Nitroaromatic Antibiotics as Nitrogen Oxide Sources

In Vitro Activities of Oxazolidinone Antibiotics Alone and in Combination with C-TEMPO against Methicillin-Resistant Staphylococcus aureus Biofilms

Infections caused by methicillin-resistant Staphylococcus aureus (MRSA) are a global health concern. The propensity of MRSA to form biofilms is a significant contributor to its pathogenicity. Strategies to treat biofilms often involve small molecules that disperse the biofilm into planktonic cells. Linezolid and, by extension, theoxazolidinones have been developed to treat infections caused by Gram-positive bacteria such as MRSA. However, the clinical development of these antibiotics has mainly assessed the susceptibility of planktonic cells to the drug. Previous studies evaluating the anti-biofilm activity of theoxazolidinones have mainly focused on the biofilm inhibition of Enterococcus faecalis and methicillin-sensitive Staphylococcus aureus, with only a few studies investigating the activity of oxazolidinones for eradicating established biofilms for these species. Very little is known about the ability of oxazolidinones to eradicate MRSA biofilms. In this work, five oxazolidinones were assessed against MRSA biofilms using a minimum biofilm eradication concentration (MBEC) assay. All oxazolidinones had inherent antibiofilm activity. However, only ranbezolid could completely eradicate MRSA biofilms at clinically relevant concentrations. The susceptibility of the MRSA biofilms to ranbezolid was synergistically enhanced by coadministration with the nitroxide biofilm dispersal agent C-TEMPO. We presume that ranbezolid acts as a dual warhead drug, which combines the mechanism of action of the oxazolidinones with a nitric oxide donor or cytotoxic drug.

Aryl azoles based scaffolds for disrupting tumor microenvironment

Thirty-nine aryl azoles, thirteen triazoles and twenty-seven tetrazoles, have been synthetized and biologically evaluated to determine their activity as tumor microenvironment disruptors. Antiproliferative studies have been performed on tumor cell lines HT-29, A-549 and MCF-7 and on non-tumor cell line HEK-293. It has been studied in HT-29 the expression levels of biological targets which are involved in tumor microenvironment processes, such as PD-L1, CD-47, c-Myc and VEGFR-2. In addition, antiproliferative activity was evaluated when HT-29 were co-cultured with THP-1 monocytes and the secretion levels of IL-6 were also determined in these co-cultures. The angiogenesis effect of some selected compounds on HMEC-1 was also evaluated as well as their action against vasculogenic mimicry on HEK-293. Compounds bearing an amino group in the phenyl ring and a halogen atom in the benzyl ring showed promising results as tumor microenvironment disrupting agents. The most outstanding compound decrease dramatically the population of HT-29 cells when co-cultured with THP-1 monocytes and the levels of IL-6 secreted, as well as it showed moderate effects over PD-L1, CD-47 and c-Myc.

Increased Range of Catalytic Activities of Immobilized Compared to Colloidal Gold Nanoparticles

Gold nanoparticles (AuNPs) can be described as nanozymes, species that are able to mimic the catalytic activities of several enzymes, such as oxidase/peroxidase, reductase, or catalase. Most studies in the literature focus on the colloidal suspension of AuNPs, and it is obvious that their immobilization could open the doors to new applications thanks to their increased stability in this state. This work aimed to investigate the behavior of surfaces covered by immobilized AuNPs (iAuNPs). Citrate-stabilized AuNPs (AuNPs-cit) were synthesized and immobilized on glass slides using a simple dip coating method. The resulting iAuNPs were characterized (surface plasmon resonance, microscopy, quantification of immobilized AuNPs), and their multi-enzymatic-like activities (oxidase-, peroxidase-, and catalase-like activity) were evaluated. The comparison of their activities versus AuNPs-cit highlighted their added value, especially the preservation of their activity in some reaction media, and their ease of reuse. The huge potential of iAuNPs for heterogeneous catalysis was then applied to the degradation of two model molecules of hospital pollutants: metronidazole and methylene blue.

Vimentin single cysteine residue acts as a tunable sensor for network organization and as a key for actin remodeling in response to oxidants and electrophiles

Cysteine residues can undergo multiple posttranslational modifications with diverse functional consequences, potentially behaving as tunable sensors. The intermediate filament protein vimentin has important implications in pathophysiology, including cancer progression, infection, and fibrosis, and maintains a close interplay with other cytoskeletal structures, such as actin filaments and microtubules. We previously showed that the single vimentin cysteine, C328, is a key target for oxidants and electrophiles. Here, we demonstrate that structurally diverse cysteine-reactive agents, including electrophilic mediators, oxidants and drug-related compounds, disrupt the vimentin network eliciting morphologically distinct reorganizations. As most of these agents display broad reactivity, we pinpointed the importance of C328 by confirming that local perturbations introduced through mutagenesis provoke structure-dependent vimentin rearrangements. Thus, GFP-vimentin wild type (wt) forms squiggles and short filaments in vimentin-deficient cells, the C328F, C328W, and C328H mutants generate diverse filamentous assemblies, and the C328A and C328D constructs fail to elongate yielding dots. Remarkably, vimentin C328H structures resemble the wt, but are strongly resistant to electrophile-elicited disruption. Therefore, the C328H mutant allows elucidating whether cysteine-dependent vimentin reorganization influences other cellular responses to reactive agents. Electrophiles such as 1,4-dinitro-1H-imidazole and 4-hydroxynonenal induce robust actin stress fibers in cells expressing vimentin wt. Strikingly, under these conditions, vimentin C328H expression blunts electrophile-elicited stress fiber formation, apparently acting upstream of RhoA. Analysis of additional vimentin C328 mutants shows that electrophile-sensitive and assembly-defective vimentin variants permit induction of stress fibers by reactive species, whereas electrophile-resistant filamentous vimentin structures prevent it. Together, our results suggest that vimentin acts as a break for actin stress fibers formation, which would be released by C328-aided disruption, thus allowing full actin remodeling in response to oxidants and electrophiles. These observations postulate C328 as a "sensor" transducing structurally diverse modifications into fine-tuned vimentin network rearrangements, and a gatekeeper for certain electrophiles in the interplay with actin.

Recent advances in targeted antibacterial therapy basing on nanomaterials

Bacterial infection has become one of the leading causes of death worldwide, particularly in low-income countries. Despite the fact that antibiotics have provided successful management in bacterial infections, the long-term overconsumption and abuse of antibiotics has contributed to the emergence of multidrug resistant bacteria. To address this challenge, nanomaterials with intrinsic antibacterial properties or that serve as drug carriers have been substantially developed as an alternative to fight against bacterial infection. Systematically and deeply understanding the antibacterial mechanisms of nanomaterials is extremely important for designing new therapeutics. Recently, nanomaterials-mediated targeted bacteria depletion in either a passive or active manner is one of the most promising approaches for antibacterial treatment by increasing local concentration around bacterial cells to enhance inhibitory activity and reduce side effects. Passive targeting approach is widely explored by searching nanomaterial-based alternatives to antibiotics, while active targeting strategy relies on biomimetic or biomolecular surface feature that can selectively recognize targeted bacteria. In this review article, we summarize the recent developments in the field of targeted antibacterial therapy based on nanomaterials, which will promote more innovative thinking focusing on the treatment of multidrug-resistant bacteria.

Dual Action of Eeyarestatin 24 on Sec-Dependent Protein Secretion and Bacterial DNA

Eeyarestatin 24 (ES24) is a promising new antibiotic with broad-spectrum activity. It shares structural similarity with nitrofurantoin (NFT), yet appears to have a distinct and novel mechanism: ES24 was found to inhibit SecYEG-mediated protein transport and membrane insertion in Gram-negative bacteria. However, possible additional targets have not yet been explored. Moreover, its activity was notably better against Gram-positive bacteria, for which its mechanism of action had not yet been investigated. We have used transcriptomic stress response profiling, phenotypic assays, and protein secretion analyses to investigate the mode of action of ES24 in comparison with NFT using the Gram-positive model bacterium Bacillus subtilis and have compared our findings to Gram-negative Escherichia coli. Here, we show the inhibition of Sec-dependent protein secretion in B. subtilis and additionally provide evidence for DNA damage, probably caused by the generation of reactive derivatives of ES24. Interestingly, ES24 caused a gradual dissipation of the membrane potential, which led to delocalization of cytokinetic proteins and subsequent cell elongation in E. coli. However, none of those effects were observed in B. subtilis, thereby suggesting that ES24 displays distinct mechanistic differences with respect to Gram-positive and Gram-negative bacteria. Despite its structural similarity to NFT, ES24 profoundly differed in our phenotypic analysis, which implies that it does not share the NFT mechanism of generalized macromolecule and structural damage. Importantly, ES24 outperformed NFT in vivo in a zebrafish embryo pneumococcal infection model. Our results suggest that ES24 not only inhibits the Sec translocon, but also targets bacterial DNA and, in Gram-negative bacteria, the cell membrane.

Oxidative damage by 1,10-phenanthroline-5,6-dione and its silver and copper complexes lead to apoptotic-like death in Trichomonas vaginalis

Trichomoniasis is a neglected, parasitic, sexually transmitted infection. Resistance to the only approved drugs is increasing worldwide, leaving millions of people without alternative medications. Thus, the search for new therapeutic options against this infection is necessary. Previously, our group reported that 1,10-phenanthroline-5,6-dione (phendione) and its silver(I) and copper(II) complexes (abbreviated as Ag-phendione and Cu-phendione, respectively) presented activity against the amitochondriate parasite Trichomonas vaginalis, with Cu-phendione being the most effective (IC₅₀ = 0.84 μM). Methods: qRT-PCR, SEM, flow cytometry. The current study on the effects of Cu-phendione on the antioxidant metabolism of T. vaginalis by qRT-PCR revealed that the complex causes a decrease in the relative expression of mRNA of NADH oxidase, flavin reductase, superoxide dismutase, peroxiredoxin, iron-sulfur flavoprotein, rubrerythrin and osmotically inducible proteins. In contrast, the mRNA expression of flavodiiron protein was increased. Detoxification-related enzymes were downregulated, impairing oxygen metabolism in trophozoites and triggering a subsequent accumulation of the superoxide anion. Although no DNA fragmentation was observed, the treatment of parasites with Cu-phendione led to a significant reduction in cell size and a concomitant increase in granularity. The complex promoted phosphatidylserine exposure at the plasma membrane (as judged by Annexin V binding) and propidium iodide was unable to passively permeate the parasites. All of these outcomes are classical hallmarks of cell death by apoptosis. In essence, the trichomonacidal effect of Cu-phendione operates through redox homeostasis imbalance, which is a mode of action that is quite distinct from that caused by metronidazole.

Antileishmanial Activities of ( Z )-2-(Nitroimidazolylmethylene)-3( 2H )-Benzofuranones: Synthesis, In Vitro Assessment, and Bioactivation by NTR 1 and 2

The antileishmanial activity of a series of ( Z )-2-(heteroarylmethylene)-3(2 H )-benzofuranone derivatives, possessing 5-nitroimidazole or 4-nitroimidazole moieties, was investigated against Leishmania major promastigotes and some analogues exhibited prominent activities. Compounds with IC 50 values lower than 20 μM were further examined against L. donovani axenic amastigotes.

pH-Sensitive Drug Delivery System Based on Chitin Nanowhiskers-Sodium Alginate Polyelectrolyte Complex

Polyelectrolyte complexes (PECs), based on partially deacetylated chitin nanowhiskers (CNWs) and anionic polysaccharides, are characterized by their variability of properties (particle size, ζ-potential, and pH-sensitivity) depending on the preparation conditions, thereby allowing the development of polymeric nanoplatforms with a sustained release profile for active pharmaceutical substances. This study is focused on the development of hydrogels based on PECs of CNWs and sodium alginate (ALG) for potential vaginal administration that provide controlled pH-dependent antibiotic release in an acidic vaginal environment, as well as prolonged pharmacological action due to both the sustained drug release profile and the mucoadhesive properties of the polysaccharides. The desired hydrogels were formed as a result of both electrostatic interactions between CNWs and ALG (PEC formation), and the subsequent molecular entanglement of ALG chains, and the formation of additional hydrogen bonds. Metronidazole (MET) delivery systems with the desired properties were obtained at pH 5.5 and an CNW:ALG ratio of 1:2. The MET-CNW-ALG microparticles in the hydrogel composition had an apparent hydrodynamic diameter of approximately 1.7 µm and a ζ-potential of -43 mV. In vitro release studies showed a prolonged pH-sensitive drug release from the designed hydrogels; 37 and 67% of MET were released within 24 h at pH 7.4 and pH 4.5, respectively. The introduction of CNWs into the MET-ALG system not only prolonged the drug release, but also increased the mucoadhesive properties by about 1.3 times. Thus, novel CNW-ALG hydrogels are promising carriers for pH sensitive drug delivery carriers.

Bioactive Nitrosylated and Nitrated N-(2-hydroxyphenyl)acetamides and Derived Oligomers: An Alternative Pathway to 2-Amidophenol-Derived Phytotoxic Metabolites

Incubation of Aminobacter aminovorans, Paenibacillus polymyxa, and Arthrobacter MPI764 with the microbial 2-benzoxazolinone (BOA)-degradation-product 2-acetamido-phenol, produced from 2-aminophenol, led to the recently identified N-(2-hydroxy-5-nitrophenyl) acetamide, to the hitherto unknown N-(2-hydroxy-5-nitrosophenyl)acetamide, and to N-(2-hydroxy-3-nitrophenyl)acetamide. As an alternative to the formation of phenoxazinone derived from aminophenol, dimers- and trimers-transformation products have been found. Identification of the compounds was carried out by LC/HRMS and MS/MS and, for the new structure N-(2-hydroxy-5-nitrosophenyl)acetamide, additionally by 1D- and 2D-NMR. Incubation of microorganisms, such as the soil bacteria Pseudomonas laurentiana, Arthrobacter MPI763, the yeast Papiliotrema baii and Pantoea ananatis, and the plants Brassica oleracea var. gongylodes L. (kohlrabi) and Arabidopsis thaliana Col-0, with N-(2-hydroxy-5-nitrophenyl) acetamide, led to its glucoside derivative as a prominent detoxification product; in the case of Pantoea ananatis, this was together with the corresponding glucoside succinic acid ester. In contrast, Actinomucor elegans consortium synthesized 2-acetamido-4-nitrophenyl sulfate. 1 mM bioactive N-(2-hydroxy-5-nitrophenyl) acetamide elicits alterations in the Arabidopsis thaliana expression profile of several genes. The most responsive upregulated gene was pathogen-inducible terpene synthase TPS04. The bioactivity of the compound is rapidly annihilated by glucosylation.

Identification of nitrofuranylchalcone tethered benzoxazole-2-amines as potent inhibitors of drug resistant Mycobacterium tuberculosis demonstrating bactericidal efficacy

Ever increasing drug resistance has become an impeding threat that continues to hamper effective tackling of otherwise treatable tuberculosis (TB). Such dismal situation necessitates identification and exploration of multitarget acting newer chemotypes with bactericidal efficacy as a priority, that could efficiently hinder uncontrolled spread of TB. In this context, herein we present design, synthesis and bio-evaluation of chalcone tethered bezoxazole-2-amines as promising anti-TB chemotypes. Preliminary screening of 24 compounds revealed initial hits 3,4,5-trimethoxyphenyl and 5-nitrofuran-2-yl derivative exhibiting selective inhibition of Mycobacterium tuberculosis (Mtb) H37Rv. Further, structural optimization of hit compounds generated 12 analogues, amongst which 5-nitrofuran-2-yl derivatives displayed potent inhibition of not only drug-susceptible (DS) Mtb but also clinical isolates of drug-resistant (DR) Mtb strains equipotently. Moreover, cell viability test against Vero cells found these compounds with favourable selectivity. Time kill analysis led to the identification of the lead compound (E)-1-(4-((5-chlorobenzo[d]oxazol-2-yl)amino)phenyl)-3-(5-nitrofuran-2-yl)prop-2-en-1-one, that demonstrated bactericidal killing of Mtb bacilli. Together with acceptable microsomal stability, the lead compound of the series manifested all desirable traits of a promising antitubercular agent.

The Diverse Biological Activity of Recently Synthesized Nitro Compounds

The search for new and efficient pharmaceuticals is a constant struggle for medicinal chemists. New substances are needed in order to treat different pathologies affecting the health of humans and animals, and these new compounds should be safe, effective and have the fewest side effects possible. Some functional groups are known for having biological activity; in this matter, the nitro group (NO₂) is an efficient scaffold when synthesizing new bioactive molecules. Nitro compounds display a wide spectrum of activities that include antineoplastic, antibiotic, antihypertensive, antiparasitic, tranquilizers and even herbicides, among many others. Most nitro molecules exhibit antimicrobial activity, and several of the compounds mentioned in this review may be further studied as lead compounds for the treatment of H. pylori, P. aeruginosa, M. tuberculosis and S. mutans infections, among others. The NO₂ moiety triggers redox reactions within cells causing toxicity and the posterior death of microorganisms, not only bacteria but also multicellular organisms such as parasites. The same effect may be present in humans as well, so the nitro groups can be considered both a pharmacophore and a toxicophore at the same time. The role of the nitro group itself also has a deep effect on the polarity and electronic properties of the resulting molecules, and hence favors interactions with some amino acids in proteins. For these reasons, it is fundamental to analyze the recently synthesized nitro molecules that show any potential activity in order to develop new pharmacological treatments that enhance human health.

Functionalized Nitroimidazole Scaffold Construction and Their Pharmaceutical Applications: A 1950–2021 Comprehensive Overview

Nitroimidazole represents one of the most essential and unique scaffolds in drug discovery since its discovery in the 1950s. It was K. Maeda in Japan who reported in 1953 the first nitroimidazole as a natural product from Nocardia mesenterica with antibacterial activity, which was later identified as Azomycin 1 (2-nitroimidazole) and remained in focus until now. This natural antibiotic was the starting point for synthesizing numerous analogs and regio-isomers, leading to several life-saving drugs and clinical candidates against a number of diseases, including infections (bacterial, viral, parasitic) and cancers, as well as imaging agents in medicine/diagnosis. In the present decade, the nitroimidazole scaffold has again been given two life-saving drugs (Delamanid and Pretomanid) used to treat MDR (multi-drug resistant) tuberculosis. Keeping in view the highly successful track-record of the nitroimidazole scaffold in providing breakthrough therapeutic drugs, this comprehensive review focuses explicitly on presenting the activity profile and synthetic chemistry of functionalized nitroimidazole (2-, 4- and 5-nitroimidazoles as well as the fused nitroimidazoles) based drugs and leads published from 1950 to 2021. The present review also presents the miscellaneous examples in each class. In addition, the mutagenic profile of nitroimidazole-based drugs and leads and derivatives is also discussed.

Defenses of multidrug resistant pathogens against reactive nitrogen species produced in infected hosts

Bacterial pathogens have sophisticated systems that allow them to survive in hosts in which innate immunity is the frontline of defense. One of the substances produced by infected hosts is nitric oxide (NO) that together with its derived species leads to the so-called nitrosative stress, which has antimicrobial properties. In this review, we summarize the current knowledge on targets and protective systems that bacteria have to survive host-generated nitrosative stress. We focus on bacterial pathogens that pose serious health concerns due to the growing increase in resistance to currently available antimicrobials. We describe the role of nitrosative stress as a weapon for pathogen eradication, the detoxification enzymes, protein/DNA repair systems and metabolic strategies that contribute to limiting NO damage and ultimately allow survival of the pathogen in the host. Additionally, this systematization highlights the lack of available data for some of the most important human pathogens, a gap that urgently needs to be addressed.

Exploration of Nitroaromatic Antibiotics via Sanger's Reagent: Synthesis, In Silico, and Antimicrobial Evaluation

Facile synthesis of molecular hybrids containing a 2,4-dinitrophenyl moiety was achieved via nucleophilic aromatic substitution of the fluoride anion of Sanger's reagent (2,4-dinitrofluorobenzene) with various N, S, and O nucleophiles, considered as bioactive moieties. Antimicrobial evaluation of the new hybrids was carried out using amoxicillin and nystatin as antibacterial and antifungal reference standards, respectively. MIC test results identified the compounds 3, 4, and 7 as the most active hybrids against standard strains and multidrug-resistant strains (MDR) of Staphylococcus aureus, Escherichia coli, and Pseudomonas aurginosa. Most of the hybrids displayed two times the antibacterial activity of AMOX against MDR Pseudomonas aeruginosa, E. coli, and a standard strain of P. aeruginosa (ATCC 29853), while demonstrating a weak antifungal profile against Candida albicans. Selectivity profiles of the promising compounds 3, 4, 6, 7, 8, and 11 on WI-38 human cells were characterized, which indicated that compound 3 is the safest one (CC₅₀ 343.72 μM). The preferential anti-Gram-negative activity of our compounds led us to do docking studies on DNA gyrase B. Docking revealed that the potential antimicrobial compounds fit well into the active site of DNA gyrase B. Furthermore, in silico absorption, distribution, metabolism, and excretion (ADME) predictions revealed that most of the new compounds have high gastrointestinal absorption and a good oral bioavailability with no BBB permeability.

Recent advancement in drug development of nitro(NO2 )-heterocyclic compounds as lead scaffolds for the treatment of Mycobacterium tuberculosis

Tuberculosis (TB) is an infectious disease caused predominantly by Mycobacterium tuberculosis (Mtb). It was responsible for approximately 1.4 million deaths worldwide in 2019. The lack of new drugs to treat drug-resistant strains is a principal factor for the slow rise in TB infections. Our aim is to aid the development of new TB treatments by describing improvements (last decade, 2011-2021) to nitro(NO₂ )-based compounds that have shown activity or pharmacological properties (e.g., anti-proliferative, anti-kinetoplastid) against Mtb. For all compounds, we have included final correlations of minimum inhibitory concentrations against Mtb (H₃₇ Rv).

The zebrafish embryo as an in vivo model for screening nanoparticle-formulated lipophilic anti-tuberculosis compounds

With the increasing emergence of drug-resistant Mycobacterium tuberculosis strains, new and effective antibiotics against tuberculosis (TB) are urgently needed. However, the high frequency of poorly water-soluble compounds among hits in high-throughput drug screening campaigns is a major obstacle in drug discovery. Moreover, in vivo testing using conventional animal TB models, such as mice, is time consuming and costly, and represents a major bottleneck in lead compound discovery and development. Here, we report the use of the zebrafish embryo TB model for evaluating the in vivo toxicity and efficacy of five poorly water-soluble nitronaphthofuran derivatives, which were recently identified as possessing anti-TB activity in vitro. To aid solubilization, compounds were formulated in biocompatible polymeric micelles (PMs). Three of the five PM-formulated nitronaphthofuran derivatives showed low toxicity in vivo, significantly reduced bacterial burden and improved survival in infected zebrafish embryos. We propose the zebrafish embryo TB-model as a quick and sensitive tool for evaluating the in vivo toxicity and efficacy of new anti-TB compounds during early stages of drug development. Thus, this model is well suited for pinpointing promising compounds for further development.

Sodium borohydride and thiol mediated nitrite release from nitroaromatic antibiotics

Nitroaromatic antibiotics are used to treat a variety of bacterial and parasitic infections. These prodrugs require reductive bioactivation for activity, which provides a pathway for the release of nitrogen oxide species such as nitric oxide, nitrite, and/or nitroxyl. Using sodium borohydride and 2-aminoethanol as model reductants, this work examines release of nitrogen oxide species from various nitroaromatic compounds through several characterization methods. Specifically, 4- and 5-nitroimidazoles reproducibly generate higher amounts of nitrite (not nitric oxide or nitroxyl) than 2-nitroimidazoles during the reaction of model hydride donors or thiols. Mass spectrometric analysis shows clean formation of products resulting from nucleophile addition and nitro group loss. 2-Nitrofurans generate nitrite upon addition of sodium borohydride or 2-aminoethanethiol, but these complex reactions do not produce clean organic products. A mechanism that includes nucleophile addition to the carbon βto the nitro group to generate a nitronate anion followed by protonation and nitrous acid elimination explains the observed products and labeling studies. These systematic studies give a better understanding of the release mechanisms of nitrogen oxide species from these compounds allowing for the design of more efficient therapeutics.

Fragmentation of Polyfunctional Compounds Recorded Using Automated High-Throughput Desorption Electrospray Ionization

Using desorption electrospray ionization (DESI) as part of an automated high-throughput system, tandem mass spectra of the compounds in a pharmaceutical library were recorded in the positive mode under standardized conditions. Quality control filtering yielded an MS/MS library of 16 662 spectra. Fragmentation of subsets of the compounds in the library chosen to contain a single instance of a particular functional group (amide, piperazine, sulfonamide) was predicted by experts, and the results were compared with the experimental data. Expert performance was good to excellent for all the cases evaluated. Substituents on the functional groups were found to exert important secondary control over the fragmentation, with the main effect observed being product ion stabilization by aromatic substitution, which was consistent across the different groups evaluated. These substituent effects are generally explicable in terms of standard physical organic chemistry considerations of product ion stability as controlling fragmentation. A somewhat unexpected feature was the incidence of homolytic cleavages, driven by the stability of substituted amine radical cations. The findings of this study are intended to lay the groundwork for machine learning approaches to performing MS/MS spectrum → structure and structure → MS/MS spectrum operations on the same experimental data set. The effort involved and the success achieved in computer-aided interpretation, now underway, will be compared with the expert performance as described here.