EnantioselectivePhotolysis and Quantitative Chiral Analysis of Tryptophan Complexed With Alkali-Metalized L-Serine in the Gas Phase

Mass spectrometric detection of enantioselectivity in three-component complexation, copper(ii)-chiral tetradentate ligand-free amino acid in solution

Enantioselective coordination of a free amino acid as the second ligand of a Cu(ii) complex with a novel chiral tetradentate ligand was evaluated quantitatively in a short measurement time by ESI-MS/deuterium-labelled quasienantiomers.

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 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.

Enantioselective Collision-Activated Dissociation of Gas-Phase Tryptophan Induced by Chiral Recognition of Protonated L-Alanine Peptides

Enantioselective dissociation in the gas phase is important for enantiomeric enrichment and chiral transmission processes in molecular clouds regarding the origin of homochirality in biomolecules. Enantioselective collision-activated dissociation (CAD) of tryptophan (Trp) and the chiral recognition ability of L-alanine peptides (L-Ala n ; n = 2-4) were examined using a linear ion trap mass spectrometer. CAD spectra of gas-phase heterochiral H+(D-Trp)(L-Ala n ) and homochiral H+(L-Trp)(L-Ala n ) noncovalent complexes were obtained as a function of the peptide size n. The H2O-elimination product was observed in CAD spectra of both heterochiral and homochiral complexes for n = 2 and 4, and in homochiral H+(L-Trp)(L-Ala3), indicating that the proton is attached to the L-alanine peptide, and H2O loss occurs from H+(L-Ala n ) in the noncovalent complexes. H2O loss did not occur in heterochiral H+(D-Trp)(L-Ala3), where NH3 loss and (H2O + CO) loss were the primary dissociation pathways. In heterochiral H+(D-Trp)(L-Ala3), the protonation site is the amino group of D-Trp, and NH3 loss and (H2O + CO) loss occur from H+(D-Trp). L-Ala peptides recognize D-Trp through protonation of the amino group for peptide size n = 3. NH3 loss and (H2O + CO) loss from H+(D-Trp) proceeds via enantioselective CAD in gas-phase heterochiral H+(D-Trp)(L-Ala3) at room temperature, whereas L-Trp dissociation was not observed in homochiral H+(L-Trp)(L-Ala3). These results suggest that enantioselective dissociation induced by chiral recognition of L-Ala peptides through protonation could play an important role in enantiomeric enrichment and chiral transmission processes of amino acids.

Enantiomeric Excess Determination for Monosaccharides Using Chiral Transmission to Cold Gas-Phase Tryptophan in Ultraviolet Photodissociation

Chiral transmission between monosaccharides and amino acids via photodissociation in the gas phase was examined using a tandem mass spectrometer fitted with an electrospray ionization source and a cold ion trap in order to investigate the origin of the homochirality of biomolecules in molecular clouds. Ultraviolet photodissociation mass spectra of cold gas-phase noncovalent complexes of the monosaccharide enantiomers glucose (Glc) and galactose (Gal) with protonated L-tryptophan H⁺(L-Trp) were obtained by photoexcitation of the indole ring of L-Trp. L-Trp dissociated via C_α-C_β bond cleavage when noncovalently complexed with D-Glc; however, no dissociation of L-Trp occurred in the homochiral H⁺(L-Trp)(L-Glc) noncovalent complex, where the energy absorbed by L-Trp was released through the evaporation of L-Glc. This enantioselective photodissociation of Trp was due to the transmission of chirality from Glc to Trp via photodissociation in the gas-phase noncovalent complexes, and was applied to the quantitative chiral analysis of monosaccharides. The enantiomeric excess of monosaccharides in solution could be determined by measuring the relative abundance of the two product ions in a single photodissociation mass spectrum of the cold gas-phase noncovalent complex with H⁺(L-Trp), and by referring to the linear relationships derived in this work. Graphical Abstract ᅟ.