Mechanism of the formation of a dehydrated ion by an unusual loss of oxygen at the 4′-carbonyl group of abscisic acid methyl ester in electron ionization mass spectrometry

Methyl ester of abscisic acid (ABA), a plant hormone, gives a dehydrated ion at m/z 260 in electron ionization mass spectrometry (EI–MS). This dehydrated ion had been considered to be derived only from the elimination of the tertiary hydroxyl group at C-1′. We found that 34% of the dehydrated ion was formed by elimination of the oxygen atom at the 4′-carbonyl group, and the remaining 66% by elimination of the 1′-hydroxyl group. This unusual elimination of the carbonyl oxygen was shown with [4′-18O]ABA methyl ester. Involvement of the 4′-carbonyl oxygen in dehydration was observed in methyl ester of phaseic acid (PA), a natural metabolite of ABA, but not in 1′-deoxy-ABA methyl ester or isophorone. This suggested that the 1′-hydroxyl group was necessary for the elimination of the 4′-carbonyl oxygen. ABA methyl esters labeled with stable isotopes showed that hydrogen atoms at the 1′-hydroxyl group and at C-4 or -5 or -3′ or - 5′ or -7′ were eliminated with the 4′-carbonyl oxygen. These results allow us to propose a formation mechanism of the dehydrated ion derived from the elimination of 4′-carbonyl oxygen and hydrogen atoms at C-4 and 1′-oxygen in ABA methyl ester as follows: first, ionization at the 1′-hydroxyl group occurs to give an ion radical, and the proton at the 1′-oxygen migrates to the 4′-carbonyl oxygen after the bond fission between C-1′–C-6′; second, migration of the proton at C-4 to the 1′-oxygen is followed by migration of the protons at C-5 and C-7′ to C-4 and C-5, respectively; finally, the proton at the 1′-oxygen migrates to the 4′-hydroxyl group, and H2O at C-4′ is eliminated to give the dehydrated ion. Our findings point out that a dehydrated ion is not always derived from the elimination of a hydroxyl group. Copyright © 2005 John Wiley & Sons, Ltd.