Chirality effects on the IRMPD spectra of basket resorcinarene/nucleoside complexes

Thermodynamic Reversal and Structural Correlation of 24-Crown-8/Protonated Tryptophan and 24-Crown 8/Protonated Serine Noncovalent Complexes in the Gas Phase vs in Solution: Quantum Chemical Analysis

We investigate the structures of 24-crown-8/H+/l-tryptophan (CR/TrpH+) and 24-crown-8/H+/l-serine (CR/SerH+) noncovalent host-guest complex both in the gas phase and in an aqueous solution by quantum chemical methods. The Gibbs free energies of the complex in the two phases are calculated to determine the thermodynamically most favorable conformer in each phase. Our predictions indicate that both the carboxyl and the ammonium in CR/TrpH+ and the ammonium in the CR/SerH+ complexes in the lowest Gibbs free energy configurations form hydrogen bonds (H-bonds) with the CR host in the gas phase, while the conformer with the "naked" (devoid of H-bond with the CR host) -CO2H (and/or -OH) is much less favorable (Gibbs free energy higher by >3.6 kcal/mol). In the solution phase, however, a "thermodynamic reversal" occurs, making the higher Gibbs free energy gas-phase CR/TrpH+ and CR/SerH+ conformers thermodynamically more favorable under the influence of solvent molecules. Consequently, the global minimum Gibbs free energy structure in solution is structurally correlated with the thermodynamically much less gas-phase conformer. Discussions are provided concerning the possibility of elucidating host-guest-solvent interactions in solution from the gas-phase host-guest configurations in molecular detail.

Online-Monitoring of the Enantiomeric Ratio in Microfluidic Chip Reactors Using Chiral Selector Ion Vibrational Spectroscopy

A novel experimental approach for the rapid online monitoring of the enantiomeric ratio of chiral analytes in solution is presented. The charged analyte is transferred to the gas phase by electrospray. Diastereomeric complexes are formed with a volatile chiral selector in a buffer-gas-filled ion guide held at room temperature, mass-selected, and subsequently spectrally differentiated by cryogenic ion trap vibrational spectroscopy. Based on the spectra of the pure complexes in a small diastereomer-specific spectral range, the composition of diastereomeric mixtures is characterized using the cosine similarity score, from which the enantiomeric ratio in the solution is determined. The method is demonstrated for acidified alanine solutions and using three different chiral selectors (2-butanol, 1-phenylethanol, 1-amino-2-propanol). Among these, 2-butanol is the best choice as a selector for protonated alanine, also because the formation ratio of the corresponding diastereomeric complexes is found to be independent of the nature of the enantiomer. Subsequently, a microfluidic chip is implemented to mix enantiomerically pure alanine solutions continuously and determine the enantiomeric ratio online with minimal sample consumption within one minute and with competitive accuracy.

Simultaneous quantitative chiral analysis of four isomers by ultraviolet photodissociation mass spectrometry and artificial neural network

Although mass spectrometry (MS) has its unique advantages in speed, specificity and sensitivity, its application in quantitative chiral analysis aimed to determine the proportions of multiple chiral isomers is still a challenge. Herein, we present an artificial neural network (ANN) based approach for quantitatively analyzing multiple chiral isomers from their ultraviolet photodissociation mass spectra. Tripeptide of GYG and iodo-L-tyrosine have been applied as chiral references to fulfill the relative quantitative analysis of four chiral isomers of two dipeptides of L/DHisL/DAla and L/DAspL/DPhe, respectively. The results show that the network can be well-trained with limited sets, and have a good performance in testing sets. This study shows the potential of the new method in rapid quantitative chiral analysis aimed at practical applications, with much room for improvement in the near future, including selecting better chiral references and improving machine learning methods.

Quantitation of Enantiomeric Excess in an Achiral Environment Using Trapped Ion Mobility Mass Spectrometry

We present a novel, straightforward method to determine the enantiomeric excess (ee) of tryptophan (Trp) and N-tert-butyloxycarbonyl-O-benzylserine (BBS) solutions without chiral additives. For this, lithium carbonate, sodium carbonate, or silver acetate was added to solutions of Trp or BBS. Singly negatively charged dimer and trimer clusters were then formed by electrospray ionization and analyzed using trapped ion mobility spectrometry (TIMS) and time-of-flight mass spectrometry. When a solution contains both enantiomers, homo- and heterochiral clusters are generated which can be separated in the TIMS-tunnel based on their different mobilities using a nitrogen buffer gas. The ratio of homochiral to heterochiral clusters shows a binomial distribution and can be calibrated with solutions of known ee to yield ee measurements of samples with better than 1% accuracy. Samples can be prepared rapidly, and measurements are completed in less than 5 min. Current instrumental limitations restrict this method to rigid molecules with large functional groups adjacent to the chiral centers. Nevertheless, we expect this method to be applicable to many pharmaceuticals and provide the example of 1-methyltryptophan to demonstrate this.

Infrared multiple photon dissociation (IRMPD) spectroscopy and its potential for the clinical laboratory

Infrared multiple photon dissociation (IRMPD) spectroscopy is a powerful tool used to probe the vibrational modes-and, by extension, the structure-of an ion within an ion trap mass spectrometer. Compared to traditional FTIR spectroscopy, IRMPD spectroscopy has advantages including its sensitivity and its relative ability to handle complex mixtures. While IRMPD has historically been a technique for fundamental analyses, it is increasingly being applied in a more analytical fashion. Notable recent demonstrations pertinent to the clinical laboratory and adjacent interests include analysis of modified amino acids/residues and carbohydrates, structural elucidation (including isomeric differentiation) of metabolites, identification of novel illicit drugs, and structural studies of various biomolecules and pharmaceuticals. Improvements in analysis time, coupling to commercial instruments, and integration with separations methods are all drivers toward the realization of these analytical applications. Additional improvements in these areas, along with advances in benchtop tunable IR sources and increased cross-discipline collaboration, will continue to drive innovation and widespread adoption. The goal of this tutorial article is to briefly present the fundamentals and instrumentation of IRMPD spectroscopy, as an overview of the utility of this technique for helping to answer questions relevant to clinical analysis, and to highlight limitations to widespread adoption, as well as promising directions in which the field may be heading.

Unprotected Galactosamine as a Dynamic Key for a Cyclochiral Lock

The discrimination of d-galactosamine (G), representative of the amino-sugar class of compounds, has been probed through nano-ESI-FT-ICR mass spectrometry by isolating the relevant [C·H·G]+ proton-bound complexes with the enantiomers of the cyclochiral resorcin[4]arene C and allowing them to react toward three primary amines (B = EtNH2, iPrNH2, and (R)- and (S)-sBuNH2). The system under investigation presents several features that help to unveil the behavior of unprotected G in such a supramolecular architecture: (i) the hydrophobic derivatization of the C convex side forces the polar guest G to be coordinated by the cyclochiral concave region; (ii) protonated d-galactosamine exists as an anomeric mixture, dynamically interconverting throughout the experimental time-window; and (iii) different basicities of B allow the experiment to subtly tune the reactivity of the [C·H·G]+ complexes. Three [C·H·G]+ aggregate-types were found to exist, differing in both their origin and reactivity. The most reactive adducts ([C·H·G]ESI+), generated in the electrospray environment, undergo a G-to-B ligand exchange in competition with a partial isomerization to the unreactive [C·H·G]GAS+-type complexes. Finally, the poorly reactive [C·H·G]SOL+ aggregates are formed in solution over an hours-long time scale. A cyclochirality effect on the reactivity was found to depend on the considered [C·H·G]+ aggregate-type.

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.

Noncovalent Complexes of Cyclodextrin with Small Organic Molecules: Applications and Insights into Host-Guest Interactions in the Gas Phase and Condensed Phase

Cyclodextrins (CDs) have drawn a lot of attention from the scientific communities as a model system for host–guest chemistry and also due to its variety of applications in the pharmaceutical, cosmetic, food, textile, separation science, and essential oil industries. The formation of the inclusion complexes enables these applications in the condensed phases, which have been confirmed by nuclear magnetic resonance (NMR) spectroscopy, X-ray crystallography, and other methodologies. The advent of soft ionization techniques that can transfer the solution-phase noncovalent complexes to the gas phase has allowed for extensive examination of these complexes and provides valuable insight into the principles governing the formation of gaseous noncovalent complexes. As for the CDs’ host–guest chemistry in the gas phase, there has been a controversial issue as to whether noncovalent complexes are inclusion conformers reflecting the solution-phase structure of the complex or not. In this review, the basic principles governing CD’s host–guest complex formation will be described. Applications and structures of CDs in the condensed phases will also be presented. More importantly, the experimental and theoretical evidence supporting the two opposing views for the CD–guest structures in the gas phase will be intensively reviewed. These include data obtained via mass spectrometry, ion mobility measurements, infrared multiphoton dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations.

Chiral Differentiation of Non-Covalent Diastereomers Based on Multichannel Dissociation Induced by 213-nm Ultraviolet Photodissociation

Here we present the implementation of 213-nm ultraviolet photodissociation (UVPD) in a FT-ICR mass spectrometer for chiral differentiation in the gas phase. The L/D amino acid-substituted serine octamer ions were selected as examples of diastereoisomers for chiral analysis. Several kinds of fragment ions were observed in these experiments, including fragment ions that are similar to the ones observed in corresponding collision-activated dissociation (CAD) experiments, fragment ions generated with different protonation sites by only destroying non-covalent bonds, and unique non-covalent cluster radical ions. The latter two kinds of fragment ions are found to be more sensitive to the chirality of the substituted units. Further experiments suggest that the formation of radical ions is mainly affected by chromophores on side chains of the substituted units and micro surroundings of the characterized non-covalent diastereoisomers. A comparing experiment performed by only changing the wavelength of UV laser to 266 nm shows that the 213-nm UV laser has the priority in the diversity of fragmentation pathways and potential of further application in chiral differentiation experiments.

Chiral differentiation of d- and l-isoleucine using permethylated β-cyclodextrin: infrared multiple photon dissociation spectroscopy, ion-mobility mass spectrometry, and DFT calculations

Chiral differentiation of protonated isoleucine (Ile) using permethylated β-cyclodextrin (perCD) in the gas-phase was studied using infrared multiple photon dissociation (IRMPD) spectroscopy, ion-mobility, and density functional theory (DFT) calculations. The gaseous protonated non-covalent complexes of perCD and d-Ile or l-Ile produced by electrospray ionization were interrogated by laser pulses in the wavenumber region of 2650 to 3800 cm-1. The IRMPD spectra showed remarkably different IR spectral features for the d-Ile or l-Ile and perCD non-covalent complexes. However, drift-tube ion-mobility experiments provided only a small difference in their collision cross-sections, and thus a limited separation of the d- and l-Ile complexes. DFT calculations revealed that the chiral distinction of the d- and l-complexes by IRMPD spectroscopy resulted from local interactions of the protonated Ile with perCD. Furthermore, the theoretical results showed that the IR absorption spectra of higher energy conformers (by ∼13.7 kcal mol-1) matched best with the experimentally observed IRMPD spectra. These conformers are speculated to be formed from kinetic-trapping of the solution-phase conformers. This study demonstrated that IRMPD spectroscopy provides an excellent platform for differentiating the subtle chiral difference of a small amino acid in a cyclodextrin-complexation environment; however, drift-tube ion-mobility did not have sufficient resolution to distinguish the chiral difference.

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.

Cisplatin Primary Complex with l-Histidine Target Revealed by IR Multiple Photon Dissociation (IRMPD) Spectroscopy

The primary complex obtained from cisplatin and l-histidine in water has been detected and isolated by electrospray ionization. The so-obtained cis-[PtCl(NH₃ )₂ (histidine)]⁺ complex has been characterized in detail by high-resolution mass spectrometry (MS), tandem MS, IR multiple photon dissociation (IRMPD) spectroscopy, and by quantum chemical calculations. The structural features revealed by IRMPD spectroscopy indicate that platinum binds to the imidazole group, which presents tautomeric forms. Thus, depending on the position of the amino acid pendant on the imidazole ring, isomeric complexes are formed that are remarkably different with respect to the ease with which they undergo fragmentation when activated either by energetic collisions or by multiple IR photon absorption. It is shown here how IRMPD kinetics can allow their relative proportions to be estimated.

Chirality-dependent structuration of protonated or sodiated polyphenylalanines: IRMPD and ion mobility studies

Ion mobility experiments are combined with Infra-Red Multiple Photon Dissociation (IRMPD) spectroscopy and quantum chemical calculations for assessing the role of chirality in the structure of protonated and sodiated di- or tetra-peptides. Sodiated systems show a strong chirality dependence of the competition between Na(+)O and Na(+)π interactions. Chirality effects are more subtle in protonated systems and manifest themselves by differences in the secondary interactions such hydrogen bonds between neutral groups or those involving the aromatic rings.

Stereochemical preference of 2'-deoxycytidine for chiral bis(diamido)-bridged basket resorcin[4]arenes

Bis(diamido)-bridged basket resorcin[4]arene (all-S)-1 and its (all-R)-1 enantiomer proved able to interact with 2'-deoxycytidine (2) and pyrimidine nucleoside analogs in dimethyl sulfoxide (DMSO) solution. In such a solvent, the resorcinarene hosts adopt a preferential 1,3-alternate-like conformation, with a larger cavity delimited by two syn 3,5-dimethoxyphenyl moieties, and two external pockets, each delimited by the other 3,5-dimethoxyphenyl group and its diamido arm (the wing). Complexation phenomena were investigated by nuclear magnetic resonance (NMR) methods, including (1)H NMR DOSY and 1D ROESY experiments, and molecular modeling. Heteroassociation constants of [(all-S)-1·2] and [(all-R)-1·2] diastereoisomeric complexes were obtained from diffusion data by single point measurements, and from nonlinear fitting of 1H NMR chemical shifts. Selective proton relaxation rate measurements allowed us to significantly discriminate the two complexes by identifying two different interaction sites of the guest in the resorcin[4]arene host, depending on its configuration.

Probing the competition among different coordination motifs in metal-ciprofloxacin complexes through IRMPD spectroscopy and DFT calculations

The vibrational spectra of ciprofloxacin complexes with monovalent (Li(+), Na(+), K(+), Ag(+)) and polyvalent (Mg(2+), Al(3+)) metal ions are recorded in the range 1000-1900 cm(-1) by means of infrared multiple-photon dissociation (IRMPD) spectroscopy. The IRMPD spectra are analyzed and interpreted in the light of density functional theory (DFT)-based quantum chemical calculations in order to identify the possible structures present under our experimental conditions. For each metal-ciprofloxacin complex, four isomers are predicted, considering different chelation patterns. A good agreement is found between the measured IRMPD spectrum and the calculated absorption spectrum of the most stable isomer for each complex. Metal ion size and charge are found to drive the competition among the different coordination motifs: small size and high charge density metal ions prefer to coordinate the quinolone between the two carbonyl oxygen atoms, whereas large-size metal ions prefer the carboxylate group as a coordination site. In the latter case, an intramolecular hydrogen bond compensates the weaker interaction established by these cations. The role of the metal cation on the stabilization of ionic and nonionic structures of ciprofloxacin is also investigated. It is found that large-size metal ions preferentially stabilize charge separated motifs and that the increase of metal ion charge has a stabilizing effect on the zwitterionic form of ciprofloxacin.