Quantification of Amino Acid Enantiomers Using Electrospray Ionization and Ultraviolet Photodissociation

The enantioselectivity of tryptophan (Trp) for amino acids, such as alanine (Ala), valine (Val), and serine (Ser), was investigated using ultraviolet (UV) photoexcitation and tandem mass spectrometry. Product ion spectra of cold gas-phase amino acid enantiomers that were hydrogen-bonded to Na⁺(L-Trp) were measured using a variable-wavelength UV laser and a tandem mass spectrometer equipped with an electrospray ionization source and a cold ion trap. Na⁺(L-Trp), formed via amino acid detachment, and the elimination of CO₂ from the clusters were observed in the product ion spectra. For photoexcitation at 265 nm, the relative abundance of Na⁺(L-Trp) compared to that of the precursor ion observed in the product ion spectrum of heterochiral Na⁺(L-Trp)(D-Ala) was larger than that observed in the product ion spectrum of homochiral Na⁺(L-Trp)(L-Ala). A difference between the Val enantiomers in the relative abundance of the precursor and product ions was observed in the case of photoexcitation at 272 nm. The elimination of CO₂ was not observed for L-Ser for the 285 nm photoexcitation, which was the main reaction pathway for D-Ser. Photoexcited Trp chiral recognition was applied to identify and quantify the amino acid enantiomers in solution. Ala, Val, and Ser enantiomers in solution were quantified from their relative abundances in single product ion spectra measured using photoexcitation at 265, 272, and 285 nm, respectively, for hydrogen-bonded Trp within the clusters.