Collision Cross Sections Research Articles

Probing the Intramolecular Folding of Nucleic Acids with Native Ion Mobility Mass Spectrometry: Strategies and Caveats.

The goal of native mass spectrometry is to obtain information on noncovalent interactions in solution through mass spectrometry measurements in the gas phase. Characterizing intramolecular folding requires using structural probing techniques such as ion mobility spectrometry. However, inferring solution structures of nucleic acids is difficult because the low-charge state ions produced from aqueous solutions at physiological ionic strength get compacted during electrospray. Here we explored whether native supercharging could produce higher charge states that would better reflect solution folding, and whether the voltage required for collision-induced unfolding (CIU) could reflect preserved intramolecular hydrogen bonds. We studied pH-responsive i-motif structures with different loops, and unstructured controls. We also implemented a multivariate curve resolution procedure to extract physically meaningful pure components from the CIU data and reconstruct unfolding curves. We found that the relative unfolding voltages reflect to some extent, but not always unambiguously, the number of intramolecular hydrogen bonds that were present in solution. Reaching phosphate charging densities over 0.25 makes it easier to discriminate between structures, and the use of native supercharging agents is thus essential. We also uncovered several caveats in data interpretation: (1) when different structures (for example the i-motif with and without hairpin) unfold via different pathways, the unfolding voltages do not necessarily reflect the number of hydrogen bonds, (2) unstructured controls also undergo unfolding, and the base composition influences the unfolding voltage, (3) changing the solution pH also unexpectedly changed the unfolding voltage, and (4) the ion mobility patterns become more complicated when two structures are present simultaneously, such as an i-motif and a harpin, because of opposite effects on the collision cross section upon activation.

Cross-Sections for Projectile Ionization, Electron Capture, and System Breakdown of C5+ and Li2+ Ions with Atomic Hydrogen

For many disciplines of science, all conceivable collisional cross-sections and reactions must be precisely known. Although recent decades have seen a trial of large-scale research to obtain such data, many essential atomic and molecular cross-section data are still missing, and the reliability of the existing cross-sections has to be validated. In this paper, we present projectile ionization, electron capture, and system breakdown cross-sections in carbon (C5+) ions and lithium (Li2+) ion collisions with atomic hydrogen based on the Monte Carlo models of classical and quasi-classical trajectories. According to our expectation, the QCTMC results show higher results in comparison to standard CTMC data, emphasizing the role of the Heisenberg correction constraint, especially in the low-energy regime. On the other hand, at high energy, the Heisenberg correction term has less influence as the projectile momentum increases. We present the total cross-sections of projectile ionization, electron capture, and system breakdown in C5+ ions and Li2+ ion collisions with atomic hydrogen in the impact energy range from 10 keV to 160 keV, which is of interest in astrophysical plasmas, atmospheric sciences, plasma laboratories, and fusion research.

Analytical method for predicting interference in ion mobility spectrometric detection of drug compounds with phthalates

In ion mobility spectrometry (IMS), ions are distinguished by their mobility through a gas, which depends on their size, shape, and charge. Ions with very similar reduced ion mobility (Ko) values are not separated. This technique is widely used for the onsite detection of drug compounds. However, the presence of phthalates can interfere with IMS-based detection of drug compounds because some of them may have Ko values similar to those of drug compounds. In this study, the interference of phthalates in the IMS detection of drug compounds was investigated using drug compounds and phthalates in the molecular weight range of 134–335 g mol−1. The relationships between Ko and the m/z value, collision cross section (CCS), maximum cross section (CSmax), and minimum cross section (CSmin) of the product ions were examined. The CSmax and CSmin values were calculated from the energy-minimized structures of the ions. It was found that the m/z, CCS, and CSmax values were not closely related to the Ko value. However, the CSmin values showed a strong correlation with the Ko values, exhibiting excellent linear relationships with R2 values of 0.997 for the drug compounds and 0.990 for the phthalates. By comparing the CSmin values of drug compounds and phthalates, the probability of their peaks overlapping in the IMS spectra can be determined. For chemicals that are difficult to identify, the overlap of the peaks with the target chemicals can be determined using their CSmin values.

Formation of complex organics by covalent and non-covalent interactions of the sequential reactions of 1-4 acrylonitrile molecules with the benzonitrile radical cation.

Benzonitrile molecules are present in ionizing environments including interstellar clouds and solar nebulae, where their ions can form adducts with neutral molecules such as acrylonitrile leading to the formation of a variety of nitrogen-containing complex organics. Herein, we report on the formation of complex organics by the sequential reactions of 1-4 acrylonitrile (C3NH3) molecules with the benzonitrile radical cation (C7NH5+˙). The results reveal the formation of the covalently bonded N-acrylonitrile-benzonitrile radical cation (C10N2H8+˙) with a rate coefficient of 2.2 (±0.4) × 10-11 cm3 s-1 at 423 K and a calculated collision cross-section of 73.8 Å2 in good agreement with the measured cross-section of 70.7 Å2 of the C10N2H8+˙ adduct. Subsequent reversible association of 1-3 acrylonitrile molecules with the N-acrylonitrile-benzonitrile radical cation (C10N2H8+˙) at lower temperatures (250-200 K) results in the formation of the N-rich clusters (C10N2H8+˙)(C3NH3)1-3 which can be enhanced in the very cold cores of the interstellar medium (ISM) and could offer unique potential candidates for the substantial amount of nitrogen carriers detected in the emission spectra of the ISM. The observed N-acrylonitrile-benzonitrile covalent adduct and its associated acrylonitrile clusters could have significant implications in the formation of different types of complex organics in different regions of outer space. It is anticipated that the current results would have direct implications in the search for nitrogen-containing complex organics in space.

Monte Carlo Calculation of Electron Energy Degradation in a Pure O2 Atmosphere

Electron degradation serves as a significant energy source in planetary upper atmospheres. In this study, a Monte Carlo model is constructed to investigate the degradation of electrons in a pure O2 atmosphere under the local approximation. Both elastic and inelastic collision processes between electrons and O2 molecules are considered in the model. The yield spectra, characterizing the occurrence of various inelastic collisions for a specified pair of incident and post-collision energies, are obtained from the model. When combined with the information of e-O2 collision cross sections, the yields of each inelastic process are then determined. Furthermore, we derive the mean energy per ion pair and the efficiencies of various inelastic processes, along with the yields of secondary (and higher-order) electrons. The calculations presented here are beneficial for understanding the ionization and heating budgets in planetary atmospheres containing a significant amount of O2 such as Earth and Jovian icy satellites (Europa, Ganymede, and Callisto).

MS-DIAL 5 multimodal mass spectrometry data mining unveils lipidome complexities

Lipidomics and metabolomics communities comprise various informatics tools; however, software programs handling multimodal mass spectrometry (MS) data with structural annotations guided by the Lipidomics Standards Initiative are limited. Here, we provide MS-DIAL 5 for in-depth lipidome structural elucidation through electron-activated dissociation (EAD)-based tandem MS and determining their molecular localization through MS imaging (MSI) data using a species/tissue-specific lipidome database containing the predicted collision-cross section values. With the optimized EAD settings using 14 eV kinetic energy, the program correctly delineated lipid structures for 96.4% of authentic standards, among which 78.0% had the sn-, OH-, and/or C = C positions correctly assigned at concentrations exceeding 1 μM. We showcased our workflow by annotating the sn- and double-bond positions of eye-specific phosphatidylcholines containing very-long-chain polyunsaturated fatty acids (VLC-PUFAs), characterized as PC n-3-VLC-PUFA/FA. Using MSI data from the eye and n-3-VLC-PUFA-supplemented HeLa cells, we identified glycerol 3-phosphate acyltransferase as an enzyme candidate responsible for incorporating n-3 VLC-PUFAs into the sn1 position of phospholipids in mammalian cells, which was confirmed using EAD-MS/MS and recombinant proteins in a cell-free system. Therefore, the MS-DIAL 5 environment, combined with optimized MS data acquisition methods, facilitates a better understanding of lipid structures and their localization, offering insights into lipid biology.

Photochemical and Collision-Induced Cross-Linking of Asp, Glu, Asn, and Gln Residues in Peptide-Nitrile Imine Conjugate Ions in the Gas Phase.

Peptide conjugates furnished with a 2,5-diaryltetrazolecarbonyl tag at the C-terminal lysine, which we call peptide-tet-K, were found to undergo efficient cross-linking of Asp, Glu, Asn, and Gln residues to transient nitrile-imine intermediates produced by photodissociation and collision-induced dissociation (CID) of the tetrazole ring in gas-phase ions. UV photodissociation (UVPD) at 213 nm achieved cross-linking conversion yields of 37 and 61% for DAAAK-tet-K and EAAAK-tet-K, respectively. The yields for NAAAK-tet-K and QAAAK-tet-K were 29 and 57%, respectively. Even higher cross-link yields were found for CID-MS3 of stable denitrogenated ions, (peptide-tet-K-N2 + H)+, that were in the 69-83% range. Different types of cross-links were distinguished by CID-MSn that showed a distinct series of backbone fragment ions, loss of N-terminal groups, and loss of phenylhydrazine from the modified nitrile imines. The Asp and Glu side-chain carboxyl groups were major participants in cross-linking that resulted in proton and oxygen transfer to the nitrile imine group. Other types of cross-linking involved Asn and Gln CONH2 groups and backbone amides. Cyclic ion mobility-mass spectrometry was used to separate NAAAK-tet-K and QAAAK-tet-K conformers and products of their collision-induced denitrogenation. Linear nitrile-imine and cross-linked ion structures were identified by comparing the experimental collision cross sections (CCSexp) to those for structures obtained by combined Born-Oppenheimer molecular dynamics and density functional theory (DFT) calculations. The formation of cross-links was found to be energetically favorable and involved proton-facilitated nucleophilic attack at the nitrile-imine carbon atom.

Cross-Interactions of Aβ Peptides Implicated in Alzheimer's Disease Shape Amyloid Oligomer Structures and Aggregation.

A defining hallmark of Alzheimer's disease (AD) is the synaptic aggregation of the amyloid β (Aβ) peptide. In vivo, Aβ production results in a diverse mixture of variants, of which Aβ40, Aβ42, and Aβ43 are profusely present in the AD brain, and their relative abundance is recognized to play a role in disease onset and progression. Nonetheless, the occurrence of Aβ40, Aβ42, and Aβ43 hetero-oligomerization and the subsequent effects on Aβ aggregation remain elusive and were investigated here. Using thioflavin-T (ThT)-monitored aggregation assays and native mass spectrometry coupled to ion mobility analysis (IM-MS), we first show that all Aβ peptides are aggregation-competent and self-assemble into homo-oligomers with distinct conformational populations, which are more pronounced between Aβ40 than the longer variants. ThT assays were then conducted on binary mixtures of Aβ variants, revealing that Aβ42 and Aβ43 aggregate independently from Aβ40 but significantly speed up Aβ40 fibrillation. Aβ42 and Aβ43 were observed to aggregate concurrently and mutually accelerate fibril formation, which likely involves hetero-oligomerization. Accordingly, native MS analysis revealed pairwise oligomerization between all variants, with the formation of heterodimers and heterotrimers. Interestingly, IM-MS indicates that hetero-oligomers containing longer Aβ variants are enriched in conformers with lower collision cross-sections when compared to their homo-oligomer counterparts. This suggests that Aβ42 and Aβ43 are capable of remodeling the oligomer structure toward a higher compaction level. Altogether, our findings provide a mechanistic description for the hetero-oligomerization of Aβ variants implicated in AD, contributing to rationalizing their in vivo proteotoxic interplay.

Interpretations of the ATLAS measurements of Higgs boson production and decay rates and differential cross-sections in pp collisions at s = 13 TeV

Measurements of the Higgs boson production times decay rates and differential cross-sections have recently been performed by the ATLAS experiment in several decay channels using up to 139 fb−1 of proton-proton collision data at s = 13 TeV recorded at the Large Hadron Collider. This paper presents multiple interpretations of these Higgs boson measurements. Measurements of production-mode cross-sections, simplified template cross-sections and fiducial differential cross-sections in different decay channels are reparameterised in terms of the impact of Standard Model effective field theory operators, and constraints are reported on the corresponding Wilson coefficients. Production and decay rate measurements are interpreted in UV-complete extensions of the Standard Model, namely the two-Higgs-doublet model (2HDM) near the alignment limit and the Minimal Supersymmetric Standard Model (MSSM) for various MSSM benchmark scenarios. The constraints on the 2HDM parameters (cos(β − α), tanβ) and the MSSM parameters (mA, tanβ) are complementary to those obtained from direct searches for additional Higgs bosons.

Spatial resolution of dijet photoproduction in near-encounter ultraperipheral nuclear collisions

We present next-to-leading order perturbative QCD predictions for inclusive dijet photoproduction in ultraperipheral nucleus-nucleus collisions (UPCs) within the impact-parameter dependent equivalent photon approximation. Taking into account the finite size of both the photon-emitting nucleus and the target nucleus, we show that this process is sensitive to the transverse-plane geometry of the UPC events. We show that this leads to a sizable, 20–40% effect for large values of the zγ variable in the dijet photoproduction cross section in lead-lead UPCs at 5.02 TeV compared to the widely used pointlike approximation where the nuclear radius is accounted for only as a sharp cutoff in the photon flux calculation. This resolution of the spatial degrees of freedom is a result of having high-transverse-momentum jets in the final state, which at the large-zγ kinematics requires a highly energetic photon in the initial state, thus biasing the collisions to small impact-parameter “near-encounter” configurations. We further discuss the role of the forward-neutron event-class selection in isolating the photonuclear cross section in the nucleus-nucleus collisions, and employ the needed electromagnetic breakup survival factor in our predictions. Published by the American Physical Society 2024

Machine Learning Framework for Conotoxin Class and Molecular Target Prediction.

Conotoxins are small and highly potent neurotoxic peptides derived from the venom of marine cone snails which have captured the interest of the scientific community due to their pharmacological potential. These toxins display significant sequence and structure diversity, which results in a wide range of specificities for several different ion channels and receptors. Despite the recognized importance of these compounds, our ability to determine their binding targets and toxicities remains a significant challenge. Predicting the target receptors of conotoxins, based solely on their amino acid sequence, remains a challenge due to the intricate relationships between structure, function, target specificity, and the significant conformational heterogeneity observed in conotoxins with the same primary sequence. We have previously demonstrated that the inclusion of post-translational modifications, collisional cross sections values, and other structural features, when added to the standard primary sequence features, improves the prediction accuracy of conotoxins against non-toxic and other toxic peptides across varied datasets and several different commonly used machine learning classifiers. Here, we present the effects of these features on conotoxin class and molecular target predictions, in particular, predicting conotoxins that bind to nicotinic acetylcholine receptors (nAChRs). We also demonstrate the use of the Synthetic Minority Oversampling Technique (SMOTE)-Tomek in balancing the datasets while simultaneously making the different classes more distinct by reducing the number of ambiguous samples which nearly overlap between the classes. In predicting the alpha, mu, and omega conotoxin classes, the SMOTE-Tomek PCA PLR model, using the combination of the SS and P feature sets establishes the best performance with an overall accuracy (OA) of 95.95%, with an average accuracy (AA) of 93.04%, and an f1 score of 0.959. Using this model, we obtained sensitivities of 98.98%, 89.66%, and 90.48% when predicting alpha, mu, and omega conotoxin classes, respectively. Similarly, in predicting conotoxins that bind to nAChRs, the SMOTE-Tomek PCA SVM model, which used the collisional cross sections (CCSs) and the P feature sets, demonstrated the highest performance with 91.3% OA, 91.32% AA, and an f1 score of 0.9131. The sensitivity when predicting conotoxins that bind to nAChRs is 91.46% with a 91.18% sensitivity when predicting conotoxins that do not bind to nAChRs.

Exploring the structural heterogeneity of IgG1s: A comprehensive analysis of glycosylated and deglycosylated forms using TIMS-TOF-MS technique

Denaturation of IgG is a complex and multistep process that involves various structural changes in the protein as it loses its native conformation. IgG denaturation is crucial in therapeutic antibody development, manufacturing, and storage. Glycosylated IgG is generally more resistant to denaturing conditions due to the stabilizing effect of its glycans. In contrast, deglycosylated IgG is more vulnerable to structural disruption. In this study, we examined the intact level and large fragments of IgG glycoforms. Our investigation focused on changes in structural heterogeneity through glycosylation under denaturing conditions. Collision cross-sections (CCS) of intact level and large fragments of IgG1 molecules have been determined using reversed phase-high performance liquid chromatography-electrospray ionization-trapped ion mobility-time-of-flight mass spectrometry (RP-HPLC-ESI-TIMS-TOF-MS) technique. At the intact level, deglycosylated bevacizumab (IgG1B) and trastuzumab (IgG1T) could be differentiated at specific charge states and CCS ranges. The deglycosylated IgG1B molecule showed a broader CCS range than the IgG1T molecule. This result demonstrated that the deglycosylated IgG1T was altered from deglycosylated IgG1B in the gas phase under denaturing conditions. The lowering in the collision cross-section distribution (CCSD) values indicates a decrease in the structural heterogeneity of the intact IgG after deglycosylation. The ion mobility-mass spectrometry (IM-MS) analyses performed in this study showed that the glycans on the Fc region of IgG1s can trigger structural changes in the molecule under denaturing conditions. Depending on whether they are glycosylated or deglycosylated, various conformations of IgG1 molecules could be distinguished in RP-HPLC-TIMS-TOF-MS analysis.

Measurement of differential ZZ + jets production cross sections in pp collisions at s = 13 TeV

Diboson production in association with jets is studied in the fully leptonic final states, pp → (Z/γ*)(Z/γ*) + jets → 2ℓ2ℓ′ + jets, (ℓ, ℓ′ = e or μ) in proton-proton collisions at a center-of-mass energy of 13 TeV. The data sample corresponds to an integrated luminosity of 138 fb−1 collected with the CMS detector at the LHC. Differential distributions and normalized differential cross sections are measured as a function of jet multiplicity, transverse momentum pT, pseudorapidity η, invariant mass and ∆η of the highest-pT and second-highest-pT jets, and as a function of invariant mass of the four-lepton system for events with various jet multiplicities. These differential cross sections are compared with theoretical predictions that mostly agree with the experimental data. However, in a few regions we observe discrepancies between the predicted and measured values. Further improvement of the predictions is required to describe the ZZ+jets production in the whole phase space.

Sequential Reactions of Acetylene with the Benzonitrile Radical Cation: New Insights into Structures and Rate Coefficients of the Covalent Ion Products.

Benzonitrile radical cations generated in ionizing environments such as solar nebulae and interstellar clouds can react with neutral molecules such as acetylene to form a variety of nitrogen-containing complex organics. Herein, we present results from mass-selected ion mobility experiments and coupled-cluster and DFT calculations for the sequential reactions of acetylene with the benzonitrile radical cation (C7NH5+•). The results reveal the formation of two covalently bonded adduct ions C9NH7+• and C11NH9+• with individual rate coefficients of 2.1(±0.4) × 10-11 cm3 s-1 and 1.1(±0.9) × 10-11 cm3 s-1, respectively measured at 334.5 K. The direct addition of acetylene onto the N atom of the benzonitrile cation results in the formation of a N-acetylene-benzonitrile+• radical cation with a calculated collision cross-section of 67.5 Å2 in perfect agreement with the measured cross-section of 67.5 Å2 of the C9NH7+• adduct. The measured collision cross-section of the second covalent adduct C11NH9+• (72.2 Å2) is also in excellent agreement with the calculated cross-section (71.2 Å2) of the lowest energy isomer of the C11NH9+• ion corresponding to the 2-phenylpyridine structure. The formation of the bicyclic 2-phenylpyridine radical cation is explained by the rapid conversion of the classical radical cation C11NH9+• into a distonic ion structure that can efficiently cyclize in an exothermic transformation to form the 2-phenylpyridine radical cation. This intriguing mechanism could explain the formation of N-containing complex organics in different regions of outer space. The current results are expected to have direct implications for the search for nitrogen-containing complex organics in space.

Differential and integral elastic cross sections for electrons and positrons collisions with carbon dioxide molecules

This study investigates the elastic scattering of electrons and positrons by carbon dioxide (CO2) molecules over a wide energy range from 1 eV to 1 KeV. We have computed differential and integral elastic cross-sections (DCS and ICS) using partial wave analysis and compared the theoretical results with available experimental data. Our findings show a strong correlation with experimental results, particularly at mid to high-energy ranges, validating the employed theoretical frameworks. Additionally, the Sherman function was examined to explore spin polarisation effects during scattering events. This research provides crucial insights for applications in atmospheric physics, materials science, and molecular scattering processes, demonstrating its direct relevance to real-world problems and paving the way for further investigation into particle-molecule interactions.

Photochemical and Collision-Induced Cross-Linking in Stereochemically Distinct Scaffolds of Peptides and Nitrile Imines in Gas-Phase Ions.

Intramolecular cross-linking between peptides and nitrile-imine intermediates was studied in stereochemically distinct conjugates in which the reacting components were mounted on cis-1,2-cyclohexane and trans-1,4-cyclohexane scaffolds that we call 1,2-s-peptides and 1,4-s-peptides, respectively. The nitrile-imine intermediates were generated by N2 loss from 2,5-diaryltetrazole tags upon UV-photodissociation at 213 and 250 nm or by collision-induced dissociation, and further interrogated by CID and UVPD-MS3. Peptide fragment ion series originating from linear structures and macrocyclic cross-links were distinguished and used to quantify the cross-linking yields. The yields in MS2 varied between 27% for AAAG conjugates to 78% for GAAAK conjugates, depending on the peptide sequence. The CID-MS3 yields were in a 57-97% range, depending on the peptide sequence. Structures of 1,2-s-peptide and 1,4-s-peptide ions as well as several of their nitrile-imine intermediates and cross-links were investigated by high-resolution cyclic ion mobility in combination with Born-Oppenheimer molecular dynamics and density functional theory calculations. Matches between the experimental and calculated collision cross sections and ion relative Gibbs energies were used to assign peptide structures. Peptide conjugates C-terminated with Gly and Lys residues underwent cross-linking by the carboxyl group, as established by MS3 sequencing and corroborated by carboxyl blocking experiments that lowered the cross-linking yields. Peptide conjugates C-terminated with Arg also cross-linked via the side-chain guanidine group. A notable feature of the 1,4-s-peptide ions was the participation of low-energy twist-boat cyclohexane conformers that was enforced by strong hydrogen bonds between the peptide and nitrile imine.

Elucidating Protein Structures in the Gas Phase: Traversing Configuration Space with Biasing Methods.

Achieving accurate characterization of protein structures in the gas phase continues to be a formidable challenge. To tackle this issue, the present study employs Molecular Dynamics (MD) simulations in tandem with enhanced sampling techniques (methods designed to efficiently explore protein conformations). The objective is to identify suitable structures of proteins by contrasting their calculated Collision Cross-Section (CCS) with those observed experimentally. Significant discrepancies were observed between the initial MD-simulated and experimentally measured CCS values through Ion Mobility-Mass Spectrometry (IMS-MS). To bridge this gap, we employed two distinct enhanced sampling methods, Harmonic Biasing Potential and Adaptive Biasing Force, which help the proteins overcome energy barriers to adopt more compact configurations. These techniques leverage the radius of gyration as a reaction coordinate (guiding parameter), guiding the system toward compressed states that potentially match experimental configurations more closely. The guiding forces are only employed to overcome existing barriers and are removed to allow the protein to naturally arrive at a potential gas phase configuration. The results demonstrated close alignment (within ∼4%) between simulated and experimental CCS values despite using different strengths and/or methods, validating their efficacy. This work lays the groundwork for future studies aimed at optimizing biasing methods and expanding the collective variables used for more accurate gas-phase structural predictions.

The Influence of Non‐Thermal Collisions in Europa's Atmosphere

AbstractIn this study, we show that non‐thermal collisions can play a significant role in shaping Europa's exospheric structure. Collisions between radiolytically produced and the O produced via electron impact dissociation of affect the exospheric structure and escape rates. Specifically, O + collisions lead to the production of a non‐thermal population, and increase the escape while decreasing the O escape. These collisions are dependent on three specific physical parameters: (a) the density of , (b) the electron impact dissociation rate of , and (c) the O + collision cross section. We demonstrate here that O + collisions affect Europa's atmosphere even in the lowest limits considered. Thus, to more accurately determine the influence O + collisions have on Europa's atmosphere in preparation for the forthcoming spacecraft missions, Europa Clipper and the JUpiter ICy moons Explorer (JUICE), these physical parameters need to be better constrained.

Eliminating the Deadwood: A Machine Learning Model for CCS Knowledge-Based Conformational Focusing for Lipids.

Accurate elucidation of gas-phase chemical structures using collision cross section (CCS) values obtained from ion-mobility mass spectrometry benefits from a synergism between experimental and in silico results. We have shown in recent work that for a molecule of modest size with a proscribed conformational space we can successfully capture a conformation(s) that can match experimental CCS values. However, for flexible systems such as fatty acids that have many rotatable bonds and multiple intramolecular London dispersion interactions, it becomes necessary to sample a much greater conformational space. Sampling more conformers, however, accrues significant computational cost downstream in optimization steps involving quantum mechanics. To reduce this computational expense for lipids, we have developed a novel machine learning (ML) model to facilitate conformer filtering according to the estimated gas-phase CCS values. Herein we report that the implementation of our CCS knowledge-based approach for conformational sampling resulted in improved structure prediction agreement with experiment by achieving favorable average CCS prediction errors of ∼2% for lipid systems in both the validation set and the test set. Moreover, most of the gas-phase candidate conformations obtained by using CCS focusing achieved lower energy-minimum geometries than the candidate conformations without focusing. Altogether, the implementation of this ML model into our modeling workflow has proven to be beneficial for both the quality of the results and the turnaround time. Finally, while our approach is limited to lipids, it can be readily extended to other molecules of interest.

B-282 Cost and Benefits of Utilization of Ion Mobility Mass Spectrometry Testing for Drugs of Abuse Compared to Immunoassay Screening in a Large Urban Hospital

Abstract Background Urine drugs of abuse testing (DOAU) in US hospitals are mainly orders from the emergency department performed by immunoassay screening. Testing can be relatively expensive, and results may suffer from non-idealities in sensitivity, specificity, and from panel limitations. A potential approach to these issues for immunoassays is instead to employ mass spectrometry (MS), where a large and flexible analyte list can be evaluated providing needed specificity and reducing send out costs for confirmatory testing. We compared results and cost impacts of a DOAU assay on a prototype, ion mobility mass spectrometry system to those obtained from an array of immunoassays offered as routine tests at a large urban hospital. Methods Immunoassay screen-positive and screen-negative urine samples were isolated from among those submitted for a DOAU panel (amphetamines, barbiturates, benzodiazepines, cannabis, cocaine, opiates, and methadone) or for additional single drug tests (fentanyl, oxycodone, phencyclidine, buprenorphine, propoxyphene). Sample aliquots (3 mL) were retrieved after not more than 5 days of refrigerated storage of the original sample and were subsequently frozen. These samples were then subjected to MS analysis using the prototype system where identification of 62 compounds was based on mass and ion mobility-derived Collision Cross Section (CCS). An established library of m/z, SLIMCCSHe, peak intensity, and peak width for each analyte was used, with acceptable method performance characteristics as based on applicable CLIA validation guidelines. The MS list included compounds covered by the immunoassay classes as well as additional drugs not expected to be positive among the immunoassays test results but of value for routine toxicological assessments. Results comparisons, utility of results, sample-to-result turnaround times, and overall financial implications were compared between the two workflows. Results Among immunoassay screen-negative samples, MS analysis showed additional positive results for metabolites of parent drugs as well as novel psychoactive substances and known adulterants in >10% of cases. Among the immunoassay screen-positive results, MS analysis confirmed immunoassay results (>90%) but picked up an additional positive component in over 15% of cases. Cost per complete patient result for the immunoassay was approximately $2.80 in comparison to an estimated cost of $5 for the MS method. Confirmation testing for positive immunoassay screen results (16% of all samples) would cost an additional $70 per sample, whereas the need for confirmation testing is eliminated for the MS method. Thus, use of the MS testing workflow would be superior to an immunoassay with reflex testing workflow while at the same time providing a clinically superior service of more diverse positive test results and elimination of often overlooked false negatives. Turnaround time between immunoassay (1h) and MS (50m) were judged to be similar. Conclusions The MS approach confirmed >90% of all immunoassay positive screen results. In addition, the MS method identified additional positive results not detected by the immunoassay despite an overall similar estimated time for sample to patient. Need for development of a user friendly, MS analyzer for use in hospitals is supported based on cost-effective and analytically superior tests for DOAU compared to screens performed using immunoassay panels.