Small Mass Difference Research Articles

Surface-Induced Dissociation of Protein Complexes Selected by Trapped Ion Mobility Spectrometry.

Native mass spectrometry (nMS), particularly in conjunction with gas-phase ion mobility spectrometry measurements, has proven useful as a structural biology tool for evaluating the stoichiometry, conformation, and topology of protein complexes. Here, we demonstrate the combination of trapped ion mobility spectrometry (TIMS) and surface-induced dissociation (SID) on a Bruker SolariX XR 15 T FT-ICR mass spectrometer for the structural analysis of protein complexes. We successfully performed SID on mobility-selected protein complexes, including the streptavidin tetramer and cholera toxin B with bound ligands. Additionally, TIMS-SID was employed on a mixture of the peptides desArg1 and desArg9 bradykinin to mobility-separate and identify the individual peptides. Importantly, results show that native-like conformations can be maintained throughout the TIMS analysis. The TIMS-SID spectra are analogous to SID spectra acquired using quadrupole mass selection, indicating little measurable, if any, structural rearrangement during mobility selection. Mobility parking was used on the ion or mobility of interest and 50-200 SID mass spectra were averaged. High-quality TIMS-SID spectra were acquired over a period of 2-10 min, comparable to or slightly longer than SID coupled with ion mobility on various instrument platforms in our laboratory. The ultrahigh resolving power of the 15 T FT-ICR allowed for the identification and relative quantification of overlapping SID fragments with the same nominal m/z based on isotope patterns, and it shows promise as a platform to probe small mass differences, such as protein/ligand binding or post-translational modifications. These results represent the potential of TIMS-SID-MS for the analysis of both protein complexes and peptides.

A radiative seesaw model linking to XENON1T anomaly

We propose an attractive model that excess of electron recoil events around 1-5 keV reported by the XENON1T collaboration nicely links to the tiny neutrino masses based on a radiative seesaw scenario. Our dark matter (DM) is an isospin singlet inert boson that plays an role in generating non-vanishing neutrino mass at one-loop level, and this DM inelastically interacts with a pair of electrons at one-loop level that is required to explain the XENON1T anomaly. It is also demanded that the mass difference between an excited DM and DM has to be of the order keV. Interestingly, the small mass difference ∼keV is proportional to the neutrino masses. It suggests that we have double suppressions through the tiny mass difference and the one-loop effect. Then, we show some benchmark points to explain the XENON1T anomaly, satisfying all the constraints such as the event ratio of electrons of XENON1T, a long lived particle be longer than the age of Universe, and relic density in addition to the neutrino oscillation data and lepton flavor violations (LFVs).

Inelastic dark matter electron scattering and the XENON1T excess

Detection of electron recoils by dark matter (DM) may reveal the structure of the dark sector. We consider a scenario where a heavier DM particle inelastically scatters off an electron and is converted into a lighter DM particle. A small mass difference between the two DM particles is transferred into electron recoil energy. We investigate the DM-electron interaction mediated by a massive dark photon and evaluate the inelastic DM scattering rate, taking account of the atomic structure. It is found that the scattering rate is significantly enhanced because of the small mass splitting, which allows for a small momentum transfer matched with the size of the electron wave function. We show that there exists a viable parameter space which explains the excess of electron recoil events around 2-3 keV recently reported by the XENON1T experiment.

Collision Cross Sections of Charge-Reduced Proteins and Protein Complexes: A Database for Collision Cross Section Calibration.

The use of charge-reducing reagents to generate lower-charge ions has gained popularity in the field of native mass spectrometry (MS) and ion mobility mass spectrometry (IM-MS). This is because the lower number of charged sites decreases the propensity for Coulombic repulsions and unfolding/restructuring, helping to preserve the native-like structure. Furthermore, lowering the charge state consequently increases the mass-to-charge values (m/z), effectively increasing spacing between signals originating from small mass differences, such as different proteoforms or protein-drug complexes. IM-MS yields collision cross section (CCS, Ω) values that provide information about the three-dimensional structure of the ion. Traveling wave IM (TWIM) is an established and expanding technique within the native MS field. TWIM measurements require CCS calibration, which is achieved via the use of standard species of known CCS. Current databases for native-like proteins and protein complexes provide CCS values obtained using normal (i.e., non-charge-reducing) conditions. Herein, we explored the validity of using "normal" charge calibrants to calibrate for charge-reduced proteins and show cases where it is not appropriate. Using a custom linear field drift cell that enables the determination of ion mobilities from "first principles", we directly determined CCS values for 19 protein calibrant species under three solution conditions (yielding a broad range of charge states) and two drift gases. This has established a database of CCS and reduced-mobility (K0) values, along with their associated uncertainties, for proteins and protein complexes over a large m/z range. TWIM validation of this database shows improved accuracy over existing methods in calibrating CCS values for charge-reduced proteins.

Search for charged excitations of dark matter by KamLAND-Zen experiment

There are many scenarios in which dark matter is a part of a multiplet with an electrically charged state. If WIMP dark matter is accompanied by a charged state separated by a small mass difference, it can form stable bound states with nuclei. The region of observable energy deposition via this process of bound state formation is O(1˜10 MeV). KamLAND-Zen is a large scintillator detector designed for neutrino-less double-beta decay search. This detector is also useful to detect dark matter bound state formations with nuclei. The result from the KamLAND-Zen 400 dataset is reported.

Search for Supersymmetry with a Compressed Mass Spectrum in Events with a Soft τ Lepton, a Highly Energetic Jet, and Large Missing Transverse Momentum in Proton-Proton Collisions at sqrt[s]=13 TeV.

The first search for supersymmetry in events with an experimental signature of one soft, hadronically decaying τ lepton, one energetic jet from initial-state radiation, and large transverse momentum imbalance is presented. These event signatures are consistent with direct or indirect production of scalar τ leptons (τ[over ˜]) in supersymmetric models that exhibit coannihilation between the τ[over ˜] and the lightest neutralino (χ[over ˜]_{1}^{0}), and that could generate the observed relic density of dark matter. The data correspond to an integrated luminosity of 77.2 fb^{-1} of proton-proton collisions at sqrt[s]=13 TeV collected with the CMS detector at the LHC in 2016 and 2017. The results are interpreted in a supersymmetric scenario with a small mass difference (Δm) between the chargino (χ[over ˜]_{1}^{±}) or next-to-lightest neutralino (χ[over ˜]_{2}^{0}), and the χ[over ˜]_{1}^{0}. The mass of the τ[over ˜] is assumed to be the average of the χ[over ˜]_{1}^{±} and χ[over ˜]_{1}^{0} masses. The data are consistent with standard model background predictions. Upper limits at 95%confidence level are set on the sum of the χ[over ˜]_{1}^{±}, χ[over ˜]_{2}^{0}, and τ[over ˜] production cross sections for Δm(χ[over ˜]_{1}^{±},χ[over ˜]_{1}^{0})=50 GeV, resulting in a lower limit of 290GeV on the mass of the χ[over ˜]_{1}^{±}, which is the most stringent to date and surpasses the bounds from the LEP experiments.

Angular resolved mass-energy analysis of species emitted from a dc magnetron sputtered NiW-target

A Ni81W19 target was dc sputter eroded at constant target power density from a tiltable magnetron at different Ar pressures. The combination with a stationary mass-energy analyzer allowed investigating the abundance of different species within the plasma as well as the ion energy distribution functions of 40Ar+, 58Ni+, and 184W+ at any given angle θ between 0° and 90° from the target normal. Ar+ ions are detected at θ angles close to the target normal, whereas metal atoms are observed at larger θ angles. Ni is emitted at smaller θ angles compared to W. Both investigated metal ion energy distributions exhibit a high energy tail with energies up to 50 eV. Increasing the Ar pressure first affects the trajectories of Ni before W atoms. This can be understood by considering the smaller mass difference between Ni and Ar compared to W and Ar. This enables more effective energy transfer and larger scattering angles of Ni undergoing collisions with Ar compared to W. Subsequent film depositions on a spherical-shell substrate holder, covering angles between 0° and 80° from the target normal, allowed for a comparison of the angular dependent film- and plasma-compositions. This correlative analysis suggests that selective resputtering of Ni by energetic Ar neutrals, reflected from the target, leads to the observed difference between the target and film composition during sputter deposition from a multielement NiW target.

Ion Mobility and Surface Collisions: Submicrometer Capillaries Can Produce Native-like Protein Complexes.

The use of submicrometer capillaries for nanoelectrospray ionization of native proteins and protein complexes effectively reduces the number of nonspecific salt adducts to biological molecules, therefore increasing the apparent resolution of a mass spectrometer without any further instrument modifications or increased ion activation. However, the increased interaction between proteins and the surface of the capillary has been shown to promote protein expansion and therefore loss of native structure. Here, we compare the effect of micrometer and submicrometer sized capillaries on the native structures of the protein complexes streptavidin, concanavalin A, and C-reactive protein under charge reducing conditions. We observe that the use of submicrometer capillaries did not result in a significantly higher charge state distribution, indicative of expansion, when compared to micrometer sized capillaries for complexes in 100 mM ammonium acetate and 100 mM triethylammonium acetate and for streptavidin in 200 mM ammonium acetate with no charge reduction. Additionally, no significant differences in collision cross sections were observed using ion mobility mass spectrometry. Finally, the dissociation behaviors of protein complexes ionized using micrometer and submicrometer capillaries were compared to determine if any structural perturbation occurred during ionization. Protein complexes from both capillary sizes displayed similar surface-induced dissociation patterns at similar activation energies. The results suggest that submicrometer capillaries do not result in significant changes to protein complex structure under charge reducing conditions and may be used for native mass spectrometry experiments. Submicrometer capillaries can be used to resolve small mass differences of biological systems on a QTOF platform; however, a laser tip puller is required for pulling reproducible submicrometer capillaries, and disruption in spray due to clogging was observed for larger protein complexes.

Understanding Electrolyte Degradation Mechanisms over the Lifespan of Li-Ion Cells Using Differential Thermal Analysis (DTA) and High Precision Coulometry (HPC)

Introduction Differential thermal analysis (DTA) is a non-destructive method designed to assess liquid electrolyte degradation mechanisms in full lithium-ion cells over their lifespan [1]. The signal obtained from a DTA measurement can be used to determine the amount of liquid electrolyte, amount of salt in the liquid phase, and the overall composition of the solvent components of the liquid electrolyte. Combined with short term high precision cycling or long-term aging studies, this technique can highlight how the salt consumption rate and chemical stability of the electrolyte correlates to the lifespan and performance of a cell. Experimental Method DTA measures the latent heat of fusion of the liquid electrolyte in a cell. This is achieved by tracking the temperature difference between the cell and an inert reference while cooling the cell down below the electrolyte freezing temperature and subsequently heating it back to room temperature. As a cell ages, the change in the amount of liquid electrolyte can be approximated by calculating the area under the curve of a DTA signal. Furthermore, the change in salt concentration of the liquid electrolyte can be determined by the peak shifts of a signal. The ability to measure the compositional changes of the liquid electrolyte over the lifespan of a cell in a non-destructive manner makes DTA a very useful tool for assessing the chemical stability of electrolyte in a full lithium-ion cell. Results A binary solvent system (EMC-DEC) was selected to demonstrate that data collected from the DTA system correlates with phase diagram measurements [2]. Figure 1a shows the signals obtained from Li-ion pouch cells filled with either pure EMC, pure DEC, or a mixture of EMC: DEC 66:33 by weight. Figure 1b shows the phase diagram obtained from a set of differential scanning calorimetry (DSC) measurements of EMC:DEC binary solutions [2]. The colored arrows provide a guide to the eye to show correlation between DTA signal melting features and phase transitions in the phase diagram. It is important to note that most DTA features occur slightly below the phase diagram temperatures as the DTA measurements were performed in full lithium-ion cells, with electrodes and pouch cell material, not just the electrolytes themselves. Results for an electrolyte blend of EC: EMC 3:7 with 1 M LiPF6 with 2% VC show that DTA signals are sensitive to very small (< 0.15 g) electrolyte mass differences (Figure 2). As more electrolyte is present in the cell, the peak position remains constant, while the intensity of each peak increases. The inset of Figure 2 shows that this change in the area under the curve correlates linearly with electrolyte mass. Each peak can be attributed to physical transitions, such as a liquidus, solidus or salt-solvent complex. Figure 3 shows how changing the concentration of salt in a solution can impact a typical DTA curve. Cells were made with variable salt concentrations (0, 0.5, 1, and 1.5 M LiPF6) in DEC. The most noticeable trend is in the liquidus feature, where the peaks shift with changing salt concentration. This presentation will show how DTA measurements can be used to help understand electrolyte degradation with aging for Li-ion cells. It will also be shown how a combination of HPC and DTA measurements can be used to better understand degradation mechanisms that will ultimately lead to cell failure. References R. P. Day, J. Xia, R. Petibon, J. Rucska, H. Wang, A. T. B. Wright, J. R. Dahn, Differential Thermal Analysis of Li-Ion Cells as an Effective Probe of Liquid Electrolyte Evolution during Aging. Journal of The Electrochemical Society. 162, A2577-A2581 (2015).S. Ding, K. Xu, S. Zhang, T.R. Jow, Liquid/Solid Phase Diagrams of Binary Carbonates for Lithium Batteries Part 2, Journal of The Electrochemical Society. 148, A299-A304 (2001). Figure 1

Structural Properties and Thermoelectric Performance of the Double-Filled Skutterudite (Sm,Gd)y(FexNi1-x)4Sb12.

The structural and thermoelectric properties of the filled skutterudite (Sm,Gd)y(FexNi1-x)4Sb12 were investigated and critically compared to the ones in the Sm-containing system with the aim of unravelling the effect of double filling on filling fraction and thermal conductivity. Several samples (x = 0.50–0.90 and y = 0.15–0.48) were prepared by melting-sintering, and two of them were densified by spark plasma sintering in order to study their thermoelectric features. The crystallographic study enables the recognition of the role of the filler size in ruling the filling fraction and the compositional location of the p/n crossover: It has been found that the former lowers and the latter moves toward lower x values with the reduction of the filler ionic size, as a consequence of the progressively weaker interaction of the filler with the Sb12 cavity. The analysis of thermoelectric properties indicates that, despite the Sm3+/Gd3+ small mass difference, the contemporary presence of these ions in the 2a site significantly affects the thermal conductivity of both p- and n-compositions. This occurs by reducing its value with respect to the Sm-filled compound at each temperature considered, and making the overall thermoelectric performance of the system comparable to several multi-filled (Fe, Ni)-based skutterudites described in the literature.

Mini Review on Vector-Like Leptonic Dark Matter, Neutrino Mass, and Collider Signatures

We review a class of models in which the Standard Model (SM) is augmented by vector-like leptons: one doublet and a singlet, which are odd under an unbroken discrete Z_2 symmetry. As a result, the neutral component of these additional vector-like leptons are stable and behave as dark matter. We study the phenomenological constraints on the model parameters and elucidate the parameter space for relic density, direct detection and collider signatures of dark matter. In such models, we further add a scalar triplet of hypercharge two and study the consequences. In particular, after electro weak symmetry breaking (EWSB), the triplet scalar gets an induced vacuum expectation value (vev), which yield Majorana masses not only to the light neutrinos but also to vector-like leptonic doublet DM. Due to the Majorana mass of DM, the Z mediated elastic scattering with nucleon is forbidden and hence allowing the model to survive from stringent direct search bound. The DM without scalar triplet lives in a small singlet-doublet leptonic mixing region (sin θ ≤ 0.1) due to large contribution from singlet component and have small mass difference (∆m ∼ 10 GeV) with charged companion, the NLSP (next to lightest stable particle), to aid co-annihilation for yielding correct relic density. Both these observations change to certain extent in presence of scalar triplet to aid observability of hadronically quiet leptonic final states at LHC, while one may also confirm/rule-out the model through displaced vertex signal of NLSP, a characteristic signature of the model in relic density and direct search allowed parameter space.

Comparing methods to normalize insulin secretion shows the process may not be needed.

Glucose-stimulated insulin secretion (GSIS) is a well-accepted method to investigate the physiological and pathophysiological function of islets. However, there is little consensus about which method is best for normalizing and presenting GSIS data. In this study, we evaluated the sufficiency of islet area, total protein, total DNA, and total insulin content as parameters to normalize GSIS data. First, we tested if there is a linear correlation between each parameter and the number of islets (10, 20, 30, and 40 islets). Islet area, total protein, and insulin content produced excellent linear correlations with islet number (R2 >0.9 for each) from the same islet material. Insulin secretion in 11mM glucose also correlated reasonably well for islet area (R2=0.69), protein (R2=0.49), and insulin content (R2=0.58). DNA content was difficult to reliably measure and was excluded from additional comparisons. We next measured GSIS for 18 replicates of 20 islets each, measuring 3mM and 11mM glucose to calculate the stimulation index and to compare each normalization parameter. Using these similar islet masses for each replicate, none of the parameters produced linear correlations with GSIS (R2<0.05), suggesting that inherent differences in GSIS dominate small differences in islet mass. We conclude that when comparing GSIS for islets of reasonably similar size (<50% variance), normalization does not improve the representation of GSIS data. Normalization may be beneficial when substantial differences in islet mass are involved. In such situations, we suggest that using islet cross-sectional area is superior to other commonly used techniques for normalizing GSIS data.

Measuring porosities of chromatographic columns utilizing a mass-based total pore-blocking method: Superficially porous particles and pore-blocking critical pressure mechanism

Experimentally determined total, interstitial and intraparticle porosity values are necessary to equate theory, simulation and experimental column performance. This paper reports a study of a mass-based technique for determining total, interstitial and intraparticle porosity measurements based on the total pore-blocking (TPB) method. Commercially available superficially porous particle (SPP) columns, in a variety of small-pore and wide-pore materials, with both hydrophobic and hydrophilic surfaces, are utilized as samples. The results are compared with previously determined literature values for a number of columns and contrasted with HPLC-based elution methods. This method uses only a high-precision balance and an HPLC pump.A simple theoretical analysis of the TPB method using the Young-Laplace equation shows the pressure bounds and flow rate constraints of the method which ensure pore blocking stability. The results suggest that particles with small-pore diameters can be analyzed over a range of solvent clearing pressures and flow rates. However, wide-pore materials, typically with pore diameters in excess of 400 Å, have very low critical pressures and are difficult to determine without losing the pore blocking component. Small mass differences between clearing solvents are shown to present a challenge for measuring the interstitial volume.

Searches for pair production of charginos and top squarks in final states with two oppositely charged leptons in proton-proton collisions at sqrt{s}=13 TeV

A search for pair production of supersymmetric particles in events with two oppositely charged leptons (electrons or muons) and missing transverse momentum is reported. The data sample corresponds to an integrated luminosity of 35.9 fb−1 of proton-proton collisions at sqrt{s}=13 TeV collected with the CMS detector during the 2016 data taking period at the LHC. No significant deviation is observed from the predicted standard model background. The results are interpreted in terms of several simplified models for chargino and top squark pair production, assuming R-parity conservation and with the neutralino as the lightest supersymmetric particle. When the chargino is assumed to undergo a cascade decay through sleptons, with a slepton mass equal to the average of the chargino and neutralino masses, exclusion limits at 95% confidence level are set on the masses of the chargino and neutralino up to 800 and 320 GeV, respectively. For top squark pair production, the search focuses on models with a small mass difference between the top squark and the lightest neutralino. When the top squark decays into an off-shell top quark and a neutralino, the limits extend up to 420 and 360 GeV for the top squark and neutralino masses, respectively.

Increasing the Separation Capacity of Intact Histone Proteoforms Chromatography Coupling Online Weak Cation Exchange-HILIC to Reversed Phase LC UVPD-HRMS

Top-down proteomics is an emerging analytical strategy to characterize combinatorial protein post-translational modifications (PTMs). However, sample complexity and small mass differences between chemically closely related proteoforms often limit the resolution attainable by separations employing a single liquid chromatographic (LC) principle. In particular, for ultramodified proteins like histones, extensive and time-consuming fractionation is needed to achieve deep proteoform coverage. Herein, we present the first online nanoflow comprehensive two-dimensional liquid chromatography (nLC×LC) platform top-down mass spectrometry analysis of histone proteoforms. The described two-dimensional LC system combines weak cation exchange chromatography under hydrophilic interaction LC conditions (i.e., charge- and hydrophilicity-based separation) with reversed phase liquid chromatography (i.e., hydrophobicity-based separation). The two independent chemical selectivities were run at nanoflows (300 nL/min) and coupled online with high-resolution mass spectrometry employing ultraviolet photodissociation (UVPD-HRMS). The nLC×LC workflow increased the number of intact protein masses observable relative to one-dimensional approaches and allowed characterization of hundreds of proteoforms starting from limited sample quantities (∼1.5 μg).

Optical spectroscopy of laser-produced plasmas for standoff isotopic analysis

Rapid, in-field, and non-contact isotopic analysis of solid materials is extremely important to a large number of applications, such as nuclear nonproliferation monitoring and forensics, geochemistry, archaeology, and biochemistry. Presently, isotopic measurements for these and many other fields are performed in laboratory settings. Rapid, in-field, and non-contact isotopic analysis of solid material is possible with optical spectroscopy tools when combined with laser ablation. Laser ablation generates a transient vapor of any solid material when a powerful laser interacts with a sample of interest. Analysis of atoms, ions, and molecules in a laser-produced plasma using optical spectroscopy tools can provide isotopic information with the advantages of real-time analysis, standoff capability, and no sample preparation requirement. Both emission and absorption spectroscopy methods can be used for isotopic analysis of solid materials. However, applying optical spectroscopy to the measurement of isotope ratios from solid materials presents numerous challenges. Isotope shifts arise primarily due to variation in nuclear charge distribution caused by different numbers of neutrons, but the small proportional nuclear mass differences between nuclei of various isotopes lead to correspondingly small differences in optical transition wavelengths. Along with this, various line broadening mechanisms in laser-produced plasmas and instrumental broadening generated by the detection system are technical challenges frequently encountered with emission-based optical diagnostics. These challenges can be overcome by measuring the isotope shifts associated with the vibronic emission bands from molecules or by using the techniques of laser-based absorption/fluorescence spectroscopy to marginalize the effect of instrumental broadening. Absorption and fluorescence spectroscopy probe the ground state atoms existing in the plasma when it is cooler, which inherently provides narrower lineshapes, as opposed to emission spectroscopy which requires higher plasma temperatures to be able to detect thermally excited emission. Improvements in laser and detection systems and spectroscopic techniques have allowed for isotopic measurements to be carried out at standoff distances under ambient atmospheric conditions, which have expanded the applicability of optical spectroscopy-based isotopic measurements to a variety of scientific fields. These technological advances offer an in-situ measurement capability that was previously not available. This review will focus on isotope detection through emission, absorption, and fluorescence spectroscopy of atoms and molecules in a laser-produced plasma formed from a solid sample. A description of the physics behind isotope shifts in atoms and molecules is presented, followed by the physics behind solid sampling of laser ablation plumes, optical methods for isotope measurements, the suitable physical conditions of laser-produced plasma plumes for isotopic analysis, and the current status. Finally, concluding remarks will be made on the existing knowledge/technological gaps identified from the current literature and suggestions for the future work.

Prompt gamma-ray emission of GRB 170817A associated to GW 170817: A consistent picture

The short GRB 170817A associated to the first detection of gravitation waves from a Binary Neutron Star (BNS) merger was in many ways unusual. Possible explanations are emission from a cocoon or cocoon break out, off-axis view of a structured or uniform jet, and on-axis ultra-relativistic jet with reduced density and Lorentz factor. Here we use a phenomenological model of shock evolution and synchrotron/self-Compton emission to simulate the prompt emission of GRB 170817A and to test above proposals. We find that synchrotron emission from a mildly relativistic cocoon with a Lorentz factor of 2-3, as considered in the literature, generates a too soft, too long, and too bright prompt emission. Off-axis view of an structured jet with a Lorentz factor of about 10 can reproduce observations, but needs a very efficient transfer of kinetic energy to electrons in internal shocks, which is disfavored by particle in cell simulations. We also comment on cocoon breakout as a mechanism for generation of the prompt gamma-ray. A relativistic jet with a Lorentz factor of about 100 and a density lower than typical short GRBs seems to be the most plausible model and we conclude that GRB 170817A was intrinsically faint. Based on this result and findings of relativistic magnetohydrodynamics simulations of BNS merger in the literature we discuss physical and astronomical conditions, which may lead to such faint short GRBs. We identify small mass difference of progenitor neutron stars, their old age and reduced magnetic field, and anti-alignment of spin-orbit angular momentum induced by environmental gravitational disturbances during the lifetime of the BNS as causes for the faintness of GRB 170817A. We predict that BNS mergers at lower redshifts generate on average fainter GRBs.

Search for electroweak production of supersymmetric states in scenarios with compressed mass spectra at s=13 TeV with the ATLAS detector

A search for electroweak production of supersymmetric particles in scenarios with compressed mass spectra in final states with two low-momentum leptons and missing transverse momentum is presented. This search uses proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015-2016, corresponding to 36.1 fb$^{-1}$ of integrated luminosity at $\sqrt{s}=13$ TeV. Events with same-flavor pairs of electrons or muons with opposite electric charge are selected. The data are found to be consistent with the Standard Model prediction. Results are interpreted using simplified models of R-parity-conserving supersymmetry in which there is a small mass difference between the masses of the produced supersymmetric particles and the lightest neutralino. Exclusion limits at 95% confidence level are set on next-to-lightest neutralino masses of up to 145 GeV for Higgsino production and 175 GeV for wino production, and slepton masses of up to 190 GeV for pair production of sleptons. In the compressed mass regime, the exclusion limits extend down to mass splittings of 2.5 GeV for Higgsino production, 2 GeV for wino production, and 1 GeV for slepton production. The results are also interpreted in the context of a radiatively-driven natural supersymmetry model with non-universal Higgs boson masses.

Multiplexed proteome analysis with neutron-encoded stable isotope labeling in cells and mice.

We describe a protocol for multiplexed proteomic analysis using neutron-encoded (NeuCode) stable isotope labeling of amino acids in cells (SILAC) or mice (SILAM). This method currently enables simultaneous comparison of up to nine treatment and control proteomes. Another important advantage over traditional SILAC/SILAM is that shorter labeling times are required. Exploiting the small mass differences that correspond to subtle differences in the neutron-binding energies of different isotopes, the amino acids used in NeuCode SILAC/SILAM differ in mass by just a few milliDaltons. Isotopologs of lysine are introduced into cells or mammals, via the culture medium or diet, respectively, to metabolically label the proteome. Labeling time is ∼2 weeks for cultured cells and 3-4 weeks for mammals. The proteins are then extracted, relevant samples are combined, and these are enzymatically digested with lysyl endopeptidase (Lys-C). The resultant peptides are chromatographically separated and then mass analyzed. During mass spectrometry (MS) data acquisition, high-resolution MS1 spectra (≥240,000 resolving power at m/z = 400) reveal the embedded isotopic signatures, enabling relative quantification, while tandem mass spectra, collected at lower resolutions, provide peptide identities. Both types of spectra are processed using NeuCode-enabled MaxQuant software. In total, the approximate completion time for the protocol is 3-5 weeks.

Non-invasive in vivo determination of viable islet graft volume by 111In-exendin-3

Pancreatic islet transplantation is a promising therapy for patients with type 1 diabetes. However, the duration of long-term graft survival is limited due to inflammatory as well as non-inflammatory processes and routine clinical tests are not suitable to monitor islet survival. 111In-exendin-SPECT (single photon emission computed tomography) is a promising method to non-invasively image islets after transplantation and has the potential to help improve the clinical outcome. Whether 111In-exendin-SPECT allows detecting small differences in beta-cell mass (BCM) and measuring the actual volume of islets that were successfully engrafted has yet to be demonstrated. Here, we evaluated the performance of 111In-exendin-SPECT using an intramuscular islet transplantation model in C3H mice. In vivo imaging of animals transplanted with 50, 100, 200, 400 and 800 islets revealed an excellent linear correlation between SPECT quantification of 111In-exendin uptake and insulin-positive area of islet transplants, demonstrating that 111In-exendin-SPECT specifically and accurately measures BCM. The high sensitivity of the method allowed measuring small differences in graft volumes, including grafts that contained less than 50 islets. The presented method is reliable, convenient and holds great potential for non-invasive monitoring of BCM after islet transplantation in humans.