Mass Spectrometry Rearrangement Ions and Metabolic Pathway-Based Discovery of Indole Derivatives during the Aging Process in Citrus reticulata 'Chachi'

Abstract

The rapid analysis and characterization of compounds using mass spectrometry (MS) may overlook trace compounds. Although targeted analysis methods can significantly improve detection sensitivity, it is hard to discover novel scaffold compounds in the trace. This study developed a strategy for discovering trace compounds in the aging process of traditional Chinese medicine based on MS fragmentation and known metabolic pathways. Specifically, we found that the characteristic component of C. reticulata 'Chachi', methyl N-methyl anthranilate (MMA), fragmented in electrospray ionization coupled with collision-induced dissociation (CID) to produce the rearrangement ion 3-hydroxyindole, which was proven to exist in trace amounts in C. reticulata 'Chachi' based on comparison with the reference substance using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Combining the known metabolic pathways of 3-hydroxyindole and the possible methylation reactions that may occur during aging, a total of 10 possible indole derivatives were untargeted predicted. These compounds were confirmed to originate from MMA using purchased or synthesized reference substances, all of which were detected in C. reticulata 'Chachi' through LC-MS/MS, achieving trace compound analysis from untargeted to targeted. These results may contribute to explaining the aging mechanism of C. reticulata 'Chachi', and the strategy of using the CID-induced special rearrangement ion-binding metabolic pathway has potential application value for discovering trace compounds.

Keywords: 3-hydroxyindole; Citrus reticulata ‘Chachi’; indole derivatives; mass spectrometry; methyl N-methyl anthranilate.

Scheme 1

(a–d) Approaches for synthesis of compounds 7, 9, 11, and 12.

Figure 1

Putative diagram of indole derivatives.

Figure 2

(A) MS/MS spectra of MMA and 3-hydroxyindole at m/z 134.06 using LC–HR–MS. (B) MS/MS spectra of MDA and N-methyl-3-hydroxyindole at m/z 148.0757 using LC–HR–MS.

Figure 3

Comparison of MRM chromatograms between compounds 1–12 and C. reticulata ‘Chachi’. (A) MMA; (B) 3-hydroxyindole; (C) isatin; (D) indigo and indirubin; (E) N, N′, O, O′-Tetramethyl-leuko-indigo; (F) MDA; (G) N-methyl-3-hydroxyindole; (H) N-methylisatin; (I) N, N′-demethylindigo and N, N′-demethylindirubin; and (J) synthesis fluid of N-methyl-3-hydroxyindole.