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

Differentiation of free d-amino acids and amino acid isomers in solution using tandem mass spectrometry of hydrogen-bonded clusters

Free d-amino acids and amino acid isomers were differentiated using tandem mass spectrometry without chromatographic separation. Ultraviolet photodissociation and water adsorption of leucine (Leu) and isoleucine (Ile) enantiomers hydrogen-bonded with tryptophan (Trp) were investigated at 8 K in the gas phase. The enantiomer-selective Cα-Cβ bond cleavage of Trp was observed in the product ion spectra obtained by 285 nm photoexcitation, where the abundance of NH2CHCOOH-eliminated ion of heterochiral H+(d-Trp)(l-Leu) was higher than that of homochiral H+(l-Trp)(l-Leu). When comparing water adsorption on the surfaces of the heterochiral and homochiral clusters in a cold ion trap, the number of water molecules adsorbed on the heterochiral cluster was greater than that adsorbed on the homochiral cluster. These results indicate that the stronger intermolecular interactions within the homochiral H+(l-Trp)(l-Leu) compared to the heterochiral cluster inhibit enantiomer-selective photodissociation. Leu and Ile were differentiated by the isomer-selective Cα-Cβ bond cleavage of Trp in the clusters. Calibration curves for the differentiation of isomeric amino acids and their enantiomers were developed using monitoring isomer- and enantiomer-selective photodissociation, indicating that the molar fractions in solution could be determined from a single product ion spectrum.

D-Amino acid recognition of tripeptides studied by ultraviolet photodissociation spectroscopy of hydrogen-bonded clusters

To understand the roles of D-amino acids, evaluating their chemical properties in living organisms is essential. Herein, D-amino acid recognition of peptides was investigated using a tandem mass spectrometer equipped with an electrospray ionization source and a cold ion trap. Ultraviolet (UV) photodissociation spectroscopy and water adsorption of hydrogen-bonded protonated clusters of tryptophan (Trp) enantiomers and tripeptides (SAA, ASA, and AAS, where S and A denote L-serine and L-alanine, respectively) were carried out at 8 K in the gas phase. In the UV photodissociation spectrum of H⁺(D-Trp)ASA, the bandwidth of the S₁-S₀ transition, which corresponds to the ππ* state of the Trp indole ring, was narrower than those of the other five clusters, H⁺(D-Trp)SAA, H⁺(D-Trp)AAS, H⁺(L-Trp)SAA, H⁺(L-Trp)ASA, and H⁺(L-Trp)AAS. In the UV photoexcitation of H⁺(D-Trp)ASA(H₂O)_n, which were formed via water adsorption on gas-phase H⁺(D-Trp)ASA, the evaporation of water molecules was the main photodissociation pathway. An NH₂CHCOOH-eliminated ion and H⁺ASA were observed in the product ion spectrum. By contrast, water molecules adsorbed on the other five clusters remained on the product ions for NH₂CHCOOH elimination and Trp detachment after the UV photoexcitation. The results indicated that the indole ring of Trp was located on the surface of H⁺(D-Trp)ASA, and the amino and carboxyl groups of Trp formed hydrogen bonds in H⁺(D-Trp)ASA. For the other five clusters, the indole rings of Trp were hydrogen bonded in the clusters, and the amino and carboxyl groups of Trp were present on the cluster surfaces.

Exploring halide anion affinities to native cyclodextrins by mass spectrometry and molecular modelling

The binding affinities of cyclodextrins complexation with chlorine (Cl^-), bromine (Br^-) and iodine (I^-), were measured by mass spectrometric titrimetry, and the fitting of the binding constants was based on the concentration measurement of the cyclodextrin equilibrium. The binding constants (lg K_a) for α-, β- or γ-cyclodextrin with Cl^- were 3.99, 4.03 and 4.11, respectively. The gas-phase binding affinity of halide anions for native cyclodextrins was probed by collision-induced dissociation. In collision-induced dissociation, the centre-of-mass frame energy results revealed that in the gas phase, for the same type of cyclodextrin, the stability of the complexes decreased in order: Cl > Br > I, and for the same halide anion, the binding stability of the complex with α-, β- or γ-cyclodextrin decreased in the order: γ-cyclodextrin >β-cyclodextrin > α-cyclodextrin. The density functional theory calculations showed that halide anion binding on the primary face had a lower energy than the secondary face and hydrogen bonding was the main driving force for complex formation. The higher stability of the γ-cyclodextrin complex with the Cl anion can be attributed to the higher charge density of the Cl anion and better flexibility of γ-cyclodextrin.

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.

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.

Quantifying non-covalent binding affinity using mass spectrometry: a systematic study on complexes of cyclodextrins with alkali metal cations

RATIONALE

To date, the quantification of binding affinities for non-covalent complexes between cyclodextrin (CD) and alkali cations including Li(+) , Na(+) , K(+) , Rb(+) , and Cs(+) has not been investigated in detail by electrospray ionization mass spectrometry (ESI-MS) due to the unknown ionization efficiencies of the different species. In this study, the binding constants of CD-Cs(+) complexes were determined by an improved mass spectrometric titration methodology, which was based only on the peak intensities of equilibrium CD. Hence, the discrepancy of ionization efficiencies of CD, alkaline cation and their complex would not affect the measurement. Then the obtained lgKa values were provided as references for competitive ESI-MS. The binding constants for complexes of α-, β- or γ-CD with Li(+) , Na(+) , K(+) and Rb(+) could be derived directly and quickly.

METHODS

The lgKa values between α-, β- or γ-CD and Cs(+) data were processed by curve fitting. These lgKa values were provided as references for competitive ESI-MS. In addition, linear fit equations for complexes of α-, β- or γ-CD with Cs(+) were derived. Through the linear fit equations of competitive ESI-MS, the binding constants for complexes of Li(+) , Na(+) , K(+) and Rb(+) with α-, β- or γ-CD were acquired.

RESULTS

Results showed that the binding constant (lgKa ) values for the complexes of Cs(+) with α-, β- and γ-cyclodextrins were 3.94, 3.88 and 3.80, respectively, revealing that the binding strength decreased with the increase in diameter of cyclodextrins. The competitive ESI-MS results showed a clear trend of decreasing affinity for complexes of cyclodextrins in the order of Li(+) , Na(+) , K(+) , Rb(+) .

CONCLUSIONS

The binding constants of non-covalent cyclodextrin-alkali cation complexes have been systematically studied by an improved mass spectrometric titration and competitive ESI-MS. Also, the structural features of the complexes were discussed. Our results are valuable for better understanding of mechanisms driving inclusion chemistry under well-defined conditions.

Infrared multiple photon dissociation spectroscopy and density functional theory (DFT) studies of protonated permethylated β-cyclodextrin-water non-covalent complexes

We present infrared multiple photon dissociation (IRMPD) spectroscopy and quantum chemical calculation results for the protonated permethylated β-cyclodextrin (CD)-water non-covalent complex, the simplest β-CD non-covalent complex, in the gas-phase. The IRMPD spectrum in the region 2700-3750 cm(-1) consisted of three strong peaks at 3096, 3315, and 3490 cm(-1). These spectral features in the experimental IRMPD spectrum were compared with a large set of infrared absorption spectra predicted using density functional theory (DFT) calculations for the protonated β-CD-water complex. Complex III (see ), in which the water molecule (at the primary rim) and the proton (at the secondary rim) were separated, was found to suitably reflect the main spectral characteristics found in the experimental IRMPD spectrum. The absence of the homodromic hydrogen bond ring, due to replacement of hydroxyl groups with methoxy groups in permethylated β-CD, rendered the primary rim open compared with the unmodified β-CD 'one-gate-closed' lowest energy conformer. This study demonstrates that IRMPD studies combined with DFT theoretical calculations can be a good method for studying molecular interactions of large host-guest pairs.

Chiral Differentiation of Amino Acids by In-Source Collision-Induced Dissociation Mass Spectrometry

Chiral recognition of d- and l-amino acids is achieved by a method combining electrospray ionization (ESI) and in-source collision-induced dissociation (CID) mass spectrometry (MS). Trimeric cluster ions [Cu(II)(A)(ref)2-H](+) are formed by ESI of mixtures of d- or l-analyte amino acid (A), chiral reference (ref) and CuSO4. By increasing the applied voltage in the ESI source region, the trimeric ions become unstable and dissociate progressively. Thus chiral differentiation of the analyte can be achieved by comparing the dependence of their relative intensities to a reference ion on applied voltages. The method does not need MS/MS technique, thus can be readily performed on single-stage MS instruments by turning the voltage of sampling cone.

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 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.

Host-guest hydrogen atom transfer induced by electron capture

1,n-Alkanediammonium cations in noncovalent complexes with two dibenzo-18-crown-6-ether (DBCE) ligands undergo an unusual intramolecular tandem hydrogen atom and proton transfer to the crown ether ligand upon charge reduction by electron capture. Deuterium labeling established that both migrating hydrogens originated from the ammonium groups. The double hydrogen transfer was found to depend on the length of the alkane chain connecting the ammonium groups. Ab initio calculations provided structures for select alkanediammonium.dibenzo-18-crown-6-ether complexes and dissociation products. This first observation of an intra-complex hydrogen transfer is explained by the unusual electronic properties of the complexes and the substantial hydrogen atom affinity of the aromatic rings in the crown ligand.

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.

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.

Application of FT-ICR-MS for the study of proton-transfer reactions involving biomolecules

Fourier transform ion cyclotron resonance mass spectrometry, combined with modern ionization (fast atom bombardment , electrospray ionization, matrix-assisted laser desorption-ionization), fragmentation (collision-induced dissociation, surface-induced dissociation, one-photon ultraviolet photodissociation, infrared multiphoton dissociation, blackbody infrared radiative dissociation, electron-capture dissociation), and separation (high-performance liquid chromatography, liquid chromatography, capillary electrophoresis) techniques is now becoming one of the most attractive and frequently used instrumental platforms for gas-phase studies of biomolecules such as amino acids, bioamines, peptides, polypeptides, proteins, nucleobases, nucleosides, nucleotides, polynucleotides, nucleic acids, saccharides, polysaccharides, etc. Since it gives the possibilities to trap the ions from a few seconds up to thousands of seconds, it is often applied to study ion/molecule reactions in the gas phase, particularly proton-transfer reactions which provide important information on acid-base properties. These properties determine in part the three-dimensional structure of biomolecules, most of their intramolecular and intermolecular interactions, and consequently their biological activity. They also indicate the form (unionized, zwitterionic, protonated, or deprotonated) which the biomolecule may take in a nonpolar environment.

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.

Chiral discrimination of D- and L-amino acids using iodinated tyrosines as chiral references: effect of iodine substituent

L-Tyrosine and iodinated L-tyrosines, i.e., 3-iodo-L-tyrosine and 3,5-diiodo-L-tyrosine, are successfully used as chiral references for the chiral discrimination of aliphatic, acidic, and aromatic amino acids. Chiral discrimination is achieved by investigating the collision-induced dissociation spectra of the trimeric complex [Cu(II)(ref)(2)(A) - H](+) ion generated by electro spraying the mixture of D- or L-analyte amino acid (A), chiral reference ligand (ref) and M(II)Cl(2) (M = Ni and Cu). The relative abundances of fragment ions resulted by the competitive loss of reference and analyte amino acids are considered for measuring the degree of chiral discrimination by applying the kinetic method. The chiral discrimination ability increases as the number of iodine atom increases on the aromatic ring of the reference and the discrimination is better with Cu when compared with Ni. A large chiral discrimination is obtained for aliphatic and aromatic amino acids using iodinated L-tyrosine as the reference. Computational studies on the different stabilities of the diastereomeric complexes also support the observed differences measured by the kinetic method. The suitability of the method in the measurement of enantiomeric excess over the range of 2% to 100% ee with relative error 0.28% to 1.6% is also demonstrated.

Chiral discrimination of α-amino acids by the DNA triplet GCA

The DNA triplet GCA is successfully used for the first time as a chiral selector for the chiral discrimination and optical purity measurement of some alpha-amino acids by investigating the collision-induced dissociation spectra of the sodiated ternary complex ion formed by electrospray ionization.

Current literature in mass spectrometry

In order to keep subscribers up‐to‐date with the latest developments in their field, John Wiley & Sons are providing a current awareness service in each issue of the journal. The bibliography contains newly published material in the field of mass spectrometry. Each bibliography is divided into 11 sections: 1 Books, Reviews & Symposia; 2 Instrumental Techniques & Methods; 3 Gas Phase Ion Chemistry; 4 Biology/Biochemistry: Amino Acids, Peptides & Proteins; Carbohydrates; Lipids; Nucleic Acids; 5 Pharmacology/Toxicology; 6 Natural Products; 7 Analysis of Organic Compounds; 8 Analysis of Inorganics/Organometallics; 9 Surface Analysis; 10 Environmental Analysis; 11 Elemental Analysis. Within each section, articles are listed in alphabetical order with respect to author (6 Weeks journals ‐ Search completed at 4th. Oct. 2006)