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.