Mechanism of Formation of Dehydrated Ions from Abscisic Acid and Its 1′,4′-Diol Methyl Esters in Electron Ionization Mass Spectrometry

Hydrogen atoms lost by dehydration in electron ionization mass spectrometry of methyl esters of abscisic acid and 1′,4′-trans and 1′,4′-cis diolabscisic acids were identified using their deuterated analogs. Analysis of mass spectra of the deuterated analogs showed that, in methyl ester of abscisic acid, the 4-hydrogen atom bonded to a double bond was mainly eliminated with a 1′-hydroxyl group to give a dehydrated ion at m/z 260. This unusual dehydration could be due to a proximity effect between the 1′-hydroxyl group and 4-hydrogen. In methyl esters of 1′,4′-diolabscisic acids, the first dehydration derived from loss of the 4′-hydroxyl group was accompanied with a hydrogen atom of the 1′-hydroxyl group, and the second dehydration derived from 1′-oxygen involved 3′-,4′-,5′-, and 7′-hydrogen atoms, and partially involved the 4-hydrogen atom. This indicated that the structure of the first dehydrated ion of the 1′,4′-diolabscisic acid methyl esters was not 4′-deoxyabscisic acid methyl ester. The migration of the hydroxyl hydrogen atom to another hydroxyl group in the first dehydration has also been demonstrated in dehydration of cis-1,4-cyclohexanediol. Based on these results, we proposed dehydration mechanisms in EI MS of methyl esters of abscisic acid and 1′,4′-diolabscisic acids.