Mass spectrometry analysis of new chemical entities for pharmaceutical discovery

A technical overview of supercritical fluid chromatography-mass spectrometry (SFC-MS) and its recent applications in pharmaceutical research and development

In this paper, we review the growing development and applications of supercritical fluid chromatography-mass spectrometry (SFC-MS) for the analysis of small molecular analytes and biomarkers in drug discovery. As an alternative chromatographic technique, SFC instrumentation and methodology have dramatically advanced over the last decade. Mass spectrometry (MS) provides the powerful detection capability as it couples with SFC. A growing number of SFC-MS/MS applications were reported over the last decade and the application areas of SFC-MS/MS is rapidly expanding. The first part of this review is devoted to the different aspects of SFC-MS development and recent technological advancements. In the second part of this review, we highlight the recent application areas in pharmaceutical research and development.

Intramolecular benzyl cation transfer in the gas-phase fragmentation of protonated benzyl phenyl sulfones

In this study, the gas-phase fragmentations of protonated benzyl phenyl sulfones were investigated by electrospray ionization tandem mass spectrometry (ESI-MSⁿ ). Upon collisional activation, several characteristic fragment ions were observed, and the similar results occurred with different substituted benzyl phenyl sulfones. A mechanism involving an intramolecular benzyl cation transfer and the formation of intermediate ion was proposed and further identified by density functional theory (DFT) calculations. In addition, a reference compound, benzenesulfinic acid benzyl ester, has been synthesized, and its protonated ion has the same gas-phase behavior as compared to the protonated benzyl phenyl sulfone. This work provides access to some insight into the intramolecular benzyl-transfer reactions of benzyl phenyl sulfones in the gas phase and orients the characteristic peaks in collision-induced dissociation spectrometry (CID-MS).

Two competing ionization processes in ESI-MS analysis of N-(1,3-diphenylallyl)benzenamines: formation of the unusual [M-H]+ ion versus the regular [M+H]+ ion

A series of N-(1,3-diphenylallyl)benzenamine derivatives (M) were investigated by electrospray ionization mass spectrometry in the positive-ion mode. Both the anomalous [M-H]⁺ and the regular [M+H]⁺ were observed in the ESI mass spectra. The occurrence of [M-H]⁺ has been supported by accurate mass spectrometry, liquid chromatography mass spectrometry, and tandem mass spectrometry analysis. Calculation results indicated that formation of [M-H]⁺ is attributed to the ion-molecule reaction of M with the protonated ESI solvent molecule (e.g. CH₃OH₂⁺) via hydride abstraction from a tertiary C_sp3-H. The competing ionization processes leading to [M-H]⁺ or [M+H]⁺ were significantly affected by the concentration of formic acid in the electrospray ionization solvent and the proton affinity of the N atom.

Differentiation of isomeric methylanilines by imidization and gas chromatography/mass spectrometry analysis

RATIONALE

Carcinogenic o-methylaniline is one of the banned aromatic amines in azo dyes, but it is very difficult to distinguish it from its noncarcinogenic isomers due to their identical retention time on chromatography and similar mass spectra.

METHODS

Imidization of the isomeric methylanilines was carried out by treatment with benzaldehyde under mild conditions. The formed derivatives were analyzed by gas chromatography/mass spectrometry (GC/MS). Theoretical calculations were carried out on the Gaussian 03 program by using the density functional theory method at the B3LYP/6-311+G(d,p) level.

RESULTS

Imidization of methylanilines occurred easily and gave rise to the corresponding N-methylbenzylidene benzenamines. The isomeric derivatives were completely separated by GC, and thus the three isomeric methylanilines could be determined simultaneously. Due to the ortho effect, the derivative from o-methylaniline has a characteristic fragment ion at m/z 118 with a stable bicyclic structure, and it could be easily differentiated from the meta- and para-isomers in electron ionization mass spectrometry.

CONCLUSIONS

These results provided a promising solution for simultaneous determination of isomeric methylanilines.

Competitive benzyl cation transfer and proton transfer: collision-induced mass spectrometric fragmentation of protonated N,N-dibenzylaniline

Collision-induced dissociation of protonated N,N-dibenzylaniline was investigated by electrospray tandem mass spectrometry. Various fragmentation pathways were dominated by benzyl cation and proton transfer. Benzyl cation transfers from the initial site (nitrogen) to benzylic phenyl or aniline phenyl ring. The benzyl cations transfer to the two different sites, and both result in the benzene loss combined with 1,3-H shift. In addition, after the benzyl cation transfers to the benzylic phenyl ring, 1,2-H shift and 1,4-H shift proceed competitively to trigger the diphenylmethane loss and aniline loss, respectively. Deuterium labeling experiments, substituent labeling experiments and density functional theory calculations were performed to support the proposed benzyl cation and proton transfer mechanism. Overall, this study enriches the knowledge of fragmentation mechanisms of protonated N-benzyl compounds. Copyright © 2017 John Wiley & Sons, Ltd.

Two competitive INC-mediated reactions in the gas-phase fragmentation of protonated indolyl benzo[b]carbazoles

RATIONALE

Ion-neutral complexes (INCs) are intermediates extensively existing in gas-phase ionic reactions.

METHODS

Tandem mass spectrometry (MS(n) ) analysis of indolyl benzo[b]carbazoles was performed on an electrospray ionization quadrupole time-of-flight (ESI-Q-TOF) mass spectrometer in positive ion mode.

RESULTS

Two competing INC-mediated reactions were obtained in the fragmentation of protonated indolyl benzo[b]-carbazoles.

CONCLUSIONS

This study enriches our knowledge of the important roles of INCs in the dissociation of indole compounds in the gas phase. Copyright © 2016 John Wiley & Sons, Ltd.

Observation of the intermediates of in-source aldolization reaction in electrospray ionization mass spectrometry analysis of heteroaromatic aldehydes

Electrospray ionization mass spectrometry (ESI-MS) analyses of 2-(1,2,4-triazole-1-yl)-6-methyl-3- quinolinecarboxaldehyde were carried out by using an ion trap mass spectrometer in a positive-ion mode. Interestingly, several unusual [M + 15](+), [M + 33](+), and [M + 47](+) ions were observed with a high abundance in the ESI-MS spectrum when methanol was used as the ESI solvent. However, only the protonated molecule was obtained with acetonitrile as the ESI solvent. These unusual ions have been proposed as the intermediates of an aldolization reaction occurring in the ESI source, which have been validated by a tandem mass spectrometry experiment, high-performance liquid chromatography/mass spectrometry analysis, and theoretical calculations. A full understanding of this reaction can contribute to the avoidance of analysis errors in the ESI-MS analysis of unknown heteroaromatic aldehydes.

Intramolecular electrophilic aromatic substitution in gas-phase fragmentation of protonated N-benzylbenzaldimines

In this study, the gas-phase fragmentations of protonated N-benzylbenzaldimines were investigated by electrospray ionization tandem mass spectrometry (ESI-MS(n)). Upon collisional activation, several characteristic fragment ions are produced and their fragmentation mechanisms are rationalized by electrophilic aromatic substitution accompanied by benzyl cation transfer. (1) For N-(p-methoxybenzylidene)-1-phenylmethanimine, concomitant with a loss of HCN, a product ion at m/z 121 was observed. It is proposed to be generated from electrophilic substitution at the ipso-position by transferring benzyl cation rather than cleavage of the C-N double bond. (2) For N-(m-methoxybenzylidene)-1-phenylmethanimine, a product ion at m/z 209 was obtained, corresponding to the elimination of NH(3) carrying two hydrogens from the two aromatic rings respectively. This process can be rationalized by two sequential electrophilic substitutions and cyclodeamination reaction based on the benzyl cation transfer. Deuterium-labeled experiments, density functional theory (DFT) calculation and substituent effect results also corroborate the proposed mechanism.

On the origin of the methyl radical loss from deprotonated ferulic and isoferulic acids: electronic excitation of a transient structure

Formation of radical fragments from even-electron ions is an exception to the "even-electron rule". In this work, ferulic acid (FA) and isoferulic acid (IFA) were used as the model compounds to probe the fragmentation mechanisms and the isomeric effects on homolytic cleavage. Elimination of methyl radical and CO2 are the two competing reactions observed in the CID-MS of [FA - H](-) and [IFA - H](-), of which losing methyl radical violates the "even-electron rule". The relative intensity of their product ions is significantly different, and thereby the two isomeric compounds can be differentiated by tandem MS. Theoretical calculations indicate that both the singlet-triplet gap and the excitation energy decrease in the transient structures, as the breaking C-O bond is lengthened. The methyl radical elimination has been rationalized as the intramolecular electronic excitation of a transient structure with an elongating C-O bond. The potential energy diagrams, completed by the addition of the energy barrier of the radical elimination, have provided a reasonable explanation of the different CID-MS behaviors of [FA - H](-) and [IFA - H](-).

Gas-phase fragmentation of the protonated benzyl ester of proline: intramolecular electrophilic substitution versus hydride transfer

In this study, the gas phase chemistry of the protonated benzyl esters of proline has been investigated by electrospray ionization mass spectrometry and theoretical calculation. Upon collisional activation, the protonated molecules undergo fragmentation reactions via three primary channels: (1) direct decomposition to the benzyl cation (m/z 91), (2) formation of an ion-neutral complex of [benzyl cation + proline](+), followed by a hydride transfer to generate the protonated 4,5-dihydro-3H-pyrrole-2-carboxylic acid (m/z 114), and (3) electrophilic attack at the amino by the transferring benzyl cation, and the subsequent migration of the activated amino proton leading to the simultaneous loss of (H2O + CO). Interestingly, no hydrogen/deuterium exchange for the fragment ion m/z 114 occurs in the d-labeling experiments, indicating that the transferring hydride in path-b comes from the methenyl hydrogen rather than the amino hydrogen. For para-substituted benzyl esters, the presence of electron-donating substituents significantly promotes the direct decomposition (path-a), whereas the presence of electron-withdrawing ones distinctively inhibits that channel. For the competing channels of path-b and path-c, the presence of electron-donating substituents favors path-b rather than path-c, whereas the presence of electron-withdrawing ones favors path-c rather than path-b.

Capillary electrophoresis/mass spectrometry relevant to pharmaceutical and biotechnological applications

Advanced analytical techniques play a crucial role in the pharmaceutical and biotechnological field. In this context, capillary electrophoresis/mass spectrometry (CE/MS) has attracted attention due to efficient and selective separation in combination with powerful detection allowing identification and detailed characterization. Method developments and applications of CE/MS have been focused on questions not easily accessible by liquid chromatography/mass spectrometry (LC/MS) as the analysis of intact proteins, carbohydrates, and various small molecules, including peptides. Here, recent approaches and applications of CE/MS relevant to (bio)pharmaceuticals are reviewed and discussed to show actual developments and future prospects. Based on other reviews on related subjects covering large parts of previous works, the paper is focused on general ideas and contributions of the last 2 years; for the analysis of glycans, the period is extended back to 2006.

Design and synthesis of conformationally constrained cyclophilin inhibitors showing a cyclosporin-A phenotype in C. elegans

Cyclophilin A (CypA) is a member of the immunophilin family of proteins and receptor for the immunosuppressant drug cyclosporin A (CsA). Here we describe the design and synthesis of a new class of small-molecule inhibitors for CypA that are based upon a dimedone template. Electrospray mass spectrometry is utilised as an initial screen to quantify the protein affinity of the ligands. Active inhibitors and fluorescently labelled derivatives are then used as chemical probes for investigating the biological role of cyclophilins in the nematode Caenorhabditis elegans.

Hydride transfer reactions via ion-neutral complex: fragmentation of protonated N-benzylpiperidines and protonated N-benzylpiperazines in mass spectrometry

An ion-neutral complex (INC)-mediated hydride transfer reaction was observed in the fragmentation of protonated N-benzylpiperidines and protonated N-benzylpiperazines in electrospray ionization mass spectrometry. Upon protonation at the nitrogen atom, these compounds initially dissociated to an INC consisting of [RC(6)H(4)CH(2)](+) (R = substituent) and piperidine or piperazine. Although this INC was unstable, it did exist and was supported by both experiments and density functional theory (DFT) calculations. In the subsequent fragmentation, hydride transfer from the neutral partner to the cation species competed with the direct separation. The distribution of the two corresponding product ions was found to depend on the stabilization energy of this INC, and it was also approved by the study of substituent effects. For monosubstituted N-benzylpiperidines, strong electron-donating substituents favored the formation of [RC(6)H(4)CH(2)](+), whereas strong electron-withdrawing substituents favored the competing hydride transfer reaction leading to a loss of toluene. The logarithmic values of the abundance ratios of the two ions were well correlated with the nature of the substituents, or rather, the stabilization energy of this INC.

The locks and keys to industrial biotechnology

The sustainable use of resources by Nature to synthesize the required products at the right place, when they are needed, continues to be the role model for total synthesis and production in general. The combination of molecular and engineering science and technology in the biotechnological approach needs no protecting groups at all and has therefore been established for numerous large-scale routes to both natural and synthetic products in industry. The use of biobased raw materials for chemical synthesis, and the economy of molecular transformations like atom economy and step economy are of growing importance. As safety, health and environmental issues are key drivers for process improvements in the chemical industry, the development of biocatalytic reactions or pathways replacing hazardous reagents is a major focus. The integration of the biocatalytic reaction and downstream processing with product isolation has led to a variety of in situ product recovery techniques and has found numerous successful applications. With the growing collection of biocatalytic reactions, the retrosynthetic thinking can be applied to biocatalysis as well. The introduction of biocatalytic reactions is uniquely suited to cost reductions and higher quality products, as well as to more sustainable processes. The transfer of Nature's simple and robust sensing and control principles as well as its reaction and separation organization into useful technical systems can be applied to different fermentations, biotransformations and downstream processes. Biocatalyst and pathway discovery and development is the key towards new synthetic transformations in industrial biotechnology.