258 research outputs found
The structure and energetics of He and He nanodroplets doped with alkaline earth atoms
We present systematic results, based on density functional calculations, for
the structure and energetics of He and He nanodroplets doped with
alkaline earth atoms. We predict that alkaline earth atoms from Mg to Ba go to
the center of He drops, whereas Ca, Sr, and Ba reside in a deep dimple at
the surface of He drops, and Mg is at their center. For Ca and Sr, the
structure of the dimples is shown to be very sensitive to the He-alkaline earth
pair potentials used in the calculations. The transition
of strontium atoms attached to helium nanodroplets of either isotope has been
probed in absorption experiments. The spectra show that strontium is solvated
inside He nanodroplets, supporting the calculations. In the light of our
findings, we emphasize the relevance of the heavier alkaline earth atoms for
analyzing mixed He-He nanodroplets, and in particular, we suggest their
use to experimentally probe the He-He interface.Comment: Typeset using Revtex, 20 pages and 8 Postscript file
The Competition of Charge Remote and Charge Directed Fragmentation Mechanisms in Quaternary Ammonium Salt Derivatized PeptidesâAn Isotopic Exchange Study
Derivatization of peptides as quaternary ammonium salts (QAS) is a promising method for sensitive detection by electrospray ionization tandem mass spectrometry (Cydzik et al. J. Pept. Sci.2011, 17, 445â453). The peptides derivatized by QAS at their N-termini undergo fragmentation according to the two competing mechanisms â charge remote (ChR) and charge directed (ChD). The absence of mobile proton in the quaternary salt ion results in ChR dissociation of a peptide bond. However, Hofmann elimination of quaternary salt creates an ion with one mobile proton leading to the ChD fragmentation. The experiments on the quaternary ammonium salts with deuterated N-alkyl groups or amide NH bonds revealed that QAS derivatized peptides dissociate according to the mixed ChR-ChD mechanism. The isotopic labeling allows differentiation of fragments formed according to ChR and ChD mechanisms
Calculations of relative intensities of fragment ions in the MSMS spectra of a doubly charged penta-peptide
Spectroscopic evidence for mobilization of amide position protons during CID of model peptide ions
Backbone cleavages and sequential loss of carbon monoxide and ammonia from protonated AGG: A combined tandem mass spectrometry, isotope labeling, and theoretical study
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