Ultraviolet Photodissociation and Activated Electron Photodetachment Mass Spectrometry for Top-Down Sequencing of Modified Oligoribonucleotides

Top-Down Characterization of Bacterial Lipopolysaccharides and Lipooligosaccharides Using Activated-Electron Photodetachment Mass Spectrometry

Lipopolysaccharides (LPS) and lipooligosaccharides (LOS) are located in the outer membrane of Gram-negative bacteria and are comprised of three distinctive parts: lipid A, core oligosaccharide (OS), and O-antigen. The structure of each region influences bacterial stability, toxicity, and pathogenesis. Here, we highlight the use of targeted activated-electron photodetachment (a-EPD) tandem mass spectrometry to characterize LPS and LOS from two crucial players in the human gut microbiota, Escherichia coli Nissle and Bacteroides fragilis. a-EPD is a hybrid activation method that uses ultraviolet photoirradiation to generate charge-reduced radical ions followed by collisional activation to produce informative fragmentation patterns. We benchmark the a-EPD method for top-down characterization of triacyl LOS from E. coli R2, then focus on characterization of LPS from E. coli Nissle and B. fragilis. Notably, a-EPD affords extensive fragmentation throughout the backbone of the core OS and O-antigen regions of LPS from E. coli Nissle. This hybrid approach facilitated the elucidation of structural details for LPS from B. fragilis, revealing a putative hexuronic acid (HexA) conjugated to lipid A.

Phosphorothioate RNA Analysis by NETD Tandem Mass Spectrometry

Therapeutic RNAs are routinely modified during their synthesis to ensure proper drug uptake, stability, and efficacy. Phosphorothioate (PS) RNA, molecules in which one or more backbone phosphates are modified with a sulfur atom in place of standard nonbridging oxygen, is one of the most common modifications because of ease of synthesis and pharmacokinetic benefits. Quality assessment of RNA synthesis, including modification incorporation, is essential for drug selectivity and performance, and the synthetic nature of the PS linkage incorporation often reveals impurities. Here, we present a comprehensive analysis of PS RNA via tandem mass spectrometry (MS). We show that activated ion-negative electron transfer dissociation MS/MS is especially useful in diagnosing PS incorporation, producing diagnostic a- and z-type ions at PS linkage sites, beyond the standard d- and w-type ions. Analysis using resonant and beam-type collision-based activation reveals that, overall, more intense sequence ions and base-loss ions result when a PS modification is present. Furthermore, we report increased detection of b- and x-type product ions at sites of PS incorporation, in addition to the standard c- and y-type ions. This work reveals that the gas-phase chemical stability afforded by sulfur alters RNA dissociation and necessitates inclusion of additional product ions for MS/MS of PS RNA.

A High-Throughput Workflow for Mass Spectrometry Analysis of Nucleic Acids by Nanoflow Desalting

Broad interest in nucleic acids, both their therapeutic capabilities and understanding the nuances of their structure and resulting function, has increased in recent years. Post-transcriptional modifications, in particular, have become an important analysis target, as these covalent modifications to the sugars, nitrogenous bases, and phosphate backbone impart differential functionality to synthetic and biological nucleic acids. Characterizing these post-transcriptional modifications can be difficult with traditional sequencing workflows; however, advancements in top-down mass spectrometry address these challenges. Online desalting platforms have enabled facile sample cleanup and reliable ionization of increasingly large (100 nt) oligonucleotides, and application of existing tandem mass spectrometry techniques has yielded information-rich spectra which can be used to interrogate primary sequences. To extend the capabilities of top-down MS and its analysis of nucleic acids, we have developed a nanoflow desalting platform for high-throughput and low sample-use desalting coupled with collision-induced dissociation (CID), 213 nm ultraviolet photodissociation (UVPD), and activated-ion electron photodetachment dissociation (a-EPD) to yield high-quality MS/MS spectra. Fragments identified using an m/z-domain isotope matching strategy yielded high sequence coverage (>70%) of a yeast phenylalanine tRNA.

Radical-Induced Dissociation for Oligonucleotide Sequencing by TiO2/ZnAl-Layered Double Oxide-Assisted Laser Desorption/Ionization Mass Spectrometry

De novo sequencing of oligonucleotides remains challenging, especially for oligonucleotides with post-transcriptional or synthetic modifications. Mass spectrometry (MS) sequencing can reliably detect and locate all of the modification sites in oligonucleotides via m/z variance. However, current MS-based sequencing methods exhibit complex spectra and low ion abundance and usually require coupled instrumentation. Herein, we demonstrate a method of oligonucleotide sequencing using TiO2/ZnAl-layered double oxide (LDO)-assisted laser desorption/ionization (LDI)-MS based on radical-induced dissociation (RID). ·CH2OH radicals can be produced on the surface of a TiO2/ZnAl-LDO matrix via ultraviolet light, inducing an attack on the active site of the oligonucleotide phosphate skeleton to create typical "a-, a-B-, c·-, d-, w-, and y"-type fragments. Compared with the spectra obtained via collision-based methods, such as collision-induced dissociation and higher-energy collisional dissociation, the LDI-MS spectra based on RID exhibit single-charged signals, fewer types of fragments, and a lower proportion of unknown noise peaks. We demonstrate full sequence coverage for a 6-mer 2'-O-methyl-modified oligonucleotide and a 21-mer small interfering RNA and show that RID can sequence oligonucleotides with modifications. Importantly, the mechanism responsible for the RID of the oligonucleotide phosphate skeleton was investigated through offline experiments, demonstrating consistent results with density functional theory calculations.

Integrating Internal Fragments in the Interpretation of Top-Down Sequencing Data of Larger Oligonucleotides

In the context of direct top-down analysis or concerted bottom-up characterization of nucleic acid samples, the waning yield of terminal fragments as a function of precursor ion size poses a significant challenge to the gas-phase sequencing of progressively larger oligonucleotides. In this report, we examined the behavior of oligoribonucleotide samples ranging from 20 to 364 nt upon collision-induced dissociation (CID). The experimental data showed a progressive shift from terminal to internal fragments as a function of size. The systematic evaluation of experimental factors, such as collision energy, precursor charge, sample temperature, and the presence of chaotropic agents, showed that this trend could be modestly alleviated but not suppressed. This inexorable effect, which has been reported also for other activation techniques, prompted a re-examination of the features that have traditionally discouraged the utilization of internal fragments as a source of sequence information in data interpretation procedures. Our simulations highlighted the ability of internal fragments to produce self-consistent ladders with either end corresponding to each nucleotide in the sequence, which enables both proper alignment and correct recognition of intervening nucleotides. In turn, contiguous ladders display extensive overlaps with one another and with the ladders formed by terminal fragments, which unambiguously constrain their mutual placement within the analyte sequence. The experimental data borne out the predictions by showing ladders with extensive overlaps, which translated into uninterrupted "walks" covering the entire sequence with no gaps from end to end. More significantly, the results showed that combining the information afforded by internal and terminal ladders resulted in much a greater sequence coverage and nucleotide coverage depth than those achievable when either type of information was considered separately. The examination of a series of 58-mer oligonucleotides with high sequence homology showed that the assignment ambiguities engendered by internal fragments did not significantly exceed those afforded by the terminal ones. Therefore, the balance between potential benefits and perils of including the former makes a compelling argument for the development of integrated data interpretation strategies, which are better equipped for dealing with the changing fragmentation patterns obtained from progressively larger oligonucleotides.

Review of fragmentation of synthetic single-stranded oligonucleotides by tandem mass spectrometry from 2014 to 2022

The fragmentation of oligonucleotides by mass spectrometry allows for the determination of their sequences. It is necessary to understand how oligonucleotides dissociate in the gas phase, which allows interpretation of data to obtain sequence information. Since 2014, a range of fragmentation mechanisms, including a novel internal rearrangement, have been proposed using different ion dissociation techniques. The recent publications have focused on the fragmentation of modified oligonucleotides such as locked nucleic acids, modified nucleobases (methylated, spacer, nebularine and aminopurine) and modification to the carbon 2'-position on the sugar ring; these modified oligonucleotides are of great interest as therapeutics. Comparisons of different dissociation techniques have been reported, including novel approaches such as plasma electron detachment dissociation and radical transfer dissociation. This review covers the period 2014-2022 and details the new knowledge gained with respect to oligonucleotide dissociation using tandem mass spectrometry (without priori sample digestion) during that time, with a specific focus on synthetic single-stranded oligonucleotides.

Spacer Fidelity Assessments of Guide RNA by Top-Down Mass Spectrometry

The advancement of CRISPR-based gene editing tools into biotherapeutics offers the potential for cures to genetic disorders and for new treatment paradigms for even common diseases. Arguably, the most important component of a CRISPR-based medicine is the guide RNA, which is generally large (>100-mer) synthetic RNA composed of a "tracr" and "spacer" region, the latter of which dictates the on-target editing site as well as potential undesired off-target edits. Aiming to advance contemporary capabilities for gRNA characterization to ensure the spacer region is of high fidelity, top-down mass spectrometry was herein implemented to provide direct and quantitative assessments of highly modified gRNA. In addition to sequencing the spacer region and pinpointing modifications, top-down mass spectra were utilized to quantify single-base spacer substitution impurities down to <1% and to decipher highly dissimilar spacers. To accomplish these results in an automated fashion, we devised custom software capable of sequencing and quantifying impurities in gRNA spacers. Notably, we developed automated tools that enabled the quantification of single-base substitutions, including advanced isotopic pattern matching for C > U and U > C substitutions, and created a de novo sequencing strategy to facilitate the identification and quantification of gRNA impurities with highly dissimilar spacer regions.

Charge-State-Dependent Collision-Induced Dissociation Behaviors of RNA Oligonucleotides via High-Resolution Mass Spectrometry

Mass spectrometry (MS)-based analysis of RNA oligonucleotides (oligos) plays an increasingly important role in the development of RNA therapeutics and epitranscriptomics research. However, MS fragmentation behaviors of RNA oligomers are understood insufficiently. Herein, we characterized the negative-ion-mode fragmentation behaviors of 26 synthetic RNA oligos containing four to eight nucleotides using collision-induced dissociation (CID) on a high-resolution, accurate-mass instrument. We found that in CID spectra acquired under the normalized collision energy (NCE) of 35%, approximately 70% of the total peak intensity was attributed to sequencing ions (a-B, a, b, c, d, w, x, y, z), around 25% of the peak intensity came from precursor ions that experienced complete or partial loss of a nucleobase in the form of either a neutral or an anion, and the remainder were internal ions and anionic nucleobases. The top five sequencing ions were the y, c, w, a-B, and a ions. Furthermore, we observed that CID fragmentation behaviors of RNA oligos were significantly impacted by their precursor charge. Specifically, when the precursors had a charge from 1^- to 5^-, the fractional intensity of sequencing ions decreased, while that of precursors that underwent either neutral or charged losses of a nucleobase increased. Additionally, we found that RNA oligos containing 3'-U tended to produce precursors with HNCO and/or NCO^- losses, which presumably corresponded to isocyanic acid and cyanate anion, respectively. These findings provide valuable insights for better comprehending the mechanism behind RNA fragmentation by MS/MS, thereby facilitating the future automated identification of RNA oligos based on their CID spectra in a more efficient manner.

Top-Down Mass Spectrometry of Synthetic Single Guide Ribonucleic Acids Enabled by Facile Sample Clean-Up

In recent years, CRISPR-Cas9 genome editing has become an important technology in biomedical research and has demonstrated tremendous therapeutic potential. With Cas9 endonuclease, the use of single guide ribonucleic acids (sgRNAs) allows for sequence-specific cutting on target double-stranded deoxyribonucleic acids. Therefore, the design and quality of sgRNAs can greatly affect the efficiency and specificity of genome editing. Mass spectrometry (MS) has been a powerful tool to detect molecular features and sequence a variety of biomolecules; however, as the sizes of oligonucleotides get larger, it becomes more challenging to desalt samples and achieve high-quality intact spectra with effective fragmentation. Here, we develop a simple but effective online column-based clean-up method (reversed-phase column in a size exclusion mode) that removes formulation salts and metal adducts from larger oligonucleotides upon entering the mass spectrometer in a consistent manner. Using the top-down approach without any nuclease digestion, we characterized and sequenced 100-nucleotide-long sgRNAs by higher-energy collision dissociation (HCD), collision-induced dissociation (CID), ultraviolet photodissociation (UVPD), and activated electron photodetachment (a-EPD). In a single 10 min liquid chromatography-tandem MS (LC-MS/MS) run, CID yielded the best sequence coverage, of 67%. When adding complementary UVPD and a-EPD runs, we achieved 80% overall sequence coverage and 100% cleavages for the variable sequence, the first 20 nucleotides from the 5' end. This LC-MS/MS platform provides a facile top-down workflow to analyze and sequence larger chemically modified oligonucleotides with no sample treatment.