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

Enantioselective Complexation of Protonated Tyrosine by a Chiral Crown-ether: The Nature of the Hydrogen Bonds Makes the Difference

The complexes formed between (18-crown-6)-tetracarboxylic acid, denoted as 18C6TA, and the two enantiomers of protonated tyrosine, L- and D-Tyr, are studied in a cryogenic ion trap by combining mass spectrometry, laser spectroscopy, and DFT calculations. Both UV and IR photodissociation spectra indicate the formation of multiple isomers for each complex, some of them interconverting upon IR irradiation. Conformer-selective vibrational spectroscopy reveals that all structures involve an internally hydrogen-bonded folded structure of the crown ether. The complexes formed with L-Tyr involve two NH…O interactions with the ether oxygen atoms, accompanied by two hydrogen bonds to the crown ether carboxylic function, one from the NH group and the other from the COOH group of tyrosine. The complexes formed with D-Tyr show more conformational mobility: two out of the three lowest energy conformers observed are tripodal, with three NH…O interactions between the amino acid ammonium and the crown ether oxygens. An additional conformer shows only one NH…O interaction, but has two interactions involving one of the cavity COOH moieties, one with the NH and one with the phenol OH. The increased calculated stability of the complex made from (-) 18C6TA and L-Tyr parallels its higher abundance in the mass spectrum of an isotopically labelled racemic mixture.

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.

PLSR-colorimetric simultaneous determination of L-Tyrosine and L-Tryptophan in different pharmaceutical and biological samples using one-pot synthesized leaf shape Ag@Ag2O core-shell nanocomposites modified by β-CD

In the present study, a simple, innovative, and economically beneficial method has been proposed for the synthesis of Ag@Ag2O core-shell nanocomposites using Acanthophora muscoides algae extract. The host-guest recognition of targets was performed by modification of the Ag@Ag2O surface using β-CD. The Ag@Ag2O- β-CD NCs were used as a colorimetric sensor to determine L-Tryptophan and L-Tyrosine using a partial least square (PLS) approach. A crystalline hybrid structure of Ag core and an Ag2O shell was confirmed by XRD, FTIR, TEM and AFM research. Also, DLS analysis and surface zeta potential spectra illustrated the aggregated nature of nanocomposites in the presence of analytes. The literature review shows that the colorimetric simultaneous determination of L-Tryptophan (L-Try) and L-Tyrosine (L-Tyr) has not been reported. The Ag@Ag2O- β-CD sensor exhibited outstanding sensing capability in a broad linear range of 2.0 -200 μM for both amino acids and low detection limit of 0.32 and 0.51 μM, for L-Try and L-Tyr, respectively. The good sensitivity and excellent selectivity regarding possible interfering species, originated from the synergistic effect of host-guest recognition in combination with colorimetric sensing. Additionally, determination of analytes in various pharmaceutical, supplement and urine samples, approved the practical applicability of the constructed sensor. The computed results confirmed that colorimetric sensing in conjunction with a PLS technique was appropriate for the precise and accurate simultaneous determination of target amino acids in complex mixtures with RMSEP less than 2.5% and recovery in the range of 103-108% with R.S.D. values less than 3%.

Induced photoelectron circular dichroism onto an achiral chromophore

An achiral chromophore can acquire a chiral spectroscopic signature when interacting with a chiral environment. This so-called induced chirality is documented in electronic or vibrational circular dichroism, which arises from the coupling between electric and magnetic transition dipoles. Here, we demonstrate that a chiroptical response is also induced within the electric dipole approximation by observing the asymmetric scattering of a photoelectron ejected from an achiral chromophore in interaction with a chiral host. In a phenol-methyloxirane complex, removing an electron from an achiral aromatic π orbital localised on the phenol moiety results in an intense and opposite photoelectron circular dichroism (PECD) for the two enantiomeric complexes with (R) and (S) methyloxirane, evidencing the long-range effect (~5 Å) of the scattering chiral potential. This induced chirality has important structural and analytical implications, discussed here in the context of growing interest in laser-based PECD, for in situ, real time enantiomer determination.

Distinguishing gas phase lactose and lactulose complexed with sodiated L-arginine by IRMPD spectroscopy and DFT calculations

We present the origin of the observed differentiation of lactose and lactulose achieved by complexation with sodiated L-arginine (ArgNa+). We find that the infrared multiphoton dissociation (IRMPD) bands in 3600-3650 and >3650 cm-1 regimes for gas phase lactose and lactulose, respectively, vanish when forming host-guest complexes with ArgNa+. We interpret these differences in the IRMPD spectra by scrutinizing the interactions between the functional groups (guanidium, -CO2-Na+) in ArgNa+ and -OHs in lactose/lactulose. Our calculated structures and infrared spectra of lactose/ArgNa+ and lactulose/ArgNa+ host-guest pairs indicate that the functional groups interact with the low- and high-frequency -OH stretch modes of lactose and lactulose, respectively, in the 3600-3720 cm-1 window.

Chirality Discrimination at Binary Organic|Water Interfaces Monitored by Interfacial Tension Measurements with Preliminary Comparison with Molecular Dynamics Simulations

Chirality discrimination at a binary toluene (organic)/water(aqueous) interface between R- or S-Tol-BINAP (2,2'-Bis(di-p-tolylphosphino)-1,1'-binaphthyl) molecules and the water-soluble serine chiral specie is examined for the first time, using a combination of interfacial tension measurements and molecular dynamic simulations. Experimental interfacial measurements exhibit a clear chirality-controlled difference when a homochiral versus a heterochiral enantiomeric pairs are introduced at the interfaces. The related molecular dynamics simulations support the experimental results and provide further molecular insight of intermolecular interactions at the interfaces. The results indicate that interfacial tension measurements can capture the preferential interactions which exist between different pairs of enantiomers at the binary interfaces, opening up a new way for probing chirality discrimination at liquid-liquid interfaces.

Ion Binding Site Structure and the Role of Water in Alkaline Earth EDTA Complexes

The interactions between molecular hosts and ionic guests and their dependence on the chemical environment are challenging to disentangle from solution data alone. The vibrational spectra of cold complexes of ethylenediaminetetraacetic acid (EDTA) chelating alkaline earth dications in vacuo encode structural characteristics of these complexes and their dependence on the size of the bound ion. The correlation between metal binding geometry and the relative intensities of vibrational bands of the carboxylate groups forming the binding pocket allows us to characterize water-induced changes in molecular geometry. The evolution of these structural markers from bare ions to water adducts to aqueous solution illustrates the role of water for the structure of ion binding sites in chelators. The binding pocket of EDTA opens up in aqueous solution, bringing the bound ion closer to the mouth of the binding site and leading to an increased exposure of the ion to the chemical environment.

Do Stereochemical Effects Overcome a Charge-Induced Perturbation in Isolated Protonated Cyclo(Tyr-Tyr)?

Two diastereomers of the protonated diketopiperazine (DKP) dipeptide cyclo(Tyr-Tyr), namely, cyclo(LTyr-LTyr)H⁺ and cyclo(LTyr-DTyr)H⁺, are studied in a cryogenic ion trap by means of IR photodissociation spectroscopy combined with quantum chemical calculations. The two diastereomers have similar structures in which one of the rings is folded over the DKP ring and the other one is extended in a trans geometry, allowing a strong OH⁺···π interaction to take place. This contrasts to the observation of a stacked geometry for neutral cyclo(LTyr-LTyr) only under supersonic expansion conditions that do not exist for cyclo(LTyr-DTyr). In the protonated form, the strength of the OH⁺···π interaction is different for the two diastereomers, resulting in a ∼110 cm^-1 difference in the ν(OH⁺) frequency and a smaller but clearly identifiable difference in the protonated amide ν(NH) frequency. Stereochemical effects are therefore still evidenced despite the strong perturbation due to the excess charge.

How change in chirality prevents β-amyloid type interaction in a protonated cyclic dipeptide dimer

The protonated dimers of the diketopiperazine dipeptide cyclo (LPhe-LHis) and cyclo (LPhe-DHis) are studied by laser spectroscopy combined with mass spectrometry to shed light on the influence of stereochemistry on the clustering propensity of cyclic dipeptides. The marked spectroscopic differences experimentally observed in the hydride stretch region are well accounted for by the results of DFT calculations. Both diastereomeric protonated dimers involve a strong ionic hydrogen bond from the protonated imidazole ring of one monomer to the neutral imidazole nitrogen of the other. While this strong interaction is accompanied by a single NH⋯O hydrogen bond between the amide functions of the two moieties for the protonated dimer of cyclo (LPhe-DHis), that of cyclo (LPhe-LHis) involves two NH⋯O interactions, forming the motif of an antiparallel β sheet. Therefore, a change in chirality of the residue prevents the formation of the β sheet pattern observed in the amyloid type aggregation. These results emphasize the peculiar role of the histidine residue in peptide structure and interaction.

Vibrational Spectroscopy of Homo- and Heterochiral Amino Acid Dimers: Conformational Landscapes

The homo- and heterochiral protonated dimers of asparagine with serine and with valine were investigated using infrared multiple-photon dissociation (IRMPD) spectroscopy. Extensive quantum-chemical calculations were used in a three-tiered strategy to screen the conformational spaces of all four dimer species. The resulting binary structures were further grouped into five different types based on their intermolecular binding topologies and subunit configurations. For each dimer species, there are eight to fourteen final conformational geometries within a 10 kJ mol-1 window of the global minimum structure for each species. The comparison between the experimental IRMPD spectra and the simulated harmonic IR features allowed us to clearly identify the types of structures responsible for the observation. The monomeric subunits of the observed homo- and heterochiral dimers are compared to the corresponding protonated/neutral amino acid monomers observed experimentally in previous IRMDP/rotational spectroscopic studies. Possible chirality and kinetic influences on the experimental IRMPD spectra are discussed.

Evaluation of table-top lasers for routine infrared ion spectroscopy in the analytical laboratory

Infrared ion spectroscopy is increasingly recognized as a method to identify mass spectrometry-detected analytes in many (bio)chemical areas and its integration in analytical laboratories is now on the horizon. Commercially available quadrupole ion trap mass spectrometers are attractive ion spectroscopy platforms but operate at relatively high pressures. This promotes collisional deactivation which directly interferes with the multiple-photon excitation process required for ion spectroscopy. To overcome this, infrared lasers having a high instantaneous power are required and therefore a majority of analytical studies have been performed at infrared free electron laser facilities. Proliferation of the technique to routine use in analytical laboratories requires table-top infrared lasers and optical parametric oscillators (OPOs) are the most suitable candidates, offering both relatively high intensities and reasonable spectral tuning ranges. Here, we explore the potential of a range of commercially available high-power OPOs for ion spectroscopy, comparing systems with repetition rates of 10 Hz, 20 kHz, 80 MHz and a continuous-wave (cw) system. We compare the performance for various molecular ions and show that the kHz and MHz repetition-rate systems outperform cw and 10 Hz systems in photodissociation efficiency and offer several advantages in terms of cost-effectiveness and practical implementation in an analytical laboratory not specialized in laser spectroscopy.

Rethinking Ion Transport by Ionophores: Experimental and Computational Investigation of Single Water Hydration in Valinomycin-K+ Complexes

Valinomycin is a macrocyclic ionophore that transports K⁺ across hydrophobic membranes. Its function depends on selectivity, capture, transport, and release of the ion. While thermodynamics clearly indicate that valinomycin binds K⁺ preferentially over all other alkali ions, characterizing the capture/transport/release of K⁺ by valinomycin at the molecular level remains a challenge. The bracelet-like structure of valinomycin-K⁺ (K⁺VM) has the ion completely enveloped, facilitating transport through the cell membrane. We report that hydration by a single water molecule, (K⁺VM)(H₂O), produces three different conformers, identified by infrared spectroscopy and supporting computational studies. For two minor conformers, the water prevents the ionophore from closing, a conformation that would inhibit diffusion through the membrane. However, the dominant conformer encloses both the ion and the water, replicating the bracelet-like K⁺VM and arguably enhancing diffusion through the membrane. This potential for active participation of water in transport through the hydrophobic cellular membrane has never been previously considered.

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.