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

Abstract

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 C_α-C_β 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.