83 8'-ヒドロキシアブシジン酸のファゼイン酸への異性化反応に対するpHと3'位フッ素の効果

概要

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Effect of pH and the 3'-fluorine on isomerization of 8'-hydroxyabscisic acid(8'-HOABA), the first metabolite of abscisic acid(ABA) in plants, to phaseic acid(PA) which has a much lower activity than ABA, was investigated thermodynamically and kinetically. The 8'-HOABA/PA ratio at equilibrium was 2:98 at 25℃, while the 3'-fluoro-8'HOABA/3'α-fluoro-PA raito was 16:84, meaning that introducing a fluorine at C-3' thermodynamically reduced isomerization of 8'-HOABA to PA. The isomerization became more rapidly as pH went up;the rate constant in pH 10 was higher than that in pH 3 by a factor of 2, 000. The slow isomerization under acidic conditions resulted from not the high activation enthaply but the negative, high activation entropy, suggesting that the transition structure under acidic conditions may be solvated well or associated with a concerted mechanism. Introduction of a fluorine at C-3' reduced the reaction rate by raising the activation enthalpy. This high activation enthalpy would reflect the decrease in overlap between the reaction orbitals of C-2' and O-8' that may result from the increase in electron density at C-2' which is caused by the +Iπ effect of the 3'-fluorine. These findings suggest that the stability of 8'-HOABA can be controlled by pH and the modification of the enone moiety.
1999-10-01