be amenable to conventional strategies for enantioselective catalysis.

Vinyl-substituted azaarenes are rare and challenging substrates as dienophiles in Diels–Alder reactions; by employing Lewis acid activation, high yielding and highly selective cycloadditions with unactivated dienes are enabled.     

OvoAMtht from Methyloversatilis thermotolerans ovothiol biosynthesis is a bifunction enzyme: thiol oxygenase and sulfoxide synthase activities

Mononuclear non-heme iron enzymes are a large class of enzymes catalyzing a wide-range of reactions. In this work, we report that a non-heme iron enzyme in Methyloversatilis thermotolerans, OvoA_Mtht, has two different activities, as a thiol oxygenase and a sulfoxide synthase. When cysteine is presented as the only substrate, OvoA_Mtht is a thiol oxygenase. In the presence of both histidine and cysteine as substrates, OvoA_Mtht catalyzes the oxidative coupling between histidine and cysteine (a sulfoxide synthase). Additionally, we demonstrate that both substrates and the active site iron''s secondary coordination shell residues exert exquisite control over the dual activities of OvoA_Mtht (sulfoxide synthase vs. thiol oxygenase activities). OvoA_Mtht is an excellent system for future detailed mechanistic investigation on how metal ligands and secondary coordination shell residues fine-tune the iron-center electronic properties to achieve different reactivities.

Modulation of OvoA_Mtht''s dual activities: sulfoxide synthase and thiol oxygenase.     

Ion-Tagged Oligonucleotides Coupled with a Magnetic Liquid Support for the Sequence-Specific Capture of DNA

The isolation of specific nucleic acid sequences is a major bottleneck in molecular diagnostics. Magnetic beads/particles are typically used as solid supports for the capture of DNA targets to improve sample throughput but aggregate over time resulting in lower capture efficiency and obstruction of liquid handling devices. Herein, we describe a particle-free approach to sequence-specific DNA extraction using a magnetic liquid support and ion-tagged oligonucleotide (ITO) probes. ITO conjugates were synthesized with the highest yields ever achieved for the radical thiol-ene coupling of a substrate and oligonucleotide. In addition to distinguishing nucleotide mismatches, the ITO and magnetic liquid-based approach was more sensitive than a commercial magnetic bead-based method for the capture of target DNA from a pool of interfering genomic DNA.     

Interaction between two active structural paths for source mass motion control over mid-frequency range

The interaction between two active structural paths is analytically and experimentally studied as part of a resonating source-path-receiver system, where each path consists of a piezoelectric stack actuator in series with an elastomeric (passive) mount. An analytical model of the system is first developed, and then an experiment is constructed to verify the feasibility. Good agreement is found between the model and experiment. A performance index to characterize the active path interaction for source mass motion control up to 1000 Hz is analytically defined; it considers the passive phase interaction (caused by system dynamics) between the active mounts and the resulting system motion. Two passive system parameters (rubber path structural damping and disturbance moment arm) emerge as key design variables that drastically change the performance index, and guidelines are developed for desirable path interactions. Limited experimental validation demonstrates that active source mass motion control is achieved at 400 Hz using piezoelectric stack actuators.