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

The molecular recognition ability of tryptophan (Trp) for isomeric amino acids, such as leucine (Leu) and isoleucine (Ile), and isomeric amino acid-containing dipeptides, such as Leu-Gly, Ile-Gly, Gly-Leu, and Gly-Ile (where Gly denotes glycine), was investigated using a tandem mass spectrometer equipped with an electrospray ionization source and cold ion trap. The ultraviolet photodissociation spectra of the cold gas-phase clusters of Leu and Ile with Na⁺Trp in the wavelength range of 265-290 nm revealed that the relative intensities of Leu and Ile were only different in the wavelength range of 265-273 nm; however, no differences in the relative intensities were observed when the wavelength exceeded 274 nm. The molecular recognition ability of photoexcited Trp was used for the identification and quantification of Leu and Ile in dipeptides in solution. The mole fractions of Leu and Ile in dipeptides could be determined from the abundances observed in a single product ion spectrum of the cold gas-phase clusters of dipeptides with Na⁺Trp.