Identification and quantification of leucine and isoleucine residues in peptides using photoexcited tryptophan
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ORIGINAL ARTICLE
Identification and quantification of leucine and isoleucine residues in peptides using photoexcited tryptophan Soma Hanaichi1 · Akimasa Fujihara1 Received: 17 April 2020 / Accepted: 19 July 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract 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. Keywords Electrospray ionization · Ion trap · Tandem mass spectrometry · Isomer · Molecular recognition
Introduction Amino acid substitutions in peptides have relationships with genetic disorders. Mass spectrometry is used to investigate the structure of peptides and to elucidate the types and positions of the amino acid mutations in peptides, because it is highly sensitive, selective, and suitable for analyzing mixtures (Wada et al. 1981, 1983, 1989; Wada 1992). The amino acid substitutions could be detected using the changes in the mass numbers of the normal and abnormal peptides. The relationships between the structure and fragmentation of biological molecules have been investigated using various gas-phase ion dissociation techniques (Zubarev 2003; Shao and Lam 2017; Qi and Volmer 2017; Brodbelt et al. 2020). The differentiation of isomeric amino acid residues of peptides is one of the most significant challenges in the field of mass spectrometry (Hurtado and O’Connor 2012). To distinguish l-leucine (Leu) and l-isoleucine (Ile), which Handling editor: D. Tsikas. * Akimasa Fujihara [email protected]‑u.ac.jp 1
Department of Chemistry, Osaka Prefecture University, Osaka 599‑8531, Japan
are isomers, the differences in reactivity between them have been analyzed using several mass spectrometry-based techniques. The product ions observed in the collision-induced dissociation spectra of Leu-containing protonated peptides differed from those in the spectra of Ile-containing peptides (Johnson et al. 1988; Nakamura et al. 1990; Stults et al. 1993; Hulst and Kientz 1996; Berkout 2009). The collisioninduced dissociation of the gas-phase complexes of peptides with metal ions has been used to distingui
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