Quantitative chiral analysis of amino acids in solution using enantiomer-selective photodissociation of cold gas-phase tryptophan via chiral recognition.

To explore the origin of biomolecule homochirality in interstellar molecular clouds, enantiomer-selective photodissociation via chiral recognition between amino acids in the gas phase was 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 sodiated l-tryptophan ion, Na(l-Trp), with an amino acid such as serine (Ser), threonine (Thr), or alanine (Ala) were obtained by the photo-excitation of l-Trp in the noncovalent complexes. Dissociation of l-Trp via CO loss occurred when it was noncovalently complexed with d-Ser or d-Thr in the presence of Na. For the l-enantiomers, the energy absorbed by l-Trp was released through evaporation of l-Ser or l-Thr, and dissociation of the amino acids was suppressed. In contrast, the enantiomer-selective phenomenon was not observed in the noncovalent complex with Ala, suggesting that a side-chain OH group plays an important role in chiral recognition and enantiomer-selective photodissociation. The enantiomer-selective photodissociation was applied to the quantitative chiral analysis of amino acids. The enantiomeric excess of Ser and Thr in solution could be determined by measuring the relative abundance ratio of the enantiomer-selective photodissociation of Trp to amino acid evaporation in a single photodissociation mass spectrum obtained by photo-excitation of l-Trp used as a chiral probe in cold gas-phase noncovalent complexes with the analyte amino acids, and by referring to the linear relationships established in this work.