- Award ID(s):
- NSF-PAR ID:
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Page Range / eLocation ID:
- 13714 to 13723
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The valence photoionization of light and deuterated methanol dimers was studied by imaging photoelectron photoion coincidence spectroscopy in the 10.00–10.35 eV photon energy range. Methanol clusters were generated in a rich methanol beam in nitrogen after expansion into vacuum. They generally photoionize dissociatively to protonated methanol cluster cations, (CH 3 OH) n H + . However, the stable dimer parent ion (CH 3 OH) 2 + is readily detected below the dissociation threshold to yield the dominant CH 3 OH 2 + fragment ion. In addition to protonated methanol, we could also detect the water- and methyl-loss fragment ions of the methanol dimer cation for the first time. These newly revealed fragmentation channels are slow and cannot compete with protonated methanol cation formation at higher internal energies. In fact, the water- and methyl-loss fragment ions appear together and disappear at a ca. 150 meV higher energy in the breakdown diagram. Experiments with selectively deuterated methanol samples showed H scrambling involving two hydroxyl and one methyl hydrogens prior to protonated methanol formation. These insights guided the potential energy surface exploration to rationalize the dissociative photoionization mechanism. The potential energy surface was further validated by a statistical model including isotope effects to fit the experiment for the light and the perdeuterated methanol dimers simultaneously. The (CH 3 OH) 2 + parent ion dissociates via five parallel channels at low internal energies. The loss of both CH 2 OH and CH 3 O neutral fragments leads to protonated methanol. However, the latter, direct dissociation channel is energetically forbidden at low energies. Instead, an isomerization transition state is followed by proton transfer from a methyl group, which leads to the CH 3 (H)OH + ⋯CH 2 OH ion, the precursor to the CH 2 OH-, H 2 O-, and CH 3 -loss fragments after further isomerization steps, in part by a roaming mechanism. Water loss yields the ethanol cation, and two paths are proposed to account for m/z 49 fragment ions after CH 3 loss. The roaming pathways are quickly outcompeted by hydrogen bond breaking to yield CH 3 OH 2 + , which explains the dominance of the protonated methanol fragment ion in the mass spectrum.more » « less
Alkali and alkaline earth metal adducts of a branched glycan, XXXG, were analyzed with helium charge transfer dissociation (He‐CTD) and low‐energy collision‐induced dissociation (LE‐CID) to investigate if metalation would impact the type of fragments generated and the structural characterization of the analyte. The studied adducts included 1+ and 2+ precursors involving one or more of the cations: H+, Na+, K+, Ca2+, and Mg2+. Regardless of the metal adduct, He‐CTD generated abundant and numerous glycosidic and cross‐ring cleavages that were structurally informative and able to identify the 1,4‐linkage and 1,6‐branching patterns. In contrast, the LE‐CID spectra mainly contained glycosidic cleavages, consecutive fragments, and numerous neutral losses, which complicated spectral interpretation. LE‐CID of [M + K + H]2+and [M + Na]+precursors generated a few cross‐ring cleavages, but they were not sufficient to identify the 1,4‐linkage and 1,6‐branching pattern of the XXXG xyloglucan. He‐CTD predominantly generated 1+ fragments from 1+ precursors and 2+ product ions from 2+ precursors, although both LE‐CID and He‐CTD were able to generate 1+ product ions from 2+ adducts of magnesium and calcium. The singly charged fragments derive from the loss of H+from the metalated product ions and the formation of a protonated complementary product ion; such observations are similar to previous reports for magnesium and calcium salts undergoing electron capture dissociation (ECD) activation. However, during He‐CTD, the [M + Mg]2+precursor generated more singly charged product ions than [M + Ca]2+, either because Mg has a higher second ionization potential than Ca or because of conformational differences and the locations of the charging adducts during fragmentation. He‐CTD of the [M + 2Na]2+and the [M + 2 K]2+precursors generated singly charged product ions from the loss of a sodium ion and potassium ion, respectively. In summary, although the metal ions influence the mass and charge state of the observed product ions, the metal ions had a negligible effect on the types of cross‐ring cleavages observed.
Phosphorylated proteins play essential roles in many cellular processes, and identification and characterization of the relevant phosphoproteins can help to understand underlying mechanisms. Herein, we report a collision‐induced dissociation top‐down approach for characterizing phosphoproteins on a quadrupole time‐of‐flight mass spectrometer. β‐casein, a protein with two major isoforms and five phosphorylatable serine residues, was used as a model. Peaks corresponding to intact β‐casein ions with charged states up to 36+were detected. Tandem mass spectrometry was performed on β‐casein ions of different charge states (12+, and 15+to 28+) in order to determine the effects of charge state on dissociation of this protein. Most of the abundant fragments corresponded to y, b ions, and internal fragments caused by cleavage of the N‐terminal amide bond adjacent to proline residues (Xxx‐Pro). The abundance of internal fragments increased with the charge state of the protein precursor ion; these internal fragments predominantly arose from one or two Xxx‐Pro cleavage events and were difficult to accurately assign. The presence of abundant sodium adducts of β‐casein further complicated the spectra. Our results suggest that when interpreting top‐down mass spectra of phosphoproteins and other proteins, researchers should consider the potential formation of internal fragments and sodium adducts for reliable characterization.
We investigate the gas-phase structures and fragmentation chemistry of deprotonated carbohydrate anions using combined tandem mass spectrometry, infrared spectroscopy, regioselective labelling, and theory. Our model system is deprotonated, [lactose-H] − . We computationally characterize the rate-determining barriers to glycosidic bond (C 1 –Z 1 reactions) and cross-ring cleavages, and compare these predictions to our tandem mass spectrometric and infrared spectroscopy data. The glycosidic bond cleavage product data support complex mixtures of anion structures in both the C 1 and Z 1 anion populations. The specific nature of these distributions is predicted to be directly affected by the nature of the anomeric configuration of the precursor anion and the distribution of energies imparted. i.e. , Z 1 anions produced from the β-glucose anomeric form have a differing distribution of product ion structures than do those from the α-glucose anomeric form. The most readily formed Z 1 anions ([1,4-anhydroglucose-H] − structures) are produced from the β-glucose anomers, and do not ring-open and isomerize as the hemiacetal group is no longer present. In contrast the [3,4-anhydroglucose-H] − , Z 1 anion structures, which are most readily produced from α-glucose forms, can ring-open through very low barriers (<25 kJ mol −1 ) to form energetically and entropically favorable aldehyde isomers assigned with a carbonyl stretch at ∼1640 cm −1 . Barriers to interconversion of the pyranose [β-galactose-H] − , C 1 anions to ring-open forms were larger, but still modest (≥51 kJ mol −1 ) consistent with evidence of the presence of both forms in the infrared spectrum. For the cross-ring cleavage 0,2 A 2 anions, ring-opening at the glucose hemiacetal of [lactose-H] − is rate-limiting (>180 (α-), >197 kJ mol −1 (β-anomers)). This finding offers an explanation for the low abundance of these product anions in our tandem mass spectra.more » « less
We present optical photometry and spectroscopy of the Type II supernova ASASSN-14jb, together with Very Large Telescope (VLT) Multi Unit Spectroscopic Explorer (MUSE) integral field observations of its host galaxy and a nebular-phase spectrum. This supernova, in the nearby galaxy ESO 467-G051 ( z = 0.006), was discovered and followed-up by the all-sky automated survey for supernovae (ASAS-SN). We obtained well-sampled las cumbres network (LCOGTN) B V g r i and Swift w 2 m 1 w 1 u b v optical, near-UV/optical light curves, and several optical spectra in the early photospheric phases. The transient ASASSN-14jb exploded ∼2 kpc above the star-forming disk of ESO 467-G051, an edge-on disk galaxy. The large projected distance from the disk of the supernova position and the non-detection of any H II region in a 1.4 kpc radius in projection are in conflict with the standard environment of core-collapse supernova progenitors and suggests the possible scenario that the progenitor received a kick in a binary interaction. We present analysis of the optical light curves and spectra, from which we derived a distance of 25 ± 2 Mpc using state-of-the-art empirical methods for Type II SNe, physical properties of the SN explosion ( 56 Ni mass, explosion energy, and ejected mass), and properties of the progenitor; namely the progenitor radius, mass, and metallicity. Our analysis yields a 56 Ni mass of 0.0210 ± 0.0025 M ⊙ , an explosion energy of ≈0.25 × 10 51 ergs, and an ejected mass of ≈6 M ⊙ . We also constrained the progenitor radius to be R * = 580 ± 28 R ⊙ which seems to be consistent with the sub-Solar metallicity of 0.3 ± 0.1 Z ⊙ derived from the supernova Fe II λ 5018 line. The nebular spectrum constrains strongly the progenitor mass to be in the range 10–12 M ⊙ . From the Spitzer data archive we detect ASASSN-14jb ≈330 days past explosion and we derived a total dust mass of 10 −4 M ⊙ from the 3.6 μ m and 4.5 μ m photometry. Using the F U V , N U V , B V g r i , K s , 3.6 μ m, and 4.5 μ m total magnitudes for the host galaxy, we fit stellar population synthesis models, which give an estimate of M * ≈ 1 × 10 9 M ⊙ , an age of 3.2 Gyr, and a SFR ≈0.07 M ⊙ yr −1 . We also discuss the low oxygen abundance of the host galaxy derived from the MUSE data, having an average of 12 + log(O/H) = 8.27 +0.16 −0.20 using the O 3 N 2 diagnostic with strong line methods. We compared it with the supernova spectra, which is also consistent with a sub-Solar metallicity progenitor. Following recent observations of extraplanar H II regions in nearby edge-on galaxies, we derived the metallicity offset from the disk, being positive, but consistent with zero at 2 σ , suggesting enrichment from disk outflows. We finally discuss the possible scenarios for the unusual environment for ASASSN-14jb and conclude that either the in-situ star formation or runaway scenario would imply a low-mass progenitor, agreeing with our estimate from the supernova nebular spectrum. Regardless of the true origin of ASASSN-14jb, we show that the detailed study of the environment roughly agree with the stronger constraints from the observation of the transient.more » « less