Enantiomeric analysis of pharmaceutical compounds by ion/molecule reactions

Balancing interactions in proline-based receptors for chiral recognition of l-/d-DOPA

Proline based receptors (1-14) attached with phenylboronic acid and benzaldehyde binding groups at the N-/C- or C-/N-termini of the proline residue were created for chiral recognition of l-/d-DOPA, in an attempt to examine if balancing the two binding events would influence the recognition. By changing the positions of boronic acid and aldehyde groups substituted on the phenyl rings (1-4, 5-8) and the site at which phenylboronic acid and benzaldehyde moieties attached respectively to the N- and C-termini or C- and N-termini of the proline residue (1-4vs.5-8), and by introducing an electron-withdrawing fluorine atom in the phenyl ring of the weaker binder the benzaldehyde moiety (11vs.1, 14vs.5), we were able to show that a better balance of the two binding events does improve the chiral recognition. This finding can only be made with the current version of receptors that were equipped with two different binding groups. Together with the finding that the chiral recognition performance in mixed organic-aqueous solutions is tunable by varying the solvent composition, we have now arrived at a protocol for designing proline based receptors for extended applications in chiral recognition.

Selective Gas-Phase Mass Tagging via Ion/Molecule Reactions Combined with Single Analyzer Neutral Loss Scans to Probe Pharmaceutical Mixtures

We have demonstrated the use of a simple single ion trap mass spectrometer to identify classes of compounds as well as individual components in complex mixtures. First, a neutral reagent was used to mass tag oxygen-containing analytes using a gas-phase ion/molecule reaction. Then, a neutral loss scan was used to indicate the carboxylic acids. The lack of unit mass selectivity in the neutral loss scan required subsequent product ion scans to confirm the presence and identity of the individual carboxylic acids. The neutral loss scan technique reduced the number of data-dependent MS/MS scans required to confirm identification of signals as protonated carboxylic acids. The method was demonstrated on neat mixtures of standard carboxylic acids as well as on solutions of relevant pharmaceutical tablets and may be generalizable to other ion/molecule reactions.

Using differential ion mobility spectrometry to perform class-specific ion-molecule reactions of 4-quinolones with selected chemical reagents

Gas phase ion/molecule reactions are often used in analytical applications to support the analysis of isomers or to identify specific functional groups of organic molecules. Until now, deliberate chemical reactions have not been performed in differential ion mobility spectrometry (DMS) devices except for hydrogen exchange and cluster formation. The present work extends that of Colorado and Brodbelt (Anal Chem 66:2330-5, 1994) on ion/molecule reactions in an ion trap mass spectrometer. In this study, class-specific chemical reactions of 4-quinolone antibiotics with various chemical reagents were used to demonstrate the analytical utility of ion/molecule reactions in a DMS drift cell. For these reactions, dehydrated reactive precursor ions were initially formed and made to undergo annulation reactions with selected reagents within the timescale of the DMS separation. Careful study of the energies required for dissociation of the adducts confirmed the covalent nature of the newly formed bond; thus demonstrating the analytical utility of this approach. Graphical abstract.

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 differentiation of d- and l-alanine by permethylated β-cyclodextrin: IRMPD spectroscopy and DFT methods

The gaseous chiral differentiation of alanine by permethylated β-cyclodextrin was studied using IRMPD spectroscopy and density functional theory calculations. The protonated non-covalent complexes of permethylated β-cyclodextrin and d- or l-alanine were mass-selected and investigated by IR laser pulses in the wavelength region of 2650-3800 cm^-1. The remarkably different features of the IRMPD spectra for d- and l-alanine are described, and their origin is elucidated by quantum chemical calculations. We show that the differentiation of the experimentally observed spectral features is the result of different local interactions of d- and l-alanine with permethylated β-cyclodextrin. We also assign the extremely high-frequency (>3700 cm^-1) bands in the observed spectra to the stretch motions of completely isolated alanine -OH groups.

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.

A mass spectrometric method for rapidly assaying the chiral selectivities of the copper(I) complexes of C2-symmetric ligands

A gas-phase method for rapidly assaying the enantioselectivity of metal-centered catalysts is presented. It relies on gas-phase equilibrium measurements in a quadrupole ion trap mass spectrometer. A group of well-established C2-symmetric bis-oxazoline copper(I) complexes was used to identify chiral probe reagents that are capable of profiling the quality of the asymmetric environment provided by the metal complex. The chiral probes were then applied to a wide variety of copper(I) bis-di-imine complexes. Complexes based on a BINAM backbone exhibited selectivities that were comparable to the bis-oxazolines. Taking advantage of the mass selectivity capabilities of the ion trap mass spectrometer, the method was also applied to an un-purified mix of copper(I) complexes derived from a combinatorial synthesis of bis-di-imine ligands. This approach holds promise as a rapid screening tool for potential chiral catalysts.

Self-assembly of l-cysteine–gold nanoparticles as chiral probes for visual recognition of 3,4-dihydroxyphenylalanine enantiomers

A simple protocol to distinguish enantiomers is extremely intriguing and useful.

A novel electrochemical chiral sensor for 3,4-dihydroxyphenylalanine based on the combination of single-walled carbon nanotubes, sulfuric acid and square wave voltammetry

The combination of SWV with chiral SWCNTs and H₂SO₄ shows chiral discrimination for 3,4-dihydroxyphenylalanine, and the three are indispensable for this chiral recognition.

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.

Multifunctional Macrocyclic Receptors as Templates for Aromatic Amino Acids: A Rare Example of a Highly Selective Multi-Input Multi-Output Chemo-"Logic Gate"

Proton-bound [M⋅H⋅G]⁺ diastereomeric complexes between some chiral aromatic amino acids or dipeptides (G) and a chiral multifunctional macrocyclic receptor (M=Chirabite-A) undergo, in the gas phase, highly selective substitution and addition reactions by amines, such as 2-aminobutane and piperidine. All the [M⋅H⋅G]⁺ complexes follow time-dependent monoexponential decays. In some cases, the kinetic curves exhibit a plateau revealing the presence of unreactive [M⋅H⋅G]⁺ structures. In them, the amino acid is accommodated in the macrocycle cavity in the zwitterionic form by sharing its acidic hydrogen atoms with the pyridine nitrogen atoms of the host. The same interactions are structurally inaccessible to G=dipeptides or monofunctional amines, which then can be readily released from [M⋅H⋅G]⁺ . When the amino acid interacts with the amidocarbonyl oxygen atoms pointing outside the macrocycle cavity, it saves the canonical structure and can be readily displaced by the amine. The Chirabite-A may act as an efficient template for aromatic amino acids by releasing them or not depending upon the amino acid configuration and the basicity of the amine. These unique properties confer to the gas-phase diastereomeric [M⋅H⋅G]⁺ complexes the features of multi-input multi-output chemo-"logic gates".

Unexpected behavior of diastereomeric ions in the GasPhase: a stimulus for pondering on ee measurements by ESI-MS

The most common protocols for the quantitative determination of the enantiomeric excess (ee) of raw mixtures by ESI-MS reveal inadequate in cases where the distribution of diastereomeric derivatives diverges from the ee of original solutions. This phenomenon is attributable to a matrix effect, i.e., to the stereospecific formation of high order noncovalent adducts in the ESI droplets, which alters the actual availability of the diastereomeric species under MS analysis. In this frame, the assumption of classic protocols that the ionization correction factor q is independent on the composition of the mixture submitted to analysis is questionable. An alternative methodology is presented in this paper, which is aimed at circumventing the problem by excluding any chemical derivatization of the original raw mixture. It is based on the measurement of the actual distribution of ESI-formed proton-bound diastereomeric complexes from the enantiomeric mixture through a careful analysis of their reaction kinetics with a suitable reactant.

Differentiation of enantiomeric drugs by iodo-substituted L-amino acid references under electrospray ionization mass spectrometric conditions

RATIONALE

In chiral differentiation by mass spectrometry, use of a single reference that differentiates various classes of compounds including drugs is ideal, but so far there are no such reports in the literature. We have successfully used iodo-substituted amino acids for the chiral differentiation of ten enantiomeric pairs of drugs.

METHODS

To achieve the chiral differentiation, the trimeric Cu complex ion consisting of two chiral reference molecules and an analyte molecule was generated under positive ion electrospray ionization (ESI) conditions and subsequently subjected for collision- induced dissociation (CID) experiments using an LCQ ion trap mass spectrometer. The spectra were recorded under identical experimental conditions for both the enantiomers, and were averages of 30 scans. Cooks' kinetic method and chiral recognition ratio method (CR method) were used to arrive at the R(chiral) /CR values, respectively.

RESULTS

The R(chiral) or CR values of the studied drugs are higher for 3,5-diiodo-L-tyrosine as the reference, than for 4-iodo-L-phenylalanine, except for isoproterenol and atenolol. Both the references show the same selectivity (R- or S-selectivity) towards all the studied drugs. With 3,5-diiodo-L-tyrosine as the reference, an R(chiral) value of 12.75 is obtained for DOPA and this is the highest reported value in the literature till now. The suitability of the current method in measuring enantiomeric excess is also demonstrated for DOPA.

CONCLUSIONS

The use of 4-iodo-L-phenylalanine or 3,5-diiodo-L-tyrosine as a chiral reference for the chiral differentiation of ten enantiomeric pairs of pharmaceutically important drugs has been demonstrated. The chiral differentiation of pregabalin, tenofovir and pramipexole is reported for the first time. This study shows that it is possible to develop a single chiral reference compound for the differentiation of a group of chiral drugs having some similarities.

A review of recent advances in mass spectrometric methods for gas-phase chiral analysis of pharmaceutical and biological compounds

Chirality has been of great interest in pharmaceutical and biological sciences. The capabilities of mass spectrometry (MS) for rapid analysis of complex mixtures have encouraged its exploration for gas-phase chiral differentiation. Although particular instances of successful discrimination between enantiomers have been reported over the past three decades, a general method of quantitative chiral analysis by MS has only been demonstrated recently. This review describes the current state of the chiral MS methods without chiral chromatographic separation, which fall into five main categories: (1) the kinetic method, (2) host-guest (H-G) diastereomeric adduct formation, (3) ion/molecule (equilibrium) reactions, (4) collision-induced dissociation (CID) of diastereomeric adducts, and (5) the emerging technique for gas-phase separation using ion mobility spectrometry (IMS). It emphasizes tandem mass spectrometry (MS/MS), which provides several unique analytical advantages for quantitative chiral analysis. These include intrinsically high sensitivity, molecular specificity, and tolerance to impurities as well as the simplicity and speed of the mass spectrometric measurements. Practical prospects and current challenges in quantitative chiral MS techniques for QbD (quality-by-design)-based pharmaceutical applications are also discussed.

Enantioselective supramolecular devices in the gas phase. Resorcin[4]arene as a model system

This review describes the state-of-art in the field of the gas-phase reactivity of diastereomeric complexes formed between a chiral artificial receptor and a biologically active molecule. The presented experimental approach is a ligand-displacement reaction carried out in a nano ESI-FT-ICR instrument, supported by a thermodynamic MS-study and molecular-mechanics and molecular-dynamics (MM/MD) computational techniques. The noncovalent ion–molecule complexes are ideal for the study of chiral recognition in the absence of complicating solvent and counterion effects.

Noncovalent complexation of monoamine neurotransmitters and related ammonium ions by tetramethoxy tetraglucosylcalix[4]arene

The noncovalent complexation of monoamine neurotransmitters and related ammonium and quaternary ammonium ions by a conformationally flexible tetramethoxy glucosylcalix[4]arene was studied by electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry. The glucosylcalixarene exhibited highest binding affinity towards serotonin, norepinephrine, epinephrine, and dopamine. Structural properties of the guests, such as the number, location, and type of hydrogen bonding groups, length of the alkyl spacer between the ammonium head-group and the aromatic ring structure, and the degree of nitrogen substitution affected the complexation. Competition experiments and guest-exchange reactions indicated that the hydroxyl groups of guests participate in intermolecular hydrogen bonding with the glucocalixarene.

Chiral discrimination of drugs by DNA tetranucleotides under electrospray ionisation conditions

The DNA tetranucleotides, extended versions of GCA at the 3'-end or 5'-end, were used as chiral selectors for the chiral discrimination of atenolol, DOPA, tamsulosin, valacyclovir and zolmitriptan. Chiral discrimination was achieved by investigating the collision-induced dissociation spectra of the [X+Y-2H](2-) ion generated by electrospraying a solution mixture of tetranucleotide (X) and R- or S-analyte drug (Y). The relative abundances of the precursor ion and the product ion, resulting from the loss of drug, were considered for measuring the degree of chiral discrimination. Among all the tetranucleotides studied, AGCA showed the highest chiral discrimination. The present study emphasised the position of an adenine base in the tetranucleotide in chiral discrimination. The suitability of the method for the measurement of optical purity was also demonstrated in the case of zolmitriptan.

Chirality-induced conformational preferences in peptide-metal ion binding revealed by IR spectroscopy

Chirality reversal of a residue in a peptide can change its mode of binding to a metal ion, as shown here experimentally by gas-phase IR spectroscopy of peptide-metal ion complexes. The binding conformations of Li(+), Na(+), and H(+) with the LL and DL stereoisomers of PhePhe were compared through IR ion spectroscopy using the FELIX free-electron laser. For the DL isomer, both Li(+) and Na(+) exclusively coordinate to the amide O atom, the carboxyl O atom, and one of the aromatic rings (the OOR conformation), while for the LL isomer, a mixture of the OOR and NOR conformations was found. The stereochemically induced change in conformation is shown to reflect the strength of an NH···π interaction remote from the metal ion site. Protonated PhePhe shows no stereochemically induced variation in binding geometry.

Chiral discrimination of β-3-homo-amino acids using the kinetic method

Chiral discrimination of seven enantiomeric pairs of β-3-homo-amino acids was studied by using the kinetic method and trimeric metal-bound complexes, with natural and unnatural α-amino acids as chiral reference compounds and divalent metal ions (Cu(2+) and Ni(2+) ) as the center ions. The β-3-homo-amino acids were selected for this study because, first of all, chiral discrimination of β-amino acids has not been extensively studied by mass spectrometry. Moreover, these β-3-homo-amino acids studied have different aromatic side chains. Thus, the emphasis was to study the effect of the side chain (electron density of the phenyl ring, as well as the difference between phenyl and benzyl side chains) for the chiral discrimination. The results showed that by the proper choice of a metal ion and a chiral reference compound, all seven enantiomeric pairs of β-3-homo-amino acids could be differentiated. Moreover, it was noted that the β-3-homo-amino acids with benzyl side chains provided higher enantioselectivity than the corresponding phenyl ones. However, increasing or decreasing the electron density of the aromatic ring by different substituents in both the phenyl and benzyl side chains had practically no role for chiral discrimination of β-3-homo-amino acids studied. When copper was used as the central metal, the phenyl side chain containing reference molecules (S)-2-amino-2-phenylacetic acid (L-Phg) and (S)-2-amino-2-(4-hydroxyphenyl)-acetic acid (L-4'-OHPhg) gave rise to an additional copper-reduced dimeric fragment ion, [Cu(I) (ref)(A)](+) . The inclusion of this ion improved noticeably the enantioselectivity values obtained.

Chiral and structural analysis of biomolecules using mass spectrometry and ion mobility-mass spectrometry

This report describes the strategies for gas-phase chiral and structural characterization of biomolecules using mass spectrometry (MS) and ion mobility-MS (IM-MS) techniques. Because both MS and IM-MS do not directly provide chiral selectivity, methodologies for adding a chiral selector are discussed in the context of (i) host-guest (H-G) associations, (ii) diastereomeric collision-induced dissociation (CID) methods, (iii) ion-molecule reactions, and (iv) the kinetic method. MS techniques for the analysis of proteins and protein complexes are briefly described. New advances in performing rapid 2D gas-phase separations on the basis of IM-MS are reviewed with a particular emphasis on the different forms of IM instrumentation and how they are used for chiral and/or structural biomolecular studies. This report is not intended to be a comprehensive review of the field, but rather to underscore the contemporary techniques that are commonly or increasingly being used to complement measurements performed by chiroptical methodologies.

Effective enantiomeric separations of racemic primary amines by the isopropyl carbamate-cyclofructan6 chiral stationary phase

A new chiral stationary phase (CSP) was developed by bonding isopropyl-carbamate functionalized cyclofructan6 (IP-CF6) to the silica gel. It was evaluated by injecting 119 racemic primary amine-containing compounds. This CSP showed pronounced enantioselectivity toward all types of primary amines, separating 93% of all tested compounds. Baseline separation was achieved even for some simple aliphatic racemic amines that contained no other functionality. The polar organic mode was shown to be the effective mobile phase owing to higher efficiency. This new chiral stationary phase showed great potential for preparative-scale separations. It is also interesting that the chiral selector, R-naphthylethyl-carbamate functionalized CF6 (RN-CF6), was found to provide complementary selectivity for the relatively few amine analytes that did not separate on IP-CF6. Thus between the two CSPs, 98% of attempted amine compounds were separated.

Chiral differentiation of some cyclopentane and cyclohexane beta-amino acid enantiomers through ion/molecule reactions

Chiral differentiation of four enantiomeric pairs of beta-amino acids, cis-(1R,2S)-, cis-(1S,2R)-, trans-(1R,2R)-, and trans-(1S,2S)-2-aminocyclopentanecarboxylic acids (cyclopentane beta-amino acids), and cis-(1R,2S)-, cis-(1S,2R)-, trans-(1R,2R)-, and trans-(1S,2S)-2-aminocyclohexanecarboxylic acids (cyclohexane beta-amino acids) was performed successfully by using host-guest complexes and ion/molecule reactions. The experiments were conducted by using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The effect of a chiral host molecule was tested by using three different host compounds; (+)-(18-Crown-6)-2,3,11,12-tetracarboxylic acid, (-)-(18-Crown-6)-2,3,11,12-tetracarboxylic acid, and beta-cyclodextrin. This is the first time that small enantiomeric pairs with two chiral centers have been differentiated using ion/molecule reactions and host-guest complexes.

Exploration of mononucleotides as fixed ligands towards chiral discrimination of hexose monosaccharides by the kinetic method

The most recent version of the kinetic method, i.e. fixed ligand method, is applied towards chiral discrimination of three pairs of enantiomeric hexose monosaccharides under mass spectral conditions. Naturally occurring mononucleotides are used as fixed ligands (FL) and the amino acids are selected as the chiral references (ref) to discriminate the analyte (A), the enantiomers of glucose, mannose and galactose. Chiral discrimination is achieved by investigating the collision-induced dissociation spectra of trimeric complex ion, [Ni(II)(FL)(ref)(A)-H](+) generated by electrospraying the solution mixture of D- or L- analyte (A), FL, amino acid (ref) and NiCl(2). The relative abundance of product ions resulting from the competitive loss of reference amino acid and analyte are considered for measuring the degree of chiral discrimination by applying the kinetic method. L-Asp, L-Thr, L-Glu, L-Trp and L-Ser are found as suitable reference compounds. Among the tested mononucleotides (5'AMP, 5'GMP, 5'CMP, 5'UMP and 5'TMP), 5'GMP is found to be the best for the studied analytes. Chiral discrimination is found to depend on the nature of the monosaccharide, the functional groups present in the side chain of reference amino acids and the configuration of reference amino acids.

Halogen-substituted phenylalanines as enantioselective selectors for enantioselective discrimination of amino acids: effect of halogen

Halogen-substituted phenylalanines with a halogen X (X = F, Cl, Br or I) in the para position in the aromatic ring of L-phenylalanine are used as enantioselective selectors to explore the effect of the halogen substituent on the enantioselective discrimination of amino acids. Enantioselective discrimination is achieved by investigating the collision-induced dissociation spectra of the trimeric complex ion, [CuII(ref)2(A)-H]+, generated by electrospraying a solution of a mixture of D- or L-analyte amino acid (A), enantioselective reference ligand (ref) and CuCl2. The relative abundances of fragment ions resulting from the competitive loss of reference and analyte amino acids are considered for measuring the degree of enantioselective discrimination by applying the kinetic method. The enantioselectivity of the p-halogenated derivatives of L-Phe increases from fluorine to iodine for the studied amino acids (except for acidic amino acids). The validity of the present method has also been checked by cross enantioselective experiments using p-iodo-D-phenylalanine as the reference in place of p-iodo-L-phenylalanine. The enantioselectivity of fluoro-substituted L-phenylalanine is less than that obtained with L-phenylalanine. The high inductive effect of the fluorine atom decreases the strength of the pi-pi stacking interaction. The presence of halogen affects the enantioselectivity by inductive and steric effects.

Determination of enantiomeric compositions of DOPA by tandem mass spectrometry using the kinetic method with fixed ligands

A fixed ligand (FL) version of the kinetic method was applied to rapid, simple, and accurate chiral analysis of DOPA, which is an important drug used for treatment of Parkinson's disease. Singly charged clusters containing the transition metal ion Cu(II), pyridyl ligands which serve as a fixed ligand, some amino acid as a reference, and the analyte DOPA were generated by electrospray ionization. The cluster ion of interest was mass-selected, and the kinetics of its competitive unimolecular dissociations was investigated in an ion trap mass spectrometer. The chiral selectivity (R(chiral)), the ratio of the two fragment ion abundances when the cluster contains one pure enantiomer of the analyte expressed relative to that for the other enantiomer, varies with fixed ligands, references, and transition metals. Chiral discrimination was optimized in 1,10-phenanthroline as a FL, L-Phe and L-Pro as a reference, and Cu(II) as a central metal ion. Quantitative determinations of the enantiomeric composition of DOPA were achieved using two-point calibration curves. The linear relationship between the logarithm of the fragment ion abundance ratio (ln R) and enantiomeric compositions (ee%) of the DOPA allows the determination of the chiral purity of enantiomeric mixtures.

Chiral discrimination of alpha-amino acids by DNA tetranucleotides under electrospray ionization conditions

A set of DNA tetranucleotides, which are 3'- or 5'-end extended versions of GCA, was used as chiral selectors for the discrimination of enantiomers of alpha-amino acids. The [X+Y-2H](2-) ions of the 1:1 complexes were generated by electrospraying a mixture of tetranucleotide (X) and amino acid (Y) solution. Chiral discrimination was achieved by studying the collision-induced dissociation spectra of the [X+Y-2H](2-) ion and the ratio of relative abundance of precursor ion to that of the product ion was used to measure the extent of discrimination. Among the tetranucleotides used, GCAA and GGCA exhibited better discrimination, in which GCAA showed D-selectivity and GGCA showed L-selectivity for the studied amino acids. In addition, binding constants were measured for the 1:1 complexes of phenylalanine enantiomers with GCAA and GGCA. Ltd.

Differentiation of diastereomeric cyclic beta-amino acids by varying the neutral reagent in ion/molecule reactions studied by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

Stereochemical differentiation of diasteromeric pairs of cis- and trans-2-aminocyclohexane-, -2-amino-4-cyclohexene-, and -2-aminocyclopentanecarboxylic acids was investigated with host-guest complexes where tetraethyl resorcarene was the host molecule. Diastereoselectivity was evaluated by ion/molecule reactions and collision-induced dissociation with electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICRMS). The effect of varying the neutral reagent (n-propylamine, i-propylamine, diethylamine, and triethylamine) in ion/molecule reactions was evaluated. Both steric interactions and proton affinity of the neutral reagents influenced the reaction rates. High proton affinity of the neutral reagent apparently had a twofold effect. If the proton affinity of the neutral reagent was too high, the reaction tended to become too exothermic and part of the host-guest complex decomposed instead of transforming to a new host-guest complex, effecting a decrease in the reaction rate. The remaining portion of the host-guest complexes meanwhile reacted very fast with the neutral reagent due to high proton affinity causing an increase in the reaction rate. n-Propylamine and i-propylamine proved to be the best neutral reagents, providing clear diastereoselectivity for beta-amino acids in ion/molecule reactions. Interestingly, diastereoselectivity was better for flexible cyclohexane beta-amino acids (2 and 3) than for more rigid cyclopentane beta-amino acids (6 and 7). The results of ab initio and hybrid density functional theory calculations on the structures of the host-guest complexes of saturated beta-amino acids were in good agreement with the experimental results.

Chiral discrimination of alpha-amino acids by the DNA triplet GCA using amino acids as a co-selector

The DNA triplet GCA is successfully used as a chiral selector for the chiral discrimination of amino acids using amino acids themselves as a co-selector. Chiral discrimination was achieved by investigating the collision-induced dissociation spectra of the [X(A) + X(R) + 2Y - 2H](2-) ion generated by electrospraying a mixture of analyte amino acid (X(A)), reference amino acid (X(R)) and GCA (Y). The relative abundances of fragment ions resulting from the competitive loss of reference and X(A)'s are considered for measuring the degree of chiral discrimination. GCA successfully shows D-selectivity for all the amino acids, except Tyr and Lys. The success of the method lies in the selection of a suitable 10(R) that has closer GCA binding affinity to that of analyte. The degree of discrimination by GCA is improved in the presence of the reference, and the chirality of the reference does not change the selectivity of GCA. The suitability of the method for the measurement of optical purity is also demonstrated.

Diastereochemical differentiation of beta-amino acids using host-guest complexes studied by Fourier transform ion cyclotron resonance mass spectrometry

Host-guest complexes where tetraethyl resorcarene was the host molecule were used to study the stereoselectivity of diasteromeric pairs of di-endo- and di-exo-2,3-disubstituted norbornane and norbornene amino acids by ion-molecule reactions and collision-induced dissociation with electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). Both methods showed stereoselectivity for the diastereomeric pairs. Particularly high selectivity was achieved for di-endo- and di-exo-2,3-disubstituted norbornane amino acids with ion-molecule reactions. Also, ab initio and hybrid density functional theory calculations were performed to study the different structures of the host-guest complexes. Hydrogen bonding was crucial for the calculated lowest energy structures, and sterical considerations satisfactorily explained the ion-molecule reaction results.

Gas-Phase Enantioselectivity of Chiral Amido[4]resorcinarene Receptors

Diastereomeric proton-bound [1(L)HA]+ complexes between selected amino acids (A=phenylglycine (Phg), tryptophan (Trp), tyrosine methyl ester (TyrOMe), threonine (Thr), and allothreonine (AThr)) and a chiral amido[4]resorcinarene receptor (1(L)) display a significant enantioselectivity when undergoing loss of the amino acid guest A by way of the enantiomers of 2-aminobutanes (B) in the gas phase. The enantioselectivity of the B-to-A displacement is ascribed to a combination of thermodynamic and kinetic factors related to the structure and the stability of the diastereomeric [1(L)HA]+ complexes and of the reaction transition states. The results of the present and previous studies allow classification of the [1(L)HA]+ complexes in three main categories wherein: i) guest A does not present any additional functionalities besides the amino acid one (alanine (Ala), Phg, and phenylalanine (Phe)); ii) guest A presents an additional alcohol function (serine (Ser), Thr, and AThr); and iii) guest A contains several additional functionalities on its aromatic ring (tyrosine (Tyr), TyrOMe, Trp, and 3,4-dihydroxyphenylalanine (DOPA)). Each category exhibits a specific enantioselectivity depending upon the predominant [1(L)HA]+ structures and the orientation of the 2-aminobutane reactant in the relevant adducts observed. The results may contribute to the understanding of the exceptional selectivity and catalytic properties of enzyme mimics towards unsolvated biomolecules.

Structural relationships in small molecule interactions governing gas-phase enantioselectivity and zwitterionic formation

Gas-phase zwitterionic amino acids were formed in complexes of underivatized beta-cyclodextrin through reactions with a neutral base, n-propylamine. The reaction was performed in the analyzer cell of an electrospray ionization-Fourier transform mass spectrometer. Most of the natural amino acids were studied with three cyclodextrin hosts including alpha-, beta-, and gamma-cyclodextrin to understand better the structural features that lead to the stabilization of the zwitterionic complexes. Molecular dynamics calculations were performed to provide insight into the structural features of the complexes. The rate constants of the reactions were obtained through kinetic plots. Examination of both L- and D-enantiomers of the amino acid showed that the reaction was enantioselective. The reaction was then employed to analyze mixtures of Glu enantiomers naturally occurring in the bacteria Bacillus licheniformis.

Exceptional Gas-Phase Enantioselectivity of Chiral Tetramide Macrocycles

Diastereomeric proton-bound complexes between some phenylalanine derivatives (A) and chiral tetramide macrocycles (M) exhibit an uncommon enantioselectivity when reacting with the enantiomers of 2-aminobutanes in the gas phase (B). The measured enantioselectivity depends mainly on two distinct factors: (i) the configuration of the A guest; and (ii) the structure and the relative stability of isomeric [MHA]+ complexes. No significant effects of the B configuration are observed. The diastereomeric [MHA]+ complexes with A = 1-naphthylalanine ethyl ester exhibit the largest enantioselectivity factor ever measured in the gas phase (khomo/khetero = 0.046). The origin of such an exceptional enantioselectivity is mainly attributed to the relative stability of the diastereomeric [MHA]+ complexes, as demonstrated by the comparison of the kinetic results with those from collision-induced dissociation of the trimeric [M2HA]+ adducts and with computational evidence.

Chiral Melamine Derivatives: Design, Synthesis, and Application to Mass Spectrometry-Based Chiral Analysis

A novel class of chiral melamine derivatives has been designed and synthesized. The ability of these compounds to perform chiral recognition toward 19 natural chiral alpha-amino acids has been investigated by electrospray ionization tandem mass spectrometry for the first time. The enantioselectivities of these new chiral selectors are encouraging. To elucidate some mechanism and regularity in the chiral recognition process using chiral melamine derivatives as chiral selectors, the effect of different noncovalent interactions caused by various chiral or achiral moieties in melamine derivatives on the chiral recognition in the gas phase has been studied at the same time. The result shows that electrostatic, hydrogen bond, pi-pi stacking, and steric interaction between selector and analyte play important roles in the association and enantioselective recognition of amino acids with the chiral melamine derivatives as chiral selectors. Enantiodiscrimination for analytes with different structures and properties could be improved by modifying substituents in melamine derivatives on purpose.

Tertiary Amine Appended Derivatives of N-(3,5-Dinitrobenzoyl)leucine as Chiral Selectors for Enantiomer Assays by Electrospray Ionization Mass Spectrometry

Derivatives of the chiral selector N-(3,5-dinitrobenzoyl)leucine were prepared and used as chiral selectors for enantiomer discrimination in single-stage electrospray ionization mass spectrometric experiments. The chiral selectors were designed to remove the ionization site from the sites required for effective chiral recognition. Addition of a chiral analyte to a solution of the two pseudoenantiomeric chiral selectors, which differ in absolute stereochemistry and the length of the tether connecting the tertiary amine site used for ionization via protonation and the rest of the chiral selector, affords selector-analyte complexes in the electrospray ionization mass spectrum where the ratio of these complexes is dependent on the enantiomeric composition of the analyte. The relationship between the ratio of the selector-analyte complexes in the electrospray ionization mass spectrum and the enantiomeric composition of the analyte can be used to relate the extent of enantioselectivity that is being observed and for quantitative enantiomeric composition determinations. Investigations into the scope and limitations of this method, plus a comparison to the enantioselectivities observed by chiral HPLC using a N-(3,5-dinitrobenzoyl)leucine-derived chiral stationary phase, is presented.