Mechanism for odd‐electron anion generation of dihydroxybenzoic acid isomers in matrix‐assisted laser desorption/ionization mass spectrometry with density functional theory calculations

RATIONALE Proton and radical are transferred between matrices and matrix and analyte in matrix-assisted laser desorption/ionization (MALDI) and these transfers drive ionization of analytes. The odd-electron anion [M-2H]•- was generated in dihydroxybenzoic acids (DHBs) and the ion abundance of the 2,5-DHB was the highest among six DHB isomers. We were interested in the mechanism of the ion generation of the odd-electron anion. METHODS The observed [M-2H]•- and [M-3H]- ions, which were generated with the hydrogen radical removed from the phenolic hydroxyl groups (OH) in DHB isomers, were analyzed using negative-ion MALDI-MS. The enthalpy for ion generation and their stable structures were calculated using the density functional theory (DFT) calculation program Gaussian 09 with the B3LYP functional and the 6-31+G(d) basis set. RESULTS The number of observed [M-2H]•- and [M-3H]- ions of the DHB isomers was dependent on the positions of the phenolic OH groups in the DHB isomers because the carboxy group interacts with the ortho OH group due to neighboring group participation, as confirmed from the stable structures of the [M-2H]•- anions calculated with the Gaussian 09 program. The DHB isomers were placed into three categories according to the number of the ions. CONCLUSIONS Odd-electron anions ([M-2H]•- ) and [M-2H• -H]- ([M-3H]- ) ions were generated from DHB isomers due to removal of the hydrogen radical from the phenolic groups. The enthalpy for ion generation revealed that ion formation proceeds via a two-step pathway through the [M-M]- ion as an intermediate. © 2016 The Authors. Rapid Communications in Mass Spectrometry Published by John Wiley & Sons Ltd.