Mass spectrometric quantitation of chiral drugs by the kinetic method

Enantioselective Reduction of Noncovalent Complexes of Amino Acids with CuII via Resonant Collision-Induced Dissociation: Collision Energy, Activation Duration Effects, and RRKM Modeling

Formation of noncovalent complexes is one of the approaches to perform chiral analysis with mass spectrometry. Enantiomeric distinction of amino acids (AAs) based on the relative rate constants of competitive fragmentations of quaternary copper complexes is an efficient method for chiral differentiation. Here, we studied the complex [CuII,(Phe,PhG,Pro-H)]+ (m/z 493) under resonant collision-induced dissociation conditions while varying the activation time. The precursor ion can yield two main fragments through the loss of the non-natural AA phenylglycine (PhG): the expected product ion [CuII,(Phe,Pro-H)]+ (m/z 342) and the reduced product ion [CuI,(Phe,Pro)]+ (m/z 343). Enantioselective reduction describes the difference in relative abundance of these ions, which depends on the chirality of the precursor ion: the formation of the reduced ion m/z 343 is favored in homochiral complexes (DDD) compared to heterochiral complexes (such as LDD). Energy-resolved mass spectrometry data show that reduction, which arises from rearrangement, is favored at a low collision energy (CE) and long activation time (ActT), whereas direct cleavage preferentially occurs at a high CE and short ActT. These results were confirmed with kinetic modeling based on RRKM theory. For this modeling, it was necessary to set a pre-exponential factor as a reference, so that the E0 values obtained are relative values. Interestingly, these simulations showed that the critical energy E0 required to form the reduced ion is comparable in both homochiral and heterochiral complexes. However, the formation of product ion m/z 342 through direct cleavage is associated with a lower E0 in heterochiral complexes. Consequently, enantioselectivity would not be caused by enhanced reduction in homochiral complexes but rather by direct cleavage being favored in heterochiral complexes.

Exploration of chiral drugs as references for chiral discrimination of valsartan and voriconazole by tandem mass spectrometry

The use of mass spectrometry for chiral recognition and quantification has attracted great interest owing to its speed, sensitivity, specificity, and tolerance. However, searching for chiral selectors in chiral analyses using mass spectrometry is still problematic. In this study, chiral drugs could be applied as references for the chiral recognition and enantiomeric quantification of valsartan and voriconazole. Two novel pairs of metal-bound diastereomeric complex ions were detected by mass spectrometry, namely, nickel (II)-bound dimeric ions [NiII (2R,5S-emtricitabine) (S-valsartan)-H]+ and [NiII (2R,5S-emtricitabine) (R-valsartan)-H]+ and copper (II)-bound dimeric ions [CuII (S,S,S-enalaprilat) (2S,3R-voriconazole)-H]+ and [CuII (S,S,S-enalaprilat) (2R,3S-voriconazole)-H]+ . The resulting diastereomers were successfully identified based on the relative intensities of their characteristic fragments using tandem mass spectrometry. The logarithm of the characteristic fragment ion abundance ratio exhibited a good linear relationship with the enantiomeric excess. Density functional theory calculations were also performed to elucidate the mechanism of the structural differences observed in the MS results. This established approach proves that chiral drugs can serve as ligands for the rapid recognition and quantitative analysis of other chiral drugs without a chiral chromatographic column or complex sample pretreatment.

Tandem mass spectrometry approaches for recognition of isomeric compounds mixtures

The present review aims to collect the published literature pertaining the recognition of isobaric compounds (isomers or stereoisomers) using the features of tandem mass spectrometry (MS) experiments without any chromatographic separation or chemical modification (derivatization or isotopic enrichment) of the analytes. MS/MS methods possess high selectivity, wide dynamic range and high throughput capabilities. Generally, tandem MS has limited capability for distinguishing isomers that fragment similarly. However, some MS/MS methods have been developed and positively applied to isomers discrimination. Among the literature on this topic, the applications that fit on the review subject can be summarized as follow: (1) chiral discrimination by the kinetic method, (2) the use energy-resolved tandem mass spectra and the survival yield (SY) representation, (3) the kinetics evaluation of the ion-molecule interaction and (4) the postprocessing mathematical algorithm to resolve the isomers in MS/MS signal.

Nanomaterials-enriched sensors for detection of chiral pharmaceuticals

Advancements in nanoscience and nanotechnology have opened new pathways to fabricate novel nanostructures with interesting properties that would be used for different applications. In this respect, nanostructures comprising chirality are one of the most rapidly developing research fields encompassing chemistry, physics and biology. Chirality, also known as mirror asymmetry, describes the geometrical property of an object that is not superimposable on its mirror image. This characteristic plays a crucial role because these identical forms of chiral species in pharmaceuticals or food additives may exhibit different effects on living organisms. Therefore, chiral analysis is an important field of modern chemical analysis in health-related industries that are reliant on the production of enantiomeric compounds involving pharmaceuticals. This review covers the recent advances dealing with the synthesis, design and advantageous analytical performance of nanomaterials-enriched sensors used for chiral pharmaceuticals. We conclude this review with the challenges existing in this research field and our perspectives on some potential strategies with cutting-edge approaches for the rational design of sensors for chiral pharmaceuticals. We expect this comprehensive review will inspire future studies in nanomaterials-enriched chiral sensors.

Enantiomer Differentiation of Amino Acid Stereoisomers by Structural Mass Spectrometry Using Noncovalent Trinuclear Copper Complexes

Previous work has demonstrated that copper complexation strategies can be used with tandem MS (MS/MS) and, more recently, ion mobility-mass spectrometry (IM-MS) to differentiate chiral isomers based upon enantiomeric-specific binding. In this study, we investigate the separation of chiral amino acids (AAs) forming trinuclear complexes that can be directly resolved by IM-MS analyses. Twenty standard AAs of both d- and l-chirality were investigated. Specific AAs including d/l-histidine, d/l-proline, d/l-glutamine, d/l-tyrosine, and d/l-tryptophan were evaluated as "chiral selectors" that, when combined with copper, were found to promote selective complexation with specific AA enantiomers. Significant enantiomer differentiation was observed in the IM spectra for hydrophobic AAs acids with peak-to-peak resolutions ranging from 0.63 to 1.15. Among the chiral selectors investigated, histidine provided the best enantioselectivity, followed by tryptophan, suggesting the aromatic structure plays an important role in forming chiral-specific ion complexes. Unlike MS/MS methods where chiral selectors with l-stereochemistry enhance the differentiation, the chirality of the selector was found to have no significant effect on observed IM separation with both d- and l-selectors providing similar resolutions but with inverted IM arrival time ordering. To investigate the structural differences between resolvable chiral complexes, a combination of MS/MS, collision cross-section (CCS) measurements, and molecular mechanics techniques was used. Candidate trinuclear structures of the stoichiometry [(Cu²⁺)₃(d/lIle)₃(lHis)₂ - 5H]⁺ were constructed with guidance from empirical MS/MS results. Of the 48 theoretical structures generated, one enantiomeric cluster pair yielded close correlation (<1%) with experimental CCS measurements, suggesting the most enantioselective ion complexes observed in this work are bridged by three coppers.

Chiral probe for mass spectrometric identification and quantitation of levodropropizine and its enantiomer, dextrodropropizine

Acyl chlorides react rapidly with hydroxyl and amino groups in the absence of catalysts and therefore hold great promise for chiral mass spectrometry. Herein, a tandem mass spectrometry method based on derivatization with (-)-camphanic acid chloride as a simple chiral probe was developed for the highly sensitive detection and quantitation of levodropropizine and its enantiomer, namely, dextrodropropizine. The diastereomeric derivatization products were quantified based on the relative abundances of their fragment ions produced upon collision-induced dissociation in positive-ion mode. The reactive site was elucidated using nuclear magnetic resonance spectroscopy and two-dimensional total correlation spectroscopy, and the reaction mechanism was proved by stoichiometry studies. The degree of isomer recognition was investigated at different collision energies and determined as R_chiral  ≈ 1.5. Thus, this study gives the way to the mass spectrometric identification and quantitation of levodropropizine and its enantiomer, and the developed method represents a new and practical technique for rapid and sensitive determination and quality control of enantiomers of chiral drugs.

Distinguishment of Glycan Isomers by Trapped Ion Mobility Spectrometry

The in-depth study of glycan has drawn large research interests since it is one of the main biopolymers on the earth with a variety of biological functions. However, the distinguishment of glycans is still difficult due to the similarity of the monosaccharide building block, the anomer, and the linkage of glycosidic bonds. In this study, four novel and representative copper-bound diastereoisomeric complex ions were simultaneously detected in a single measurement by trapped ion mobility mass spectrometry, including mononuclear copper-bound dimeric ions [(Cu²⁺)(A)(l-Ser)-H]⁺ and [(Cu²⁺)(A)(l-His)-H]⁺, the mononuclear copper-bound trimeric ion [(Cu²⁺)(A)(l-Ser)(l-His)-H]⁺, and the binuclear copper-bound tetrameric ion [(Cu²⁺)₂(A)(l-Ser)₂(l-His)-3H]⁺ (where A denotes an oligosaccharide, and l-Ser and l-His denote l-serine and l-histidine, respectively). By combining the collision cross sections of complex ions, 23 oligosaccharide isomers were successfully distinguished including two pairs of sialylated glycan linkage isomers. In addition, due to the unique dissociation pathways of the trimeric ion, both the relative and absolute quantification of the individual isomer in the mixture could be determined using a mass spectrometry-based kinetic method. Finally, the method established above was successfully applied to the identification and quantification of glycan isomers in dairy beverages and juice. The method in the present study was sensitive to the fine difference of glycan isomers and might have wide applicability in glycoscience.

Enantio-separation of pregabalin by ternary complexation using trapped ion mobility spectrometry

Rationale The rapid identification of small-molecule chiral drugs is challenging due to subtle structural differences. Different enantiomers of chiral drugs may produce inverse biological effects through their different pharmacokinetics. Therefore, it is highly desirable to distinguish the chirality of drug molecules.

METHODS

The chirality of pregabalin was distinguished by studying the ion mobility spectra of the ternary non-covalent complexes formed with cyclodextrins (CDs), pregabalin, and alkali-earth cations using trapped ion mobility spectrometry (TIMS). The ternary non-covalent complex ions were determined by electrospray ionization of mixed solutions. The analyzed sample was simply mixed, without derivatization or sample pretreatment. The relative contents of pregabalin enantiomers were derived using a calibration curve method.

RESULTS

The ion mobility spectra of several ternary non-covalent complexes formed with α-, β-, and γ-CD, pregabalin, and alkali-earth cations were obtained. We compared their ability to distinguish the chirality of pregabalin. The best peak-to-peak resolution (R_p-p ) was estimated to be 2.20 for [2β-CD + pregabalin + Sr]²⁺ , which can be ascribed as baseline separation. The derived relative contents for S-pregabalin were in agreement with the actual contents.

CONCLUSIONS

A novel and convenient method for discriminating the chirality of the pregabalin molecule by TIMS was developed and optimized. The chirality of pregabalin was recognized by studying the ion mobility spectra of the ternary non-covalent complexes, such as [2β-CD + pregabalin + Sr]²⁺ . This TIMS method could also be used to quantify the relative contents of pregabalin enantiomers.

Chiral discrimination between tyrosine and β-cyclodextrin revealed by cryogenic ion trap infrared spectroscopy

Complexes of permethylated β-cyclodextrin (β-MCD) with the two enantiomers of protonated tyrosine (l- and d-TyrH⁺) are studied by cryogenic ion trap infrared photo-dissociation spectroscopy. The vibrational spectra in the OH/NH stretch and fingerprint regions are assigned based on density functional theory calculations. The spectrum of both l- and d-TyrH⁺ complexes contains features characteristic of a first structure with ammonium and acid groups of the amino acid simultaneously interacting with the β-MCD, the phenolic OH remaining free. A second structure involving additional interaction between the phenolic OH and the β-MCD is observed only for the complex with d-TyrH⁺. The larger abundance of the d-TyrH⁺ complex in the mass spectrum is tentatively explained in terms of (1) better insertion of d-TyrH⁺ within the cavity with the hydrophobic aromatic moiety less exposed to hydrophilic solvent molecules and (2) a stiff structure involving three interaction points, namely the ammonium, the phenolic OH and the carboxylic acid OH, which is not possible for the complex with l-TyrH⁺. The recognition process does not occur through size effects that induce complementarity to the host molecule but specific interactions. These results provide a comprehensive understanding of how the cyclodextrin recognises a chiral biomolecule.

Application of Infrared Multiple Photon Dissociation (IRMPD) Spectroscopy in Chiral Analysis

In recent years, methods based on photodissociation in the gas phase have become powerful means in the field of chiral analysis. Among them, infrared multiple photon dissociation (IRMPD) spectroscopy is a very attractive one, since it can provide valuable spectral and structural information of chiral complexes in addition to chiral discrimination. Experimentally, the method can be fulfilled by the isolation of target diastereomeric ions in an ion trap followed by the irradiation of a tunable IR laser. Chiral analysis is performed by comparing the difference existing in the spectra of enantiomers. Combined with theoretical calculations, their structures can be further understood on the molecular scale. By now, lots of chiral molecules, including amino acids and peptides, have been studied with the method combined with theoretical calculations. This review summarizes the relative experimental results obtained, and discusses the limitation and prospects of the method.

Mass-resolved electronic circular dichroism ion spectroscopy

DNA and proteins are chiral: Their three-dimensional structures cannot be superimposed with their mirror images. Circular dichroism spectroscopy is widely used to characterize chiral compounds, but data interpretation is difficult in the case of mixtures. We recorded the electronic circular dichroism spectra of DNA helices separated in a mass spectrometer. We studied guanine-rich strands having various secondary structures, electrosprayed them as negative ions, irradiated them with an ultraviolet nanosecond optical parametric oscillator laser, and measured the difference in electron photodetachment efficiency between left and right circularly polarized light. The reconstructed circular dichroism ion spectra resembled those of their solution-phase counterparts, thereby allowing us to assign the DNA helical topology. The ability to measure circular dichroism directly on biomolecular ions expands the capabilities of mass spectrometry for structural analysis.

Exploration of disaccharide as reference towards chiral recognition by the kinetic method

RATIONALE

A potential use of disaccharides was found as reference compounds in the kinetic method for chiral recognition and enantiomeric quantification.

METHODS

The experimental procedure consists of three steps: (1) mixing a metal salt, an analyte and a disaccharide at a molar ratio of 1:2:2 with an analyte concentration of 20 μM; (2) introducing the mixture into an electrospray ionization (ESI) mass spectrometry source; (3) isolating the ion [M^II (ref*)₂ (A) - H]⁺ and subjecting it to dissociation. Then through the relative ratio of two product ions combined with the kinetic method, chiral recognition and enantiomeric quantification can be achieved.

RESULTS

The method was verified with good chiral recognition for ten chiral amino acids and three chiral drugs. Among the ten amino acids, Tyr was observed to show best chiral selectivity (R_chiral  = 1.62) and good linearity with the correlation coefficient R²  = 0.9991 for the quantitation of the enantiomeric excess (ee) of D-Tyr. Among the tested chiral drugs, naproxen showed best chiral selectivity with R_chiral  = 1.56 and good linearity with the correlation coefficient R²  = 0.9997 for the quantitation of the ee of R-naproxen.

CONCLUSIONS

This established approach proves that a disaccharide can serve as a reference compound for chiral recognition using the kinetic method.

Ion mobility-mass spectrometry analysis of diarylquinoline diastereomers: Drugs used for tuberculosis treatment

Mycobacterium tuberculosis infection results in more than two million deaths per year and is the leading cause of mortality in people infected with HIV. A new structural class of antimycobacterials, the diarylquinolines, has been synthesized and is being highly effective against both M. tuberculosis and multidrug-resistant tuberculosis. As diarylquinolines are biologically active only under their ( R,S) stereoisomeric form, it is essential to differentiate the stereoisomers ( R,S) and ( R,R). To achieve this, tandem mass spectrometry and ion mobility spectrometry-mass spectrometry have been performed with 10 diarylquinoline diastereomers couples. In this study, we investigated cationization with alkali metal cations and several ion mobility drift gases in order to obtain diastereomer differentiations. We have shown that diastereomers of the diarylquinolines family can be differentiated separately by tandem mass spectrometry and in mixture by ion mobility spectrometry-mass spectrometry. However, although the structure of each diastereomer is close, several behaviors could be observed concerning the cationization and the ion mobility spectrometry separation. The ion mobility spectrometry isomer separation efficiency is not easily predictable; it was however observed for all diastereomeric couples with a significant improvement of separation using alkali adducts compared to protonated molecules. With the use of drift gas with higher polarizability only an improvement of separation was obtained in a few cases. Finally, a good correlation of the experimental collision cross section (relative to three-dimensional structure of ions) and the theoretical collision cross section has been shown.

Quantification of enantiomers by mass spectrometry based on chemical derivatization and spectral shape deformation quantitative theory

A facile mass spectrometric kinetic method for quantitative analysis of chiral compounds was developed by integrating mass spectrometry based on chemical derivatization and the spectral shape deformation quantitative theory. Chemical derivatization was employed to introduce diastereomeric environments to the chiral compounds of interest, resulting in different abundance distribution patterns of fragment ions of the derivatization products of enantiomers in mass spectrometry. The quantitative information of the chiral compounds of interest was extracted from complex mass spectral data by an advanced calibration model derived based on the spectral shape deformation quantitative theory. The performance of the proposed method was tested on the quantitative analysis of R-propranolol in propranolol tablets. Experimental results demonstrated that it could achieve accurate and precise concentration ratio predictions for R-propranolol with an average relative predictive error (ARPE) of about 4%, considerably better than the corresponding results of the mass spectrometric method based on chemical derivatization and the univariate ratiometric model (ARPE: about 12%). The limit of detection (LOD) and limit of quantification (LOQ) of the proposed method for the concentration ratio of R-propranolol were estimated to be 1.5% and 6.0%, respectively. The proposed method is complementary to the existing methods designed for the quantification of enantiomers such as the Cooks kinetic method.

Direct differentiation of stereoisomers of ezetimibe/ambrisentan/atorvastatin and their mechanism study by electrospray ionization quadrupole time-of-flight mass spectrometry

A mass spectrometric method is introduced for rapid and accurate chiral quantification by examining a trimeric metal complex into which a chiral reference is incorporated with the analyte. Several metal ions (Cu^II , Ni^II , Mg^II , Mn^II , Co^II , and Zn^II ) were selected as the central metal ion, and chiral drugs ezetimibe (EZM) and ambrisentan (AMB) were used as the reference to each other for isomeric differentiation by using electrospray ionization quadrupole time-of-flight mass spectrometry. Doubly charged trimeric cluster ions instead of the singly charged clusters were applied in this study. Kinetic method (KM) and chiral recognition (CR) method were used for construction of a calibration curve for chiral quantitation. The results from the two methods were found to be complementary to each other, which improved quantitative analysis of stereoisomers for EZM. Furthermore, we have successfully used S-AMB as reference for the chiral differentiation of enantiomeric atorvastatin (ATO), which is frequently combined with EZM as a codrug. Experimental results showed that the binary mixture of EZM and ATO enantiomers can be determined simultaneously without prior separation steps. The direct measurement of chiral purity within 5% was demonstrated. This mass spectrometric method represents an effective alternative to commonly used chromatographic techniques as means of chiral purity determination and is of potential use in rapid screening experiments.

Ion mobility and gas phase H/D exchange: revealing the importance of a single hydrogen bond for the chiral recognition of crown ether ammonium complexes

Two new BINOL-based chiral crown ether/ammonium complexes are studied by travelling-wave ion-mobility spectrometry. Homo- and heterochiral crown ether/ammonium complexes differ in their collision cross sections, and these differences go along with changes in hydrogen bonding as revealed by gas phase H/D-exchange experiments. Applications for the determination of enantiomeric excess are discussed.

Effects of Genetic Polymorphisms of CYP2B6 on the Pharmacokinetics of Bupropion and Hydroxybupropion in Healthy Chinese Subjects

BACKGROUND Bupropion (BUP) is an antidepressant and its pharmacological activity is mediated by its major metabolite, hydroxybupropion (HBUP). We investigated the effects of genetic polymorphisms of CYP2B6 on BUP and HBUP to provide certain evidence on the clinical rational administration of BUP. MATERIAL AND METHODS Plasma BUP and HBUP concentrations were assayed using high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). RESULTS A total of 23 healthy volunteers (eleven participants with CYP2B6*1/*1, 7 participants with CYP2B6*1/*6, 3 participants with CYP2B6*4/*6, and 2 participants with CYP2B6*1/*4) received orally administered 150 mg of BUP according to protocol. Blood samples were obtained up to 96 hours after administration. The whole blood was subject to genotyping by polymerase chain reaction and restriction fragment length polymorphism (PCR-RFLP). The concentration-time curve (AUC(0→96)), maximum plasma concentration (Cmax), and terminal half-life (t1/2) values of BUP in CYP2B6*1/*4 were lower than those of CYP2B6*1/*1. By contrast, the time to Cmax (tmax) value of the former was higher than that of the latter. The HBUP AUC(0→96) values in CYP2B6*4/*6 and CYP2B6*1/*4 increased to values 1.12-fold and 1.98-fold, compared with CYP2B6*1/*1 carriers. However, the HBUP AUC(0→96) value in CYP2B6*1/*1 was 1.51-fold higher than that in CYP2B6*1/*6. Similarly, the HBUP Cmax values in CYP2B6*4/*6 and CYP2B6*1/*4 increased by 1.12-fold and 1.97-fold, whereas the HBUP Cmax value in CYP2B6*1/*6 decreased to a value 1.64-fold lower than that in CYP2B6*1/*1. CONCLUSIONS Genetic polymorphisms of CYP2B6 influence the pharmacokinetic parameters of BUP and HBUP and thus establish rational BUP administration for Chinese patients in clinical settings.

Kinetic enantioselectivity of a protonated bis(diamido)-bridged basket resorcin[4]arene towards alanine peptides

Efficient enantiodiscrimination of some alanine-containing di- and tri-peptides by using chiral protonated bis(diamido)-bridged basket resorcin[4]arenes depends on several factors, including the basicity of the amino acid residues at the C- and N-termini of the peptide.

Chiral detection of entecavir stereoisomeric impurities through coordination with R-besivance and ZnII using mass spectrometry

In this study, a mass spectrometry (MS)-based kinetic method (KM) is shown to be successful at analyzing a multichiral center drug stereoisomer, entecavir (ETV), both qualitatively and quantitatively. On the basis of the KM, the bivalent complex ion [M^II (A)(ref*)₂ ]²⁺ (M^II  = divalent metal ion, A = analyte, and ref* = chiral reference) was set as precursor ion in MS/MS. The experiment results suggest strong chiral selectivity between ETV and its isomers when using Zn^II coordinated with the chiral reference R-besivance (R-B). The logarithm of the fragment ion abundance ratio and the enantiomeric percentage (%) exhibits a strong linear relation because of the competitive loss of the reference and analyte. The product ion pair [Zn^II (R-B)A-H]⁺ (m/z 733) and [Zn^II (R-B)₂ -H]⁺ (m/z 849), together with [R-B + H]⁺ (m/z 394) and [A + H]⁺ (m/z 278), can realize the identification of ETV and all of its chiral isomers. Theoretical calculation were also performed using the B3LYP functional with the 6-31G* and LanL2DZ basis set to clarify the mechanism of structural difference of these bivalent complex ions. The results reveal that MS-KM can be used to detect optical impurities without a chiral chromatographic column and fussy sample pretreatment. The established method has been used to determine stereoisomeric impurities of less than 0.1% in ETV crude drug, a demonstration of its simple and effective nature for rapid detection of stereoisomeric impurities.

Conversion of benzoic acid into phenol in an ITMS under CI-MSn conditions. Recognition of ortho-chlorobenzoyl derivatives

Isomeric chlorobenzoyl cations (m/z 139), under collision-induced experiments, fragment identically. Chlorobenzoyl cations can be efficiently converted into cholorophenol radical cations by the reaction with methanol in the ion trap analyzer under CI-MSn conditions. The substitution of the carbonyl group with a hydroxyl moiety is able to induce an ortho effect, which is absent in the startingortho-chlorobenzoyl cation. This transformation could be useful to recognize ortho-chlorinated benzoyl derivatives without the need of MS spectrum comparison of the whole set of isomers. The method reported in this study could be applicable to biologically active molecules that dissociate to form the chlorobenzoyl cations under CI or CI collision-induced dissociation conditions, such as indomethacin, the degradation products from the insect growth regulator 1-(2-chlorobenzoyl)-3-(4-chlorophenyl) urea, and lorazepam.

Nickel(II)-assisted enantiomeric differentiation and quantitation of tadalafil by direct electrospray ionization mass spectrometry

A facile method based on electrospray mass spectrometry was established and validated for the differentiation of enantiomeric tadalafil isomers without using chiral chromatographic separation. The enantiomers were coupled with a chiral selector to form diastereomeric complex ions. Nickel-tadalafil complexes, [Ni^II (tadalafil)(l-Trp)-H]⁺ , produced a characteristic fragment ion at m/z 524 by loss of 1-methyl-1,6-dihydropyrazine-2,5-dione via collision-induced dissociation. The relative abundance of this fragment ion to the precursor contributed to differentiate tadalafil enantiomers, and energy-resolved product-ion spectra were applied to determine the molar composition of tadalafil in the mixture (R,R and S,S) as well. In addition, the other two forms of stereomeric isomers of tadalafil (R,S and S,R) could be also distinguished and analyzed by this method. The method was validated in different types of mass spectrometers (AB quadrupole time-of-flight and Bruker ion trap) and also verified by a chiral high-performance liquid chromatography coupled with quadrupole time-of-flight. The chiral determination of tadalafil using MS method proved to be rapid (1-min run time for each sample) and to have the same accuracy and precision comparable to chiral liquid chromatography mass spectrometry methods. This method provides an alternative to commonly used chromatographic technique for chiral determination and is particularly useful in rapid screening in enantioselective synthesis and enantiomeric impurity detection in pharmaceutical industry. Copyright © 2017 John Wiley & Sons, Ltd.

Chirally sensitive collision induced dissociation of proton-bound diastereomeric complexes of tryptophan and 2-butanol

In this work we report the stereo-dependent collision-induced dissociation (CID) of proton-bound complexes of tryptophan and 2-butanol. The dissociation efficiency was measured as a function of collision energy in single collision mode. The homochiral complex was found to be less stable against CID than a heterochiral one. Additional gas dependence measurements were performed with diastereomeric complexes that confirm the findings.

Enantiomeric Excess Determination for Monosaccharides Using Chiral Transmission to Cold Gas-Phase Tryptophan in Ultraviolet Photodissociation

Chiral transmission between monosaccharides and amino acids via photodissociation in the gas phase was examined using a tandem mass spectrometer fitted with an electrospray ionization source and a cold ion trap in order to investigate the origin of the homochirality of biomolecules in molecular clouds. Ultraviolet photodissociation mass spectra of cold gas-phase noncovalent complexes of the monosaccharide enantiomers glucose (Glc) and galactose (Gal) with protonated L-tryptophan H⁺(L-Trp) were obtained by photoexcitation of the indole ring of L-Trp. L-Trp dissociated via C_α-C_β bond cleavage when noncovalently complexed with D-Glc; however, no dissociation of L-Trp occurred in the homochiral H⁺(L-Trp)(L-Glc) noncovalent complex, where the energy absorbed by L-Trp was released through the evaporation of L-Glc. This enantioselective photodissociation of Trp was due to the transmission of chirality from Glc to Trp via photodissociation in the gas-phase noncovalent complexes, and was applied to the quantitative chiral analysis of monosaccharides. The enantiomeric excess of monosaccharides in solution could be determined by measuring the relative abundance of the two product ions in a single photodissociation mass spectrum of the cold gas-phase noncovalent complex with H⁺(L-Trp), and by referring to the linear relationships derived in this work. Graphical Abstract ᅟ.

Doubly charged trimeric cluster ions: effective in mutual chiral recognition of tadalafil and three proton pump inhibitors

Mutual chiral recognition of four stereoisomers of tadalafil and three pairs of enantiomers of proton pump inhibitors (PPIs), as well as enantiomers excess analysis are achieved on the basis of the competitive fragmentation of doubly charged trimeric Ni^IIcluster ion.

Rapid identification of miglitol and its isomers by electrospray ionization tandem mass spectrometry

RATIONALE

Miglitol (1) derived from 1-deoxynojirimycin is an iminosugar that is useful in the treatment of type 2 diabetes mellitus. Isomers (2, 3, 4) that differ at the C2 and C3 positions of hydroxyl groups from miglitol are impurities resulting from the synthesis of miglitol. The impurity profile of a drug substance is critical to its safety assessment and is important for monitoring the manufacturing process. Therefore, developing a fast and simple method that can rapidly identify the configuration of miglitol and its isomers (2, 3, 4) is necessary.

METHODS

Miglitol (1) and its isomers 2-4 were derivatized with benzoboroxole (o-hydroxymethyl phenylboronic acid) at room temperature, and the cyclic boronate esters of different configurations were generated. Protonated miglitol and its isomers 2-4, as well as their derivatives, were subjected to collision-induced dissociation (CID) experiments by using electrospray ionization tandem mass spectrometry (ESI-MS/MS). Elemental compositions of all the ions were verified by electrospray ion-trap time-of-flight mass spectrometry.

RESULTS

Fragmentation of the protonated miglitol and its isomers gave the same fragment ions at m/z 190 and m/z 146. Both their fragmentation behavior and abundances were similar. Whereas the CID mass spectra of the precursor ions (m/z 322) of cyclic boronate esters showed four characteristic fragment ions, m/z 214 ([M-C7 H8 O](-) ), m/z 196 ([M-C7 H8 O-H2 O](-) ), m/z 151 ([M-C8 H13 NO3 ](-) ), and m/z 133 ([M-C8 H15 NO4 ](-) ). The abundances of these fragments are different which are related to the stereostructure of miglitol and its isomers.

CONCLUSIONS

A facile method was established for the differentiation of the spatial configuration of miglitol and its isomers using the relative abundances of the fragment ions of boronate esters generated from in-situ reaction between analytes and benzoboroxole by ESI-MS/MS. This approach could be used to rapidly identify the stereoisomers and monitor the epimerization of miglitol and its isomers in chemical reactions and manufacturing processes. Copyright © 2016 John Wiley & Sons, Ltd.

Multidimensional Analysis of 16 Glucose Isomers by Ion Mobility Spectrometry

Diastereomeric adducts comprising an enantiomerically pure monosaccharide analyte, a peptide, and/or an amino acid and a divalent metal ion (for 16 different monosaccharide isomers) are generated by electrospray ionization and analyzed by combined ion mobility spectrometry-mass spectrometry (IMS-MS) techniques. Mobility distributions of [l-Ser + M + H](+) (where l-Ser is l-serine and M is a given monosaccharide), [l-Phe-Gly + M + H](+) (where l-Phe-Gly is l-phenylalanine-glycine), and [Mn(II) + (l-Phe-Gly - H) + M](+) complex ions are used to determine collision cross sections (ccs in Å(2)), and groups of cross sections for different clusters are proposed as means of identifying the sugar isomers. Within one type of complex, variations in ccs do not always allow delineation between the 16 glucose isomers, but interestingly, when ccs of three different ions are combined as a spatial vector, enantiomers are partially resolved. As a result of this analysis, l-glucose, d-glucose, l-allose, d-allose, d-gulose, d-galactose, and l-mannose are delineated, and for all eight enantiomeric pairs, d and l entities display different coordinates. In addition, different combinations of amino acids, peptide, and metal ions are surveyed, and the potential for yielding unique coordinates for the generated diastereomeric complexes is assessed.

On-line chiral analysis using the kinetic method

Chiral analysis of constituents in solution-phase reaction mixtures can be performed by tandem mass spectrometry using the kinetic method to determine the enantiomeric excess (ee).

Enantiomer-specific analysis of multi-component mixtures by correlated electron imaging-ion mass spectrometry

Simultaneous, enantiomer-specific identification of chiral molecules in multi-component mixtures is extremely challenging. Many established techniques for single-component analysis fail to provide selectivity in multi-component mixtures and lack sensitivity for dilute samples. Here we show how enantiomers may be differentiated by mass-selected photoelectron circular dichroism using an electron-ion coincidence imaging spectrometer. As proof of concept, vapours containing ∼1% of two chiral monoterpene molecules, limonene and camphor, are irradiated by a circularly polarized femtosecond laser, resulting in multiphoton near-threshold ionization with little molecular fragmentation. Large chiral asymmetries (2-4%) are observed in the mass-tagged photoelectron angular distributions. These asymmetries switch sign according to the handedness (R- or S-) of the enantiomer in the mixture and scale with enantiomeric excess of a component. The results demonstrate that mass spectrometric identification of mixtures of chiral molecules and quantitative determination of enantiomeric excess can be achieved in a table-top instrument.

Evaluation of the α-phenylvinyl cation as a chemical ionization reagent for the differentiation of isomeric substituted phenols in an ITMS

Ion-molecule reactions between the α-phenylvinyl cation and isomeric naturally occurring phenols were investigated using a quadruple ion trap mass spectrometer. The α-phenylvinyl cation m/z 103, generated by chemical ionization from phenylacetylene, reacts with neutral aromatic compounds to form the characteristic species: [M + 103](+) adduct ions and the trans-vinylating product ions [M + 25](+) , which correspond to [M + 103](+) adduct after the loss of benzene. Isomeric differentiation of several ring-substituted phenols was achieved by using collision-induced dissociation of the [M + 103](+) adduct ions. This method also showed to be effective in the differentiation of 4-ethylguaiacol from one of its structural isomers that displays identical EI and EI/MS/MS spectra. The effects of gas-phase alkylation with phenylvinyl cation on the dissociation behavior were examined using mass spectrometry(n) and labeled derivatives. Copyright © 2015 John Wiley & Sons, Ltd.

A new chiral electrochemical sensor for the enantioselective recognition of naproxen enantiomers using l-cysteine self-assembled over gold nanoparticles on a gold electrode

A chiral electrochemical sensor for analyzing the enantiomeric composition of chiral compounds is reported.

Enantiomeric differentiation of β-amino alcohols under electrospray ionization mass spectrometric conditions

The enantiomeric differentiation of a series of chiral β-amino alcohols (A) is attempted, for the first time, by applying the kinetic method using L-proline, L-tryptophan, 4-iodo-L-phenylalanine or 3, 5-diiodo-L-tyrosine as the chiral references (Ref) and Cu(2+) or Ni(2+) ion (M) as the central metal ion. The trimeric diastereomeric adduct ions, [M+(Ref)2+A-H](+), formed under electrospray ionization conditions, are subjected for collision-induced dissociation (CID) experiments. The products ions, formed by the loss of either a reference or an analyte, detected in the CID spectra are evaluated for the enantiomeric differentiation. All the references showed enantiomeric differentiation and the R(chiral) values are better for the aromatic alcohols than for aliphatic alcohols. Notably, the R(chiral) values of the aliphatic amino alcohols enhanced when Ni(2+) is used as the central metal ion. The experimental results are well supported by computational studies carried out on the diastereomeric dimeric complexes. The computational data of amino alcohols is correlated with that of amino acids to understand the structural interaction of amino alcohols with reference molecule and central metal ion and their role on the stabilization of the dimeric complexes. Application of flow injection MS/MS method is also demonstrated for the enantiomeric differentiation of the amino alcohols.

Enantioselectivity of mass spectrometry: challenges and promises

With the fast growing market of pure enantiomer drugs and bioactive molecules, new chiral-selective analytical tools have been instigated including the use of mass spectrometry (MS). Even though MS is one of the best analytical tools that has efficiently been used in several pharmaceutical and biological applications, traditionally MS is considered as a "chiral-blind" technique. This limitation is due to the MS inability to differentiate between two enantiomers of a chiral molecule based merely on their masses. Several approaches have been explored to assess the potential role of MS in chiral analysis. The first approach depends on the use of MS-hyphenated techniques utilizing fast and sensitive chiral separation tools such as liquid chromatography (LC), gas chromatography (GC), and capillary electrophoresis (CE) coupled to MS detector. More recently, several alternative separation techniques have been evaluated such as supercritical fluid chromatography (SFC) and capillary electrochromatography (CEC); the latter being a hybrid technique that combines the efficiency of CE with the selectivity of LC. The second approach is based on using the MS instrument solely for the chiral recognition. This method depends on the behavioral differences between enantiomers towards a foreign molecule and the ability of MS to monitor such differences. These behavioral differences can be divided into three types: (i) differences in the enantiomeric affinity for association with the chiral selector, (ii) differences of the enantiomeric exchange rate with a foreign reagent, and (iii) differences in the complex MS dissociation behaviors of the enantiomers. Most recently, ion mobility spectrometry was introduced to qualitatively and quantitatively evaluate chiral compounds. This article provides an overview of MS role in chiral analysis by discussing MS based methodologies and presenting the challenges and promises associated with each approach.

Baseline resolution of isomers by traveling wave ion mobility mass spectrometry: investigating the effects of polarizable drift gases and ionic charge distribution

We have studied the behavior of isomers and analogues by traveling wave ion mobility mass spectrometry (TWIM-MS) using drift-gases with varying masses and polarizabilities. Despite the reduced length of the cell (18 cm), a pair of constitutional isomers, N-butylaniline and para-butylaniline, with theoretical collision cross-section values in helium (ΩHe ) differing by as little as 1.2 Å(2) (1.5%) but possessing contrasting charge distribution, showed baseline peak-to-peak resolution (Rp-p ) for their protonated molecules, using carbon dioxide (CO2), nitrous oxide (N2O) and ethene (C2H4 ) as the TWIM drift-gas. Near baseline Rp-p was also obtained in CO2 for a group of protonated haloanilines (para-chloroaniline, para-bromoaniline and para-iodoaniline) which display contrasting masses and theoretical ΩHe , which differ by as much as 15.7 Å(2) (19.5%) but similar charge distributions. The deprotonated isomeric pair of trans-oleic acid and cis-oleic acid possessing nearly identical theoretical ΩHe and ΩN2 as well as similar charge distributions, remained unresolved. Interestingly, an inversion of drift-times were observed for the 1,3-dialkylimidazolium ions when comparing He, N2 and N2O. Using density functional theory as a means of examining the ions electronic structure, and He and N2-based trajectory method algorithm, we discuss the effect of the long-range charge induced dipole attractive and short-range Van der Waals forces involved in the TWIM separation in drift-gases of differing polarizabilities. We therefore propose that examining the electronic structure of the ions under investigation may potentially indicate whether the use of more polarizable drift-gases could improve separation and the overall success of TWIM-MS analysis.