More Like this

1. Sequencing of Side-Chain Liquid Crystalline Copolymers by Matrix-Assisted Laser Desorption/Ionization Tandem Mass Spectrometry

   https://doi.org/10.3390/polym11071118  

   Snyder, Savannah R. ; Wei, Wei ; Xiong, Huiming ; Wesdemiotis, Chrys ( July 2019 , Polymers)

   Polyether based side-chain liquid crystalline (SCLC) copolymers with distinct microstructures were prepared using living anionic polymerization techniques. The composition, end groups, purity, and sequence of the resulting copolymers were elucidated by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) and tandem mass spectrometry (MS/MS). MALDI-MS analysis confirmed the presence of (CH3)3CO– and –H end groups at the initiating (α) and terminating (ω) chain end, respectively, and allowed determination of the molecular weight distribution and comonomer content of the copolymers. The comonomer positions along the polymer chain were identified by MS/MS, from the fragments formed via C–O and C–C bond cleavages in the polyether backbone. Random and block architectures could readily be distinguished by the contiguous fragment series formed in these reactions. Notably, backbone C–C bond scission was promoted by a radical formed via initial C–O bond cleavage in the mesogenic side chain. This result documents the ability of a properly substituted side chain to induce sequence indicative bond cleavages in the polyether backbone.
2. Controlling the strong field fragmentation of ClCHO ⁺ using two laser pulses –an ab initio molecular dynamics simulation

   https://doi.org/10.1002/jcc.25576  

   Shi, Xuetao ; Schlegel, H. Bernhard ( October 2018 , Journal of Computational Chemistry)

   For a single, intense 7 μm linearly polarized laser pulse, we found that the branching ratio for the fragmentation of ClCHO⁺→ Cl + HCO⁺, H + ClCO⁺, HCl⁺+CO depended strongly on the orientation of the molecule (J. Phys. Chem. Lett.2012,32541). The present study explores the possibility of controlling the branching ratio for fragmentation by using two independent pulses with different frequencies, alignment and delay. Born‐Oppenheimer molecular dynamics simulations in the laser field were carried out with the B3LYP/6‐311G(d,p) level of theory using combinations of 3.5, 7 and 10.5 μm sine squared pulses with field strengths of 0.03 au (peak intensity of 3.15×10¹³W/cm²) and lengths of 560 fs. A 3.5 μm pulse aligned with the C‐H bond and a 10.5 μm pulse perpendicular to the C‐H bond produced a larger branching ratio for HCl⁺+CO than a comparable single 7 μm pulse. When the 10.5 μm pulse was delayed by one quarter of the pulse envelope, the branching ratio for the high energy product, (HCl⁺+CO 73%) was a factor of three larger than the low energy product (Cl + HCO⁺, 25%). By contrast, when the 3.5 μm pulse was delayed by one quarter of the pulse envelope, the branching ratio was reversedmore »(HCl⁺+CO 38%; Cl + HCO⁺, 60%). Continuous wavelet analysis was used to follow the interaction of the laser with the various vibrational modes as a function of time. © 2018 Wiley Periodicals, Inc.

   « less
3. A plethora of isomerization processes and hydrogen scrambling in the fragmentation of the methanol dimer cation: a PEPICO study

   https://doi.org/10.1039/d1cp05155e  

   Wu, Xiangkun ; Zhou, Xiaoguo ; Bjelić, Saša ; Hemberger, Patrick ; Sztáray, Bálint ; Bodi, Andras ( January 2022 , Physical Chemistry Chemical Physics)

   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 tomore »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.« less
4. Mass spectrometry studies of the fragmentation patterns and mechanisms of protonated peptoids

   https://doi.org/10.1002/bip.23358  

   Ren, Jianhua ; Tian, Yuan ; Hossain, Ekram ; Ho, Joshua S. ; Mann, Yadwinder S. ; Zhang, Yuntao ; Browne, Michael D. ; Connolly, Michael D. ; Zuckermann, Ronald N. ( June 2020 , Biopolymers)

   Peptoids belong to a class of sequence‐controlled polymers comprising ofN‐alkylglycine. This study focuses on using tandem mass spectrometry techniques to characterize the fragmentation patterns of a set of singly and doubly protonated peptoids consisting of one basic residue placed at different positions. The singly protonated peptoids fragment by producing predominately high‐abundant C‐terminal ions called Y‐ions and low‐abundant N‐terminal ions called B‐ions. Computational studies suggest that the proton affinity (PA) of the C‐terminal fragments is generally higher than that of the N‐terminal fragments, and the PA of the former increases as the fragments are elongated. The B‐ions are likely formed upon dissociating the proton‐activated amide bonds via an oxazolone structure, and the Y‐ions are produced subsequently by abstracting a proton from the newly formed B‐ions, which is energetically favored. The doubly protonated peptoids prefer to fragment closest to either the N‐ or the C‐terminus and produce corresponding B/Y‐ion pairs. The basic residue seems to dictate the preferred fragmentation site, which may be the result of minimizing the repulsion between the two charges. Water and terminal neutral losses are a facile process accompanying the peptoid fragmentation in both charge states. The patterns appear to be highly influenced by the location ofmore »the basic residue.

   « less
5. Rapid and simple determination of average molecular weight and composition of synthetic polymers via electrospray ionization‐mass spectrometry and a Bayesian universal charge deconvolution

   https://doi.org/10.1002/rcm.9478  

   Keating, Addie R. ; Wesdemiotis, Chrys ( February 2023 , Rapid Communications in Mass Spectrometry)

   Simple, affordable, and rapid methods for identifying the molecular weight (MW) distribution and macromolecular composition of polymeric materials are limited. Current tools require extensive solvent consumption, linear calibrations, and expensive consumables. A simple method for the determination of average MW (M_n,M_w) and chain end groups is demonstrated for synthetic homopolymer standards using direct injection electrospray ionization‐mass spectrometry (ESI‐MS) and an open‐sourced charge deconvolution (CDC) algorithm.

   Five homopolymer standards in the 1–7 kDa MW range were analyzed using direct‐injection ESI‐MS on a quadrupole/time‐of‐flight mass spectrometer. The samples investigated, viz. two poly(ethylene oxide) (PEO) and two poly(styrene sulfonic acid) (PSS) standards with narrow polydispersity and one poly(d,l‐alanine) (pAla) standard with undefined polydispersity, were chosen to illustrate challenges with ESI‐MS quantitation. Using the UniDec program, weight average MWs (M_w) obtained from the charge‐deconvoluted spectra were compared to the reportedM_wdata of the standards from size exclusion chromatography (SEC) measurements.

   The MW data derived for the PSS, PEO, and pAla standards agreed well with the corresponding reportedM_wor MW range values. The method was able to provide MW, degree of polymerization (DP), and polydispersity index (PDI) information for polymers with narrow (PSS, PEO) as well as broader (pAla) molecular weight distribution; this feature provides an advantage overmore »MW analysis via matrix‐assisted laser desorption/ionization (MALDI) for ESI‐compatible materials. PSS standards differing in average MW by only a few repeat units could be confidently distinguished. Additionally, the oligomeric resolution observed for all samples studied unveiled chain‐end information not available through chromatographic analysis.

   Overall, the free and easy‐to‐use UniDec CDC algorithm provides a simple, alternative method to measuring MW and DP for polymeric materials without high solvent consumption, expensive ionization sources, or calibration curves. Information about the masses of individual oligomers and the possibility to further characterize these oligomers using tandem mass spectrometry and/or ion mobility techniques constitutes additional benefits of this approach vis‐à‐vis traditional MW and PDI elucidation through SEC.

   « less