Structural Study of Phenolic Acids by Triple Quadrupole Mass Spectrometry with Electrospray Ionization in Negative Mode and H/D Isotopic Exchange

Nayane Sinosaki,Eduardo Meurer,Oscar Santos,Sharise Roberto,Marcos Ribeiro,Camila Poliseli,Angélica Tonin,Jesuí Visentainer,Roberta Da Silveira

doi:10.21577/0103-5053.20190197

Abstract

This work reports the theoretical and experimental study of fragmentation reactions in the gas phase of five phenolic acids using triple quadrupole mass spectrometry by electrospray ionization in negative ionic mode, as well as the isotope exchange experiments. MS/MS spectra were analyzed to suggest the fragmentation mechanisms, while theoretical calculations at the theory level B3LYP/6-311+G** were performed to expose the proposed mechanisms viability for this class of compounds. As expected, compounds with aromatic methoxy substitution presented •CH3 radical elimination as the principal fragmentation pathway, forming dystonic ions. Compounds without methoxy substituents dissociate with higher energies losing the CO2, CO and H2O. The isotopic marking experiments indicated the exchange of hydrogens by deuterases in the hydroxyl protons, which corroborates with the proposed mechanisms.

Highlights

Phenolic acids are substances belonging to the phenolic compounds group
The studies available in the literature discuss the fragmentation mechanisms only for some specific phenolic acids, this work reports the use of electrospray ionization mass spectrometry (ESI-MS/MS) to investigate the pattern fragmentation of five phenolic acids: p-coumaric acid, gallic acid, caffeic acid, ferulic acid and synapic acid, under low energy collision induced dissociation (CID) associated with theoretical calculations and isotopic labeling experience
Five phenolic acids (p-coumaric, gallic, caffeic, ferulic, and synapic) were analyzed for its fragmentation patterns. Structures of these compounds are characterized by having a benzene ring, a carboxyl group and one or more hydroxyl and/or methoxyl groups. The differences between these structures are: p-coumaric acid has a C4 hydroxyl group, caffeic acid has two hydroxyl groups at the C3 and C4 positions, ferulic acid has a C4 hydroxyl group and a C3 methoxyl group, synapic acid has a C4 hydroxyl group and two methoxyl groups at the C3 and C4 positions, and gallic acid has a double bond between the aromatic group and the carboxylic acid function, plus three hydroxyl groups in the C3, C4 and C5 positions (Figure 1)

Summary

Introduction

Phenolic acids are substances belonging to the phenolic compounds group. It is extensively distributed in nature, being present in plants as free form or attached to sugars and proteins.[1]Its structure is characterized by a benzene ring, a carboxyl group and one or more hydroxyl and/or methoxyl group. The studies available in the literature discuss the fragmentation mechanisms only for some specific phenolic acids, this work reports the use of electrospray ionization mass spectrometry (ESI-MS/MS) to investigate the pattern fragmentation of five phenolic acids: p-coumaric acid, gallic acid, caffeic acid, ferulic acid and synapic acid, under low energy collision induced dissociation (CID) associated with theoretical calculations and isotopic labeling experience.

Results

Conclusion