Generation and spectroscopic identification of selenofulminic acid and its methyl and cyano derivatives (XCNSe, X=H, CH3, NC)

Evidence for the existence of nitrile selenides, potential 1,3-dipolarophiles in cycloaddition reactions, has been provided by direct spectroscopic methods. The parent nitrile selenide, selenofulminic acid (HCNSe), and its methyl and cyano derivatives have been photolytically generated in an inert solid argon matrix from 1,2,5-selenadiazoles by 280, 254, and 313 nm UV irradiation, respectively, and studied by ultraviolet spectroscopy and mid-infrared spectroscopy. Ground-state geometries have been obtained from quantum-chemical calculations at the CCSD(T)/aug-cc-pVTZ level. Nitrile selenides are predicted to be linear with a relatively weak N-Se bond.     

Stereocontrolled Synthesis of Functionalized Azaheterocycles from Carbocycles through Oxidative Ring Opening/Reductive Ring Closing Protocols

Fluorine‐containing organic scaffolds are of significant interest in medicinal chemistry. The incorporation of fluorine into biomolecules can lead to remarkable changes in their physical, chemical, and biological properties. There are already many drugs on the market, which contain at least one fluorine atom. Saturated functionalized azaheterocycles as bioactive substances have gained increasing attention in pharmaceutical chemistry. Due to the high biorelevance of organofluorine molecules and the importance of N‐heterocyclic compounds, selective stereocontrolled procedures to the access of new fluorine‐containing saturated N‐heterocycles are considered to be a hot research topic. This account summarizes the synthesis of functionalized and fluorine‐containing saturated azaheterocycles starting from functionalized cycloalkenes and based on oxidative ring cleavage of diol intermediates followed by ring expansion with reductive amination.     

Chiral separation of deprenyl-N-oxide isomers by capillary electrophoresis using various cyclodextrin derivatives

Chiral separation of deprenyl-N-oxide isomers is presented using capillary electrophoresis in the presence of various cyclodextrin (CD) derivatives. This recently identified metabolite of R-(-)-deprenyl may possess desirable pharmacological activities. The effect of the cavity size and the substituents of the CD are examined on the enantiomer resolution of the compound having an asymmetric center on a heteroatom. The importance of hydrophilic or hydrogen bonding interaction, as well as the position of the interacting groups is demonstrated. Outstanding selectivity and resolution values are achieved using the chargeable carboxymethyl-beta-CD. 2-Hydroxypropyl-beta-CD is also suitable for the enantiomer separation of the analyte. Native beta-CD and carboxyethyl-beta-CD provide only poor enantioselectivity, whereas heptakis-(2,6-di-O-methyl)-beta-CD is capable of separating only the diastereomers. No chiral resolution can be observed in the presence of gamma-CD.     

Application of Metathesis Protocols to the Stereocontrolled Synthesis of some Functionalized β-Amino Esters and Azaheterocycles

In this account our aim was to give an insight into the application of metathesis protocols (ROM, RCM, RCEYM, CM, RRM) for the synthesis of various azaheterocyclic frameworks. Due to the high biological potential and importance in peptide chemistry and drug design of β-amino acids our intention is to give a highlight on the synthetic procedures and transformation of these class of compounds with the above-mentioned metathesis strategies with emphasis on selectivity, stereocontrol, substrate-directing effect or functional group tolerance.     

13C dynamic nuclear polarization using isotopically enriched 4‐oxo‐TEMPO free radicals

The nitroxide‐based free radical 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) is a widely used polarizing agent in NMR signal amplification via dissolution dynamic nuclear polarization (DNP). In this study, we have thoroughly investigated the effects of ¹⁵N and/or ²H isotopic labeling of 4‐oxo‐TEMPO free radical on ¹³C DNP of 3 M [1‐¹³C] sodium acetate samples in 1 : 1 v/v glycerol : water at 3.35 T and 1.2 K. Four variants of this free radical were used for ¹³C DNP: 4‐oxo‐TEMPO, 4‐oxo‐TEMPO‐¹⁵N, 4‐oxo‐TEMPO‐d₁₆ and 4‐oxo‐TEMPO‐¹⁵N,d₁₆. Our results indicate that, despite the striking differences seen in the electron spin resonance (ESR) spectral features, the ¹³C DNP efficiency of these ¹⁵N and/or ²H‐enriched 4‐oxo‐TEMPO free radicals are relatively the same compared with ¹³C DNP performance of the regular 4‐oxo‐TEMPO. Furthermore, when fully deuterated glassing solvents were used, the ¹³C DNP signals of these samples all doubled in the same manner, and the ¹³C polarization buildup was faster by a factor of 2 for all samples. The data here suggest that the hyperfine coupling contributions of these isotopically enriched 4‐oxo‐TEMPO free radicals have negligible effects on the ¹³C DNP efficiency at 3.35 T and 1.2 K. These results are discussed in light of the spin temperature model of DNP. Copyright © 2016 John Wiley & Sons, Ltd.     

Evaluation and optimization of electron capture dissociation efficiency in fourier transform ion cyclotron resonance mass spectrometry

Electron capture dissociation (ECD) efficiency has typically been lower than for other dissociation techniques. Here we characterize experimental factors that limit ECD and seek to improve its efficiency. Efficiency of precursor to product ion conversion was measured for a range of peptide (∼15% efficiency) and protein (∼33% efficiency) ions of differing sizes and charge states. Conversion of precursor ions to products depends on electron irradiation period and maximizes at ∼5–30 ms. The optimal irradiation period scales inversely with charge state. We demonstrate that reflection of electrons through the ICR cell is more efficient and robust than a single pass, because electrons can cool to the optimal energy for capture, which allows for a wide range of initial electron energy. Further, efficient ECD with reflected electrons requires only a short (∼500 μs) irradiation period followed by an appropriate delay for cooling and interaction. Reflection of the electron beam results in electrons trapped in or near the ICR cell and thus requires a brief (∼50 μs) purge for successful mass spectral acquisition. Further electron irradiation of refractory precursor ions did not result in further dissociation. Possibly the ion cloud and electron beam are misaligned radially, or the electron beam diameter may be smaller than that of the ion cloud such that remaining precursor ions do not overlap with the electron beam. Several ion manipulation techniques and use of a large, movable dispenser cathode reduce the possibility that misalignment of the ion and electron beams limits ECD efficiency.     

Certification of standard reference material (SRM) 1941a, organics in marine sediment

SRM 1941a, Organics in Marine Sediment, has been recently issued with certified concentrations for 23 polycyclic aromatic hydrocarbons, 21 polychlorinated biphenyl congeners, 6 chlorinated pesticides, and sulfur. Noncertified concentrations have been also reported for additional PAHs, PCB congeners, and chlorinated pesticides and for percent total organic carbon (TOC), aliphatic hydrocarbons, and trace elements. SRM 1941a is the most extensively characterized natural matrix SRM issued by the National Institute of Standards and Technology (NIST).     

Chemoselective,Substrate‐directed Fluorination of Functionalized Cyclopentane β‐Amino Acids

This work describes a substrate‐directed fluorination of some highly functionalized cyclopentane derivatives. The cyclic products incorporating CH₂F or CHF₂ moieties in their structure have been synthesized from diexo‐ or diendo‐norbornene β‐amino acids following a stereocontrolled strategy. The synthetic study was based on an oxidative transformation of the ring carbon–carbon double bond of the norbornene β‐amino acids, followed by transformation of the resulted ?all cis“ and ?trans“ diformyl intermediates by fluorination with ?chemodifferentiation“.     

Determining secondary structure in spider dragline silk by carbon-carbon correlation solid-state NMR spectroscopy

Two-dimensional (2D) (13)C-(13)C NMR correlation spectra were collected on (13)C-enriched dragline silk fibers produced from Nephila clavipes spiders. The 2D NMR spectra were acquired under fast magic-angle spinning (MAS) and dipolar-assisted rotational resonance (DARR) recoupling to enhance magnetization transfer between (13)C spins. Spectra obtained with short (150 ms) recoupling periods were utilized to extract distinct chemical shifts for all carbon resonances of each labeled amino acid in the silk spectra, resulting in a complete resonance assignment. The NMR results presented here permit extraction of the precise chemical shift of the carbonyl environment for each (13)C-labeled amino acid in spider silk for the first time. Spectra collected with longer recoupling periods (1 s) were implemented to detect intermolecular magnetization exchange between neighboring amino acids. This information is used to ascribe NMR resonances to the specific repetitive amino acid motifs prevalent in spider silk proteins. These results indicate that glycine and alanine are both present in two distinct structural environments: a disordered 3(1)-helical conformation and an ordered beta-sheet structure. The former can be ascribed to the Gly-Gly-Ala motif while the latter is assigned to the poly(Ala) and poly(Gly-Ala) domains.