Identification and quantification of leucine and isoleucine residues in peptides using photoexcited tryptophan

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.

Gas-Phase Adsorption of N2 on Protonated Molecules and Its Application to the Structural Elucidation of Small Molecules

The gas-phase adsorption of N₂ on protonated serine (Ser, C₃H₇NO₃), threonine (Thr, C₄H₉NO₃), glycine (Gly, C₂H₅NO₂), and 2-aminoethanol (C₂H₇NO) was investigated using a tandem mass spectrometer equipped with an electrospray ionization source and a cold ion trap. N₂ molecules were adsorbed on the free X–H (X=O and N) groups of protonated molecules. Gas-phase N₂ adsorption-mass spectrometry detected the presence of free X–H groups in the molecular structures, and was applied to the structural elucidation of small molecules. When the 93 structures with an elemental composition of C₃H₇NO₃ were filtered using the gas-phase N₂ adsorption-mass spectrometry results for Ser, the number of possible molecular structures was reduced to 8 via the quantification of the X–H groups. Restricting and minimizing the number of possible candidates were effective steps in the structural elucidation process. Gas-phase N₂ adsorption-mass spectrometry combined with mass spectrometry-based techniques has the potential for being useful for elucidating the molecular structures of a variety of molecules.