Analysis of Isomerization Process of 8′-Hydroxyabscisic Acid and its 3′-Fluorinated Analog in Aqueous Solutions

8′-Hydroxyabscisic acid (8′-HOABA), the first metabolite of abscisic acid (ABA) in plants, is spontaneously isomerized to low bioactive phaseic acid (PA). We investigated thermodynamic and kinetic properties of the isomerization process in aqueous solution buffered at the various pHs, along with the effects of 3′-fluorine. The 8′-HOABA/PA ratio at equilibrium was 2:98 at 25°C, while the 3′-fluoro-8′HOABA/3′α-fluoro-PA ratio was 16:84, indicating that introduction of a fluorine at C-3′ thermodynamically reduced isomerization of 8′-HOABA to PA. The isomerization became more rapid as pH increased; the rate constant at pH 10 was higher than that at pH 3 by a factor of 2000. Introduction of a fluorine at C-3′ reduced the reaction rate by raising the activation enthalpy. This indicated that 8′-HOABA was also kinetically stabilized by the 3′-fluorine. These findings suggested that the control of pH and modification of the enone moiety of the ring would be useful to manipulate the catabolic inactivation rate of ABA.