The Variety of Thermal Pericyclic Reactions

The six-electron thermal pericyclic reactions are examined systematically as to the number and kind which are possible by varying both the σ shell and the combinations of different atoms in all orientations, on both a six-atom and a five-atom framework. A simple unifying nomenclature is offered for these reactions, which number in the thousands. Further, in order to comprehend this very large number of possible reactions, they are also organized systematically in terms of their value for basic synthesis operations: construction, elimination, refunctionalization, etc. The methodology is aimed at providing a basis of selection for the invention of useful new reactions. A discussion of reaction energetics leads also to an analysis of molecular features which can facilitate reaction.     

Supercharacter Theory Constructions Corresponding to Schur Ring Products

Diaconis and Isaacs have defined the supercharacter theories of a finite group to be certain approximations to the ordinary character theory of the group [7 Diaconis , P. , Isaacs , I. M. ( 2008 ). Supercharacters and superclasses for algebra groups . Trans. Amer. Math. Soc. 360 : 2359 – 2392 .[Crossref], [Web of Science ®] , [Google Scholar]]. We make explicit the connection between supercharacter theories and Schur rings, and we provide supercharacter theory constructions which correspond to Schur ring products of Leung and Man [12 Leung , K. H. , Man , S. H. ( 1996 ). On Schur rings over cyclic groups, II . J. Algebra 183 : 273 – 285 .[Crossref], [Web of Science ®] , [Google Scholar]], Hirasaka and Muzychuk [10 Hirasaka , M. , Muzychuk , M. ( 2001 ). An elementary abelian group of rank 4 is a CI-group . J. Combin. Theory Ser. A 94 : 339 – 362 .[Crossref], [Web of Science ®] , [Google Scholar]], and Tamaschke [20 Tamaschke , O. ( 1970 ). On Schur-rings which define a proper character theory on finite groups . Math. Z. 117 : 340 – 360 .[Crossref], [Web of Science ®] , [Google Scholar]].     

Heterometallic integer-spin analogues of S = 9/2 Mn4 cubane single-molecule magnets

A family of distorted heterometallic cubanes, [Mn (III) 3Ni (II)(hmp) 3O(N 3) 3(O 2CR) 3], where O 2CR (-) is benzoate ( 1), 3-phenylpropionate ( 2), 1-adamantanecarboxylate ( 3), or acetate ( 4) and hmp (-) is the anion of 2-pyridinemethanol, was synthesized and structurally as well as magnetically characterized. These complexes have a distorted-cubane core structure similar to that found in the S = 9/2 Mn 4 cubane family of complexes. Complexes 1, 3, and 4 crystallize in rhombohedral, hexagonal, and cubic space groups, respectively, and have C 3 molecular symmetry, while complex 2 crystallizes in the monoclinic space group Cc with local C 1 symmetry. Magnetic susceptibility and magnetization hysteresis measurements and high-frequency electron paramagnetic resonance (HFEPR) spectroscopy established that complexes 1-4 have S = 5 spin ground states with axial zero-field splitting (ZFS) parameters ( D) ranging from -0.20 to -0.33 cm (-1). Magnetization versus direct-current field sweeps below 1.1 K revealed hysteresis loops with magnetization relaxation, definitely indicating that complexes 1-4 are single-molecule magnets that exhibit quantum tunneling of magnetization (QTM) through an anisotropy barrier. Complex 2 exhibits the smallest coercive field and fastest magnetization tunneling rate, suggesting a significant rhombic ZFS parameter ( E), as expected from the low C 1 symmetry. This was confirmed by HFEPR spectroscopy studies on single crystals that gave the following parameter values for complex 2: gz = 1.98, gx = gy = 1.95, D = -0.17 cm (-1), B 4 (0) = -6.68 x 10 (-5) cm (-1), E = 6.68 x 10 (-3) cm (-1), and B 4 (2) = -1.00 x 10 (-4) cm (-1). Single-crystal HFEPR data for complex 1 gave g z = 2.02, gx = gy = 1.95, D = -0.23 cm (-1), and B 4 (0) = -5.68 x 10 (-5) cm (-1), in keeping with the C 3 site symmetry of this Mn 3Ni complex. The combined results highlight the importance of spin-parity effects and molecular symmetry, which determine the QTM rates.     

Gas phase activation energy for unimolecular dissociation of biomolecular ions determined by focused RAdiation for gaseous multiphoton ENergy transfer (FRAGMENT)

We present a novel approach for the determination of activation energy for the unimolecular dissociation of a large (>50 atoms) ion, based on measurement of the unimolecular dissociation rate constant as a function of continuous-wave CO(2) laser intensity. Following a short ( approximately 1 s) induction period, CO(2) laser irradiation produces an essentially blackbody internal energy distribution, whose 'temperature' varies inversely with laser intensity. The only currently available method for measuring such activation energies is blackbody infrared radiative dissociation (BIRD). Compared with BIRD, FRAGMENT: (a) eliminates the need to heat the surrounding ion trap and vacuum chamber to each of several temperatures (each requiring hours for temperature equilibration); (b) offers a three-fold wider range of effective blackbody temperature; and (c) extends the range of applications to include initially cold ions (e.g., gas-phase H/D exchange). Our FRAGMENT-determined activation energy for dissociation of protonated bradykinin, 1.2 +/- 0.1 eV, agrees within experimental error to the value, 1.3 +/- 0.1 eV, previously reported by Williams et al. from BIRD experiments. Copyright 1999 John Wiley & Sons, Ltd.     

Charge reduction lowers mass resolving power for isotopically resolved electrospray ionization Fourier transform ion cyclotron resonance mass spectra

Electrospray ionization of synthetic or biological macromolecules above ∼1–2 kDa in mass typically produces ions of multiple charge states. Several recent papers have illustrated charge reduction as a means to simplify low-resolution electrospray ionization mass spectra, at the cost of significant loss in signal-to-noise ratio. However, if mass resolving power is sufficiently high (as in Fourier transform ion cyclotron resonance mass spectrometry) to resolve the heavy-atom isotopic distribution, then charge reduction actually lowers mass resolving power by a factor proportional to the ion charge. For proteins or nucleic acids of 10–50 kDa in mass, reducing the charge state to unity thus lowers mass resolving power by a factor of 10–50. In other words, as long as it is possible to resolve the isotopic distributions, charge reduction has no advantages for electrospray ionization mass spectrometry and has the very serious disadvantage of greatly degraded mass resolving power. Copyright © 2001 John Wiley & Sons, Ltd.     

Shear layer instabilities in low-density transverse jets

The near-field shear layer instabilities forming in round transverse jets of variable (reduced) densities relative to the crossflow are investigated through gas-phase experiments. Jets composed of helium and nitrogen mixtures are injected from a converging nozzle mounted flush with an injection wall into air crossflow, allowing the jet-to-crossflow density ratio S to be varied between 1.00, the equidensity case, and 0.14, at constant jet Reynolds number Re _j ?=?1,800. Jet-to-crossflow momentum flux ratios J are examined in the range $\infty>J\geq5$ at incremental values of the density ratio S. The results of single-component hotwire measurements in the jet shear layer indicate that a transition to global instability likely occurs as J is brought below approximately 10, and/or as S is brought below approximately 0.45?C0.40. This observation appears to link many previous independent studies of both equidensity transverse jets and low-density free jets, which may become globally unstable under alteration of J and S, respectively. However, the dynamical character of the transition to global instability in the low-density transverse jet displays differences under independent variation of J and S, which may indicate the predominance of different modes.     

Unequivocal determination of metal atom oxidation state in naked heme proteins: Fe(III)myoglobin, Fe(III)cytochrome c, Fe(III)cytochrome b5, and Fe(III)cytochrome b5 L47R

Unambiguous determination of metal atom oxidation state in an intact metalloprotein is achieved by matching experimental (electrospray ionization 9.4 tesla Fourier transform ion cyclotron resonance) and theoretical isotopic abundance mass distributions for one or more holoprotein charge states. The ion atom oxidation state is determined unequivocally as Fe(III) for each of four gas-phase unhydrated heme proteins electrosprayed from H2O: myoglobin, cytochrome c, cytochrome b5, and cytochrome b5 L47R (i.e., the solution-phase oxidation state is conserved following electrospray to produce gas-phase ions). However, the same Fe(III) oxidation state in all four heme proteins is observed after prior reduction by sodium dithionite to produce Fe(II) heme proteins in solution: thus proving that oxygen was present during the electrospray process. Those results bear directly on the issue of similarity (or lack thereof) of solution-phase and gas-phase protein conformations. Finally, infrared multiphoton irradiation of the gas-phase Fe(III)holoproteins releases Fe(III)heme from each of the noncovalently bound Fe(III)heme proteins (myoglobin, cytochrome b5 and cytochrome b5 L47R), but yields Fe(II)heme from the covalently bound heme in cytochrome c.     

Stable isotope incorporation triples the upper mass limit for determination of elemental composition by accurate mass measurement

By comparing electrospray ionization Fourier-transform ion cyclotron resonance (FT-ICR) mass spectra and collision-induced dissociation (CID) FT-ICR mass spectra of a phospholipid (851 Da) extracted from natural abundance and 99% 13C bacterial growth media, we are able to reduce its number of possible elemental compositions (based on +/-10 ppm externally calibrated mass accuracy and biologically relevant compositional constraints) from 394 to 1. The basic idea is simply that the mass of a molecule containing N carbon atoms increases by N Da when 12C is replaced by 13C. Once the number of carbons is known, the number of possible combinations of other atoms in the molecule is greatly reduced. We demonstrate the method for a stored-waveform inverse Fourier transform-isolated phospholipid from an extract of membrane lipids from Rhodococcus rhodochrous hydrocarbon-degrading bacteria grown on either natural abundance or 99% 13C-enriched mixtures of n-hexadecane and n-octadecane. We project that this method raises the upper mass limit for unique determination of elemental composition from accurate mass measurement by a factor of at least 3, thereby extending "chemical formula" determination to identification and sequencing of larger synthetic and bio-polymers: phospholipids, oligopeptides of more than three to four amino acids, DNA or RNA of more than two nucleotides, oligosaccharides of more than three sugars, etc. The method can also be extended to determination of the number of other atoms for which heavy isotopes are available (e.g., 15N, 34S, 18O, etc.).