Gas-phase N2 adsorption on mass-selected hydrogen-bonded cluster ions

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 C_α-C_β 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 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.

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-Cβ 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.