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′‐ 18 O]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 H 2 O 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.