Development of a tandem cyclization mediated by samarium(II) iodide: sequential intramolecular conjugate addition/nucleophilic acyl substitution

The development of a one-pot tandem intramolecular conjugate addition/nucleophilic acyl substitution using samarium(II) iodide is reported. The reaction relies on the reagent’s unique ability to mediate both radical and anionic pathways, which are likely integral to the mechanism of this transformation. The tricyclic hemiacetal product was formed in good yield, with excellent diastereoselectivity, and its structure was verified by X-ray crystallographic analysis.     

Electrospray/mass spectrometric identification and analysis of 4-hydroxy-2-alkylquinolines (HAQs) produced by <Emphasis Type="Italic">Pseudomonas aeruginosa</Emphasis>

The opportunistic pathogen Pseudomonas aeruginosa produces a large array of 4-hydroxy-2-alkylquinolines (HAQs). These compounds were analyzed by LC/MS, using positive electrospray ionization, in the culture supernatant of strain PA14. Fifty-six HAQs and related compounds were detected and their [M + H](+) ions were further analyzed by collision induced dissociation (CID). These HAQs were grouped into five different series based on the presence of an hydrogen or hydroxyl group at the 3 position, an N-oxide group in place of the quinoline nitrogen, and an unsaturation on their alkyl side chain. Two new analogs of 3,4-dihydroxy-2 heptylquinoline, the Pseudomonas quinolone signal (PQS), were found with an alkyl chain longer by one and two methylene groups. Moreover, two additional series of compounds were identified in which a saturated or unsaturated alkyl side chain is located at the 3 position along with an hydroxyl group at the 3 position and a ketone at the 2 position. No HAQ N-oxides, nor any compounds from the latter two series, were detected in a pqsL mutant derivative of PA14, indicating that this gene is involved in the biosynthesis of these compounds. This work demonstrates the large repertoire of HAQ and HAQ-related compounds produced by P. aeruginosa, and provides insight into N-oxides biosynthesis and confirm the hypothesis that N-oxides are the precursors of compounds from Series 6 and 7.     

Oligodeoxyfluorosides: Strong Sequence Dependence of Fluorescence Emission

We describe the properties of a series of oligomeric polyfluorophores assembled on the DNA backbone. The 11 oligomers (oligodeoxyfluorosides, ODFs), 4-7 monomers in length, were composed of only two fluorescent monomers and a spacer in varied sequences, and were designed to test how fluorescent nucleobases can interact electronically to yield complexity in fluorescence emission. The monomer fluorophores were deoxyribosides of pyrene and perylene, which emit light in violet and blue wavelengths, respectively. The experiments show that simple variation in sequence and spacing can dramatically change fluorescence, yielding emission maxima ranging from 380 to 557 nm and visible colors from violet to orange-red. Fluorescence lifetime data, excitation spectra, and absorption data point to a number of multi-fluorophore electronic interactions, including pyrene-pyrene and perylene-perylene excimers, pyrene-perylene exciplexes, as well as monomer dye emissions, contributing to the final spectral outcomes. Thus, two simple fluorophores can be readily combined to give emissions over much of the visible spectrum, all requiring only a single excitation. The results demonstrate that fluorescent nucleobases in oligomeric form can act cooperatively as electronic units, and that fluorophore sequence in such oligomers is as important as fluorophore composition in determining fluorescence properties.     

Toward understanding the ionization of biomarkers from micrometer particles by bio-aerosol mass spectrometry

The appearance of informative signals in the mass spectra of laser-ablated bio-aerosol particles depends on the effective ionization probabilities (EIP) of individual components during the laser ionization process. This study investigates how bio-aerosol chemical composition governs the EIP values of specific components and the overall features of the spectra from the bio-aerosol mass spectrometry (BAMS). EIP values were determined for a series of amino acid, dipicolinic acid, and peptide aerosol particles to determine what chemical features aid in ionization. The spectra of individual amino acids and dipicolinic acid, as well as mixtures, were examined for extent of fragmentation and the presence of molecular ion dimers, which are indicative of ionization conditions. Standard mixtures yielded information with respect to the significance of secondary ion plume reactions on observed spectra. A greater understanding of how these parameters affect EIP and spectra characteristics of bio-aerosols will aid in the intelligent selection of viable future biomarkers for the identification of bio-terrorism agents.     

Supplementary d and f functions in molecular wave functions: Optimum and nonoptimum exponents

Energy optimization calculations have been carried out to determine the variability of optimum p, d, and f polarization function exponents in molecules containing first- and second-row elements and in normal valency and hypercoordinate species. Optimum exponents were determined for single sets of higher-order functions at both Hartree–Fock and correlated (Moller–Plesset) levels of theory using the Dunning–Hay double zeta and the McLean–Chandler triple zeta basis sets. More detailed calculations were used to test the response to nonoptimum d and f function exponents of the total energy, the optimum geometry, and harmonic stretching frequencies. The variability in optimum exponents and the size of the energy penalties incurred by adopting nonoptimum values reduces the utility of standard exponents for p, d, and f polarization functions. © 1993 John Wiley & Sons, Inc.     

Evidence for a monomeric structure of nonribosomal Peptide synthetases

Nonribosomal peptide synthetases (NRPS) are multimodular biocatalysts that bacteria and fungi use to assemble many complex peptides with broad biological activities. The same modular enzymatic assembly line principles are found in fatty acid synthases (FAS), polyketide synthases (PKS), and most recently in hybrid NRPS/PKS multienzymes. FAS as well as PKS are known to function as homodimeric enzyme complexes, raising the question of whether NRPS may also act as homodimers. To test this hypothesis, biophysical methods (size exclusion chromatography, analytical equilibrium ultracentrifugation, and chemical crosslinking) and biochemical methods (two-affinity-tag-system and complementation studies with enzymes being inactivated in different catalytic domains) were applied to NRPS subunits from the gramicidin S (GrsA-ATE), tyrocidine (TycB(1)-CAT and TycB(2-3)-AT.CATE), and enterobactin (EntF-CATTe) biosynthetic systems. These methods had revealed the dimeric structure of FAS and PKS previously, but all three NRPS systems investigated are functionally active as monomers.     

Studies on the preparation and exchange reactions of 5-deuterated uracils

The mechanism of base catalyzed proton exchange at the 5-position of uracil and its N-methylated derivatives has been studied. These reactions proceed by addition — elimination across the 5,6-double bond when the 1-nitrogen is substituted with a methyl group, or with anchimeric assistance of the N-1 anion if the 1-position is unsubstituted. The base catalyzed hydrolyses of 1,3-dimethyluracil and 3-methyluracil also appear to proceed through hydrated intermediates. A facile method for an acid catalyzed preparation of 5-deuterated uracils is described as well as a simple and accurate method for analysis of deuterium content.     

Low-Field NMR Relaxation Analysis of High-Pressure Ethane Adsorption in Mesoporous Silicas

Understanding the behaviour of short-chain hydrocarbons confined to porous solids informs the targeted extraction of natural resources from geological features, and underpins rational developments in separation, storage and catalytic conversion processes. Herein, we report the application of low-field (12.7 MHz) ¹H nuclear magnetic resonance (NMR) relaxation measurements to characterise ethane dynamics within mesoporous silica materials exhibiting mean pore diameters between 6 and 50 nm. Our measurements provide NMR-based adsorption isotherms within the range 25–50 bar and at ambient temperature, incorporating the ethane condensation point (40.7 bar at our experimental temperature of 23.6 °C). The quantitative nature of the acquired data is validated via a direct comparison of NMR-derived excess adsorption capacities with ex situ gravimetric ethane adsorption measurements, which are demonstrated to agree to within 0.2 mmol g⁻¹ of the observed ethane capacity. NMR relaxation time distributions are further demonstrated as a means to decouple interparticle and mesopore dominated adsorption phenomena, with unexpectedly rapid relaxation rates associated with interparticle ethane gas confirmed via a direct comparison with NMR self-diffusion analysis.     

Regioselective synthesis of the bridged tricyclic core of Garcinia natural products via intramolecular aryl acrylate cycloadditions

[reaction: see text] Two different routes to the tricyclic core of Garcinia-derived natural products are described. The first approach is based on a tandem Claisen/Diels-Alder rearrangement and delivers the desired lactone 14. The second approach, employing a Wessely oxidation/Diels-Alder protocol, leads to the same caged heterocycle, albeit with modified constitution.