1,721,084 research outputs found
A class-modelling approach for detecting neoplasy in human bladder tissue analysed by DESI-MS hyperspectral imaging
No full textInteractive hyperspectral approach for exploring and interpreting DESI-MS images of cancerous and normal tissue sections
No full textInteractive hyperspectral approach for exploring and interpreting DESI-MS images of cancerous and normal tissue sections
No full textInteractive hyperspectral approach for exploring and interpreting DESI-MS images of cancerous and normal tissue sections
No full textRelative Cyanide Cation (+CN) Affinities of Pyridines Determined by the Kinetic Method Using Multiple-stage (MS3) Mass Spectrometry
No full textIon-molecule reactions occurring in a pentaquadrupole mass spectrometer are used to generate and characterize
ions in which one or two pyridine molecules are bound by a +CN cation. Cyanide cation binds strongly to the
nitrogen atom of pyridine to generate a mono-adduct, which undergoes pyridine exchange reactions and from which
one can generate the dipyridine adduct in low abundance. The dimeric ions have two structures, loosely bound and
covalently bound, and both fragment to yield the constituent cyanide-bound monomers. In the case of dimers
comprised of meta-substituted alkylpyridines, there is a quantitative correlation between relative cyanide cation
affinity, as measured using the kinetic method, and literature values of relative proton affinities. These dimers
fragment analogously to the corresponding H+- and CI+- bound dimers, and on this basis are assigned analogous
structures, viz. the loosely bound form Py ,-' CN-Py,. Semi-empirical molecular orbital calculations show that
both pyridines are bound to the carbon atom of the cyanide cation. Making the assumption that the effective
temperatures of the activated cyanide-bound dimers are similar to those of the corresponding CI+- and H+-bound
dimers, relative *CN cation affinities are estimated to be 1.5 kcal mol-' (3-MePy), 1.7 kcal mol-' (CMePy), 2.6
kcal mol-' (3-EtPy), 3.5 kcal mol-' (3-n-BuPy) and 3.6 kcal mol-' (3,ldiMePy), all expressed relative to
pyridine (1 kcal = 4.184 kJ). A linear relationship between the relative +CN affinity and relative proton affinity
(PA) is derived as A +CN affinity (kcal mol-I) = 0.78 (APA), with the assumption that the 'CN dimer effective
temperature is 600 K. The estimated uncertainty is 0.5 kcal mol-'. Relative +CN affinities of pairs of pyridines
are smaller by ca. 1 kcal mol-' than the corresponding CI+ affinities. Dimers in which one of the pyridines is
meta-chlorine- or para-alkyl-substituted have the covalently bound, ring-carbon-substituted structure, in which the
'CN group is attached to the pyridine nitrogen and the second pyridine molecule is bound to a ring carbon. The
fragmentation of these isomeric dimers yields the corresponding monomers, in addition to other minor ions, but the
distribution of the cyanide cation between the two pyridines does not correlate with CI' affinity or proton affinity.
In the special cases of the 3-methylpyridine3-n-butylpyridine and the 4-methylpyridine-pyridine cyanide cation
adducts, both the loosely bound dimer and the covalently bound adduct are generated and distinguished by their
fragmentation behavior. Evidence for the formation of the covalently bound, ring-carbon-substituted structure was
also obtained in semi empirical AM1 calculations
Identification of neoplastic condition in biopsied human tissues by supervised analysis of hyperspectral DESI-MS images
No full textElectrophilic bromination of gaseous aromatic compounds: Mechanism and linear free energy effects on reaction rates
No full textElectrophilic bromination of monosubstituted aromatic compounds is effected in a pentaquadrupole mass spectrometer using BrCO+ and CH3NH2Br+ as mass‐selected reagent ions. Reaction normally occurs at the ring and the brominated product can be mass selected in turn and caused to dissociate by Br ̇ loss upon collision‐induced dissociation. Linear free energy correlations with Brown substituent σ+ constants describe the extent of gas‐phase bromine cation addition under the non‐equilibrium, low‐pressure and solvent‐free conditions which pertain in quadruple collision cells. The electrophilic addition reaction proceeds via a σ‐complex to the ring as suggested by MS3 spectra, except in the case of nitrobenzene, where substituent bromination is suggested by the occurrence of a competitive process in which the nitrosubstituent is displaced by bromine. The reactivity parameters ρ are −0.23 and −0.56 for the gaseous reagents, BrCO+ and CH3NH2Br+, respectively. Both values are much less negative than corresponding values for bromination in solution. The greater reactivity of BrCO+ is evident by the fact that it reacts even with the strongly deactivated substrates and this is consistent with a weak BrCO bond. Competitive protonation occurs in the case of CH3NH2Br+ and, unlike bromination, the rate of this reaction does not correlate with σ+ values. This is suggested to be a consequence of protonation at the ring in some cases and at the substituent in others, including acetophenone and benzonitrile. Evidence for this is that, in contrast to its lack of correlation with substituent constants, the rate of protonation correlates linearly with proton affinity. Copyright © 1993 John Wiley & Sons Ltd
Ovarian cycle lipid dynamics revealed by hyperspectral DESI-MS imaging and multivariate statistics
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