Charge-Dependent Metastable Dissociations of Multiply Charged Decafluorobiphenyl Formed by Femtosecond Laser Pulses.

Femtosecond laser ionization is a unique means to produce multiply charged organic molecules in the gas phase. The charge-dependent chemical reactions of such electron-deficient molecules are interesting from both fundamental and applied scientific perspectives. We have reported the production of quadruply charged perfluoroaromatics; however, they were so stable that we cannot obtain information about their chemical reactions. In general, it might be difficult to realize the conflicting objectives of observing multiply charged molecular ion themselves and their metastable dissociations. In this study, we report the first example showing metastable dissociations of several charge states within the measurable time range of a time-of-flight mass spectrometer. Metastable dissociations were analyzed by selecting a precursor ion with a Bradbury-Nielsen ion gate followed by time-of-flight analysis using a reflectron. We obtained qualitative information that triply and quadruply charged decafluorobiphenyl survived at least in the acceleration region but completely decomposed before entering a reflectron. In contrast, three dissociation channels for singly and one for doubly charged molecular ions were discriminated by a reflectron and determined with the help of ion trajectory simulations.

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-Cß 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-Cß 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.

Quantification of disaccharides in solution using isomer-selective ultraviolet photodissociation of hydrogen-bonded clusters in the gas phase.

Chemical properties of gas-phase hydrogen-bonded clusters were investigated as a model for interstellar molecular clouds. Cold gas-phase hydrogen-bonded clusters of tryptophan (Trp) enantiomers and disaccharide isomers, including d-maltose and d-cellobiose, were generated by electrospray ionization and collisional cooling in an ion trap at 8 K. Product ion spectra in the 265-290 nm wavelength range were obtained using tandem mass spectrometry. NH2CHCOOH loss via the Cα-Cß bond cleavage of Trp occurred frequently in homochiral H+(d-Trp)(d-maltose) compared with heterochiral H+(l-Trp)(d-maltose) at 278 nm, indicating that an enantiomeric excess of l-Trp was formed via the enantiomer-selective photodissociation. The photoreactivity differed between the enantiomers and isomers contained in the clusters at the photoexcitation of 278 nm. A calibration curve for the quantification of disaccharide isomers in solution was constructed by photoexcitation of the hydrogen-bonded clusters of disaccharide isomers with H+(l-Trp) at 278 nm. A linear relationship between the natural logarithm of the relative product ion abundance and the mole fraction of d-maltose to d-cellobiose ratio in the solution was obtained, indicating that the mole fraction could be determined from a single product ion spectrum. A calibration curve, for quantification of Trp enantiomers, was also obtained using d-maltose as a chiral auxiliary.

Chemical properties of inner and surficial regions of hydrogen-bonded clusters of biological molecules: ultraviolet photodissociation and water adsorption analyses in the gas phase.

The chemical and physical properties of cold, gas-phase hydrogen-bonded clusters of L-alanine (L-Ala), L-trialanine (L-Ala3), L-tetraalanine (L-Ala4), and tryptophan (Trp) enantiomers were investigated using tandem mass spectrometry with an electrospray ionization source and cold ion trap. From the ultraviolet (UV) photodissociation spectra at 265-290 nm, the electronic structures of homochiral H+(L-Trp)(L-Ala) at 8 K were found to be different from those of heterochiral H+(D-Trp)(L-Ala) and protonated Trp. The number of water molecules adsorbed on the surface of gas-phase H+(D-Trp)(L-Ala) was larger than that of H+(L-Trp)(L-Ala), indicating stronger intermolecular interactions of L-Ala with H+(L-Trp) than those with H+(D-Trp). The product ion spectrum obtained by 265 nm photoexcitation of H+(L-Trp)(L-Ala3)(H2O)n formed via gas-phase water adsorption on H+(L-Trp)(L-Ala3) showed that the evaporation of water molecules was the main photodissociation process. In the case of H+(L-Trp)(L-Ala4)(H2O)n, signals of H+(L-Ala4) (H2O)n formed via L-Trp evaporation were observed in the product ion spectra, and the cross-section for UV photoinduced L-Trp evaporation became larger as the number of adsorbed water molecules increased. This observation indicates that water molecules were selectively adsorbed on the H+(L-Ala4) side of H+(L-Trp)(L-Ala4) and weakened the intermolecular interactions between L-Trp and H+(L-Ala4) in the hydrogen-bonded cluster.

Quantification of Amino Acid Enantiomers Using Electrospray Ionization and Ultraviolet Photodissociation.

The enantioselectivity of tryptophan (Trp) for amino acids, such as alanine (Ala), valine (Val), and serine (Ser), was investigated using ultraviolet (UV) photoexcitation and tandem mass spectrometry. Product ion spectra of cold gas-phase amino acid enantiomers that were hydrogen-bonded to Na+(L-Trp) were measured using a variable-wavelength UV laser and a tandem mass spectrometer equipped with an electrospray ionization source and a cold ion trap. Na+(L-Trp), formed via amino acid detachment, and the elimination of CO2 from the clusters were observed in the product ion spectra. For photoexcitation at 265 nm, the relative abundance of Na+(L-Trp) compared to that of the precursor ion observed in the product ion spectrum of heterochiral Na+(L-Trp)(D-Ala) was larger than that observed in the product ion spectrum of homochiral Na+(L-Trp)(L-Ala). A difference between the Val enantiomers in the relative abundance of the precursor and product ions was observed in the case of photoexcitation at 272 nm. The elimination of CO2 was not observed for L-Ser for the 285 nm photoexcitation, which was the main reaction pathway for D-Ser. Photoexcited Trp chiral recognition was applied to identify and quantify the amino acid enantiomers in solution. Ala, Val, and Ser enantiomers in solution were quantified from their relative abundances in single product ion spectra measured using photoexcitation at 265, 272, and 285 nm, respectively, for hydrogen-bonded Trp within the clusters.

Gas-Phase Adsorption of N2 on Protonated Molecules and Its Application to the Structural Elucidation of Small Molecules.

The gas-phase adsorption of N2 on protonated serine (Ser, C3H7NO3), threonine (Thr, C4H9NO3), glycine (Gly, C2H5NO2), and 2-aminoethanol (C2H7NO) was investigated using a tandem mass spectrometer equipped with an electrospray ionization source and a cold ion trap. N2 molecules were adsorbed on the free X-H (X=O and N) groups of protonated molecules. Gas-phase N2 adsorption-mass spectrometry detected the presence of free X-H groups in the molecular structures, and was applied to the structural elucidation of small molecules. When the 93 structures with an elemental composition of C3H7NO3 were filtered using the gas-phase N2 adsorption-mass spectrometry results for Ser, the number of possible molecular structures was reduced to 8 via the quantification of the X-H groups. Restricting and minimizing the number of possible candidates were effective steps in the structural elucidation process. Gas-phase N2 adsorption-mass spectrometry combined with mass spectrometry-based techniques has the potential for being useful for elucidating the molecular structures of a variety of molecules.

Quantification of monosaccharide enantiomers using optical properties of hydrogen-bonded tryptophan.

Chiral recognition between amino acids and monosaccharides in the gas phase was investigated as a model for chemical evolution in interstellar molecular clouds. Ultraviolet (UV) photodissociation spectra and product ion spectra of cold gas-phase hydrogen-bonded clusters of protonated tryptophan (Trp) and a pentose, including ribose and arabinose, were obtained using a tandem mass spectrometer equipped with an electrospray ionization source and a temperature-controlled ion trap. The relative intensity of the signal arising from the S1-S0 transition of protonated Trp observed at approximately 285 nm in the UV photodissociation spectrum of homochiral H+(d-Trp)(d-ribose) was significantly higher than that of heterochiral H+(l-Trp)(d-ribose), corresponding to the ππ* state of the Trp indole ring. Optical properties of Trp in the clusters induced by 285-nm photoexcitation were applied to the identification and quantification of pentose enantiomers in solution. Pentose enantiomeric excess in solution was determined from relative abundances observed in a single product ion spectrum of 285-nm photoexcited hydrogen-bonded clusters of H+(l-Trp) and pentose. A mixture of two pentoses could also be quantified by this method. The geometric and electronic structures of Trp enable recognition of biological molecules through hydrogen bonding.

Molecular Adsorption on Cold Gas-Phase Hydrogen-Bonded Clusters of Chiral Molecules.

Gas-phase molecular adsorption was investigated as a model for molecular cloud formation. Molecular adsorption on cold gas-phase hydrogen-bonded clusters containing protonated tryptophan (Trp) enantiomers and monosaccharides such as methyl-α-D-glucoside, D-ribose, and D-arabinose was detected using a tandem mass spectrometer equipped with an electrospray ionization source and cold ion trap. The adsorption sites on the surface of cold gas-phase hydrogen-bonded cluster ions were quantified using gas-phase N2 adsorption-mass spectrometry. The gas-phase N2 adsorption experiments indicated that the number of adsorption sites on the surface of the hydrogen-bonded heterochiral clusters containing L-Trp and D-monosaccharides exceeded the number of adsorption sites on the homochiral clusters containing D-Trp and D-monosaccharides. H2O molecules were preferentially adsorbed on the heterochiral clusters, and larger water clusters were formed in the gas phase. Physical and chemical properties of cold gas-phase hydrogen-bonded clusters containing biological molecules were useful for investigating enantiomer selectivity and chemical evolution in interstellar molecular clouds.

Identification and quantification of leucine and isoleucine residues in peptides using photoexcited tryptophan.

The molecular recognition ability of tryptophan (Trp) for isomeric amino acids, such as leucine (Leu) and isoleucine (Ile), and isomeric amino acid-containing dipeptides, such as Leu-Gly, Ile-Gly, Gly-Leu, and Gly-Ile (where Gly denotes glycine), was investigated using a tandem mass spectrometer equipped with an electrospray ionization source and cold ion trap. The ultraviolet photodissociation spectra of the cold gas-phase clusters of Leu and Ile with Na+Trp in the wavelength range of 265-290 nm revealed that the relative intensities of Leu and Ile were only different in the wavelength range of 265-273 nm; however, no differences in the relative intensities were observed when the wavelength exceeded 274 nm. The molecular recognition ability of photoexcited Trp was used for the identification and quantification of Leu and Ile in dipeptides in solution. The mole fractions of Leu and Ile in dipeptides could be determined from the abundances observed in a single product ion spectrum of the cold gas-phase clusters of dipeptides with Na+Trp.

Charge Transfer and Metastable Ion Dissociation of Multiply Charged Molecular Cations Observed by Using Reflectron Time-of-Flight Mass Spectrometry.

The front cover artwork is provided by the group of Prof. Tomoyuki Yatsuhashi (Osaka City University) as well as Dr. Akimasa Fujihara (Osaka Prefecture University). The image shows that the potential energy of a product ion formed by metastable ion dissociation can be larger than that of a multiply charged precursor ion. Read the full text of the Article at 10.1002/cphc.202000021.

Charge Transfer and Metastable Ion Dissociation of Multiply Charged Molecular Cations Observed by Using Reflectron Time-of-Flight Mass Spectrometry.

A multiply charged molecule expands the range of a mass window and is utilized as a precursor to provide rich sequence coverage; however, reflectron time-of-flight mass spectrometer has not been well applied to the product ion analysis of multiply charged precursor ions. Here, we demonstrate that the range of the mass-to-charge ratio of measurable product ions is limited in the cases of multiply charged precursor ions. We choose C6 F6 as a model molecule to investigate the reactions of multiply charged molecular cations formed in intense femtosecond laser fields. Measurements of the time-of-flight spectrum of C6 F6 by changing the potential applied to the reflectron, combined with simulation of the ion trajectory, can identify the species detected behind the reflectron as the neutral species and/or ions formed by the collisional charge transfer. Moreover, the metastable ion dissociations of doubly and triply charged C6 F6 are identified. The detection of product ions in this manner can diminish interference by the precursor ion. Moreover, it does not need precursor ion separation before product ion analysis. These advantages would expand the capability of mass spectrometry to obtain information about metastable ion dissociation of multiply charged species.

Chiral and molecular recognition of monosaccharides by photoexcited tryptophan in cold gas-phase noncovalent complexes as a model for chemical evolution in interstellar molecular clouds.

Chiral and molecular recognition between amino acid and sugar molecules and their implications for chemical evolution were investigated using a tandem mass spectrometer equipped with an electrospray ionization source and a cold ion trap. Ultraviolet photodissociation of mass-selected and temperature-controlled gas-phase noncovalent complexes of protonated tryptophan (Trp) and monosaccharide enantiomers, such as aldohexose, aldopentose, and deoxyhexose, was examined as a model for chemical evolution in interstellar molecular clouds. Upon photoexcitation of noncovalent heterochiral H+(L-Trp)(D-aldohexose) complexes, NH2CHCOOH loss from protonated Trp via Cα-Cß bond cleavage occurred. Conversely, in homochiral H+(L-Trp)(L-aldohexose), the energy absorbed by Trp was released through the detachment of aldohexose, and dissociation of the amino acid was suppressed. In the photodissociation mass spectra of protonated Trp with aldopentose and deoxyhexose, which lacks the OH group of aldohexose, no dissociation of the molecules in the complexes or differences between enantiomers were observed. These results indicate that the OH groups in monosaccharides contribute to enantiomer-selective photodissociation in molecular clouds. The differences observed between enantiomers in the photodissociation mass spectra were applied to distinguishing and quantifying aldohexose enantiomers in solution using L-Trp as a chiral probe. The enantiomeric excesses of aldohexoses in solution could be determined from a single photodissociation mass spectrum by reference to the relative ion intensities for the NH2CHCOOH-elimination product and H+(L-Trp) formed via detachment of aldohexose. This analysis method could also distinguish and quantify two D-aldohexose mixtures, where L-Trp was employed as an isomer probe. Graphical abstract ᅟ.

Chiral and Molecular Recognition through Protonation between Aromatic Amino Acids and Tripeptides Probed by Collision-Activated Dissociation in the Gas Phase.

Chiral and molecular recognition through protonation was investigated through the collision-activated dissociation (CAD) of protonated noncovalent complexes of aromatic amino acid enantiomers with l-alanine- and l-serine-containing tripeptides using a linear ion trap mass spectrometer. In the case of l-alanine-tripeptide (AAA), NH3 loss was observed in the CAD of heterochiral H⁺(d-Trp)AAA, while H2O loss was the main dissociation pathways for l-Trp, d-Phe, and l-Phe. The protonation site of heterochiral H⁺(d-Trp)AAA was the amino group of d-Trp, and the NH3 loss occurred from H⁺(d-Trp). The H2O loss indicated that the proton was attached to the l-alanine tripeptide in the noncovalent complexes. With the substitution of a central residue of l-alanine tripeptide to l-Ser, ASA recognized l-Phe by protonation to the amino group of l-Phe in homochiral H⁺(l-Phe)ASA. For the protonated noncovalent complexes of His enantiomers with tripeptides (AAA, SAA, ASA, and AAS), protonated His was observed in the spectra, except for those of heterochiral H⁺(d-His)SAA and H⁺(d-His)AAS, indicating that d-His did not accept protons from the SAA and AAS in the noncovalent complexes. The amino-acid sequences of the tripeptides required for the recognition of aromatic amino acids were determined by analyses of the CAD spectra.

Chiral Recognition in Cold Gas-Phase Cluster Ions of Carbohydrates and Tryptophan Probed by Photodissociation.

Chiral recognition between tryptophan (Trp) and carbohydrates such as D-glucose (D-Glc), methyl-α-D-glucoside (D-glucoside), D-maltose, and D-cellobiose in cold gas-phase cluster ions was investigated as a model for chemical evolution in interstellar molecular clouds using a tandem mass spectrometer containing a cold ion trap. The photodissociation mass spectra of cold gas-phase clusters that contained Na+, Trp enantiomers, and D-maltose showed that Na+(D-Glc) was formed via the glycosidic bond cleavage of D-maltose from photoexcited homochiral Na+(D-Trp)(D-maltose), while the dissociation did not occur in heterochiral Na+(L-Trp)(D-maltose). The enantiomer-selective dissociation was also observed in the case of D-cellobiose. The enantiomer-selective glycosidic bond cleavage of disaccharides suggested that photoexcited D-Trp could prevent chemical evolution of sugar chains from D-enantiomer of carbohydrates in molecular clouds. The spectra of gas-phase clusters that contained Na+, Trp enantiomers, and D-Glc indicated that enantiomer-selective protonation of L-Trp from D-Glc could induce enantiomeric excess via collision-activated dissociation of the protonated L-Trp. In the case of protonated clusters, photoexcited H+(L-Trp) dissociated via Cα-Cß bond cleavage in the presence of D-Glc or D-glucoside, where the excited states of H+(L-Trp) contributed to the enantiomer-selective reaction in the clusters. These enantiomer selectivities in cold gas-phase clusters indicated that chirality of a molecule induced enantiomeric excess of other molecules via enantiomer-selective reactions in molecular clouds.

Enantiomer-Selective Photo-Induced Reaction of Protonated Tryptophan with Disaccharides in the Gas Phase.

In order to investigate chemical evolution in interstellar molecular clouds, enantiomer-selective photo-induced chemical reactions between an amino acid and disaccharides in the gas phase were examined using a tandem mass spectrometer containing an electrospray ionization source and a cold ion trap. Ultraviolet photodissociation mass spectra of cold gas-phase noncovalent complexes of protonated tryptophan (Trp) enantiomers with disaccharides consisting of two D-glucose units, such as D-maltose or D-cellobiose, were obtained by photoexcitation of the indole ring of Trp. NH2CHCOOH loss via cleavage of the Cα-Cß bond in Trp induced by hydrogen atom transfer from the NH3+ group of a protonated Trp was observed in a noncovalent heterochiral H+(L-Trp)(D-maltose) complex. In contrast, a photo-induced chemical reaction forming the product ion with m/z 282 occurs in homochiral H+(D-Trp)(D-maltose). For D-cellobiose, both NH2CHCOOH elimination and the m/z 282 product ion were observed, and no enantiomer-selective phenomena occurred. The m/z 282 product ion indicates that the photo-induced C-glycosylation, which links D-glucose residues to the indole moiety of Trp via a C-C bond, can occur in cold gas-phase noncovalent complexes, and its enantiomer-selectivity depends on the structure of the disaccharide.

Selection of a Single Isotope of Multiply Charged Xenon (A Xez+ , A=128-136, z=1-6) by Using a Bradbury-Nielsen Ion Gate.

The inclusion of an ion gate in a tandem mass spectrometer allows a specific precursor ion to be selected, and the fragment ions are then used for structure analysis and to investigate chemical reactions. However, the performance of an ion gate has been judged simply by whether or not the target ion was selected. In this study, we designed, manufactured, constructed, and characterized a Bradbury-Nielsen ion gate (BNG). The actual ion selection ability, i.e. the gate function, of the BNG was measured for isotopes of Xez+ (z=1-6). The gate function of the BNG was 36.5±0.5 ns in width and 3-13 ns in rise and fall times. The BNG provides a simple way to select multiply charged molecular cations of small organic molecules as well as large molecules such as proteins and peptides.

Quantitative chiral analysis of amino acids in solution using enantiomer-selective photodissociation of cold gas-phase tryptophan via chiral recognition.

To explore the origin of biomolecule homochirality in interstellar molecular clouds, enantiomer-selective photodissociation via chiral recognition between amino acids in the gas phase was examined using a tandem mass spectrometer containing an electrospray ionization source and a cold ion trap. Ultraviolet photodissociation mass spectra of cold gas-phase noncovalent complexes of sodiated l-tryptophan ion, Na+(l-Trp), with an amino acid such as serine (Ser), threonine (Thr), or alanine (Ala) were obtained by the photo-excitation of l-Trp in the noncovalent complexes. Dissociation of l-Trp via CO2 loss occurred when it was noncovalently complexed with d-Ser or d-Thr in the presence of Na+. For the l-enantiomers, the energy absorbed by l-Trp was released through evaporation of l-Ser or l-Thr, and dissociation of the amino acids was suppressed. In contrast, the enantiomer-selective phenomenon was not observed in the noncovalent complex with Ala, suggesting that a side-chain OH group plays an important role in chiral recognition and enantiomer-selective photodissociation. The enantiomer-selective photodissociation was applied to the quantitative chiral analysis of amino acids. The enantiomeric excess of Ser and Thr in solution could be determined by measuring the relative abundance ratio of the enantiomer-selective photodissociation of Trp to amino acid evaporation in a single photodissociation mass spectrum obtained by photo-excitation of l-Trp used as a chiral probe in cold gas-phase noncovalent complexes with the analyte amino acids, and by referring to the linear relationships established in this work.