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.

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 Cα-Cß 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.