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.

Abstract

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-Cβ 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.