80(P-75) Electrochemical Reaction of Abscisic Acid

Abstract

A plant hormone, abscisic acid (ABA, 1) regulates physiological adaptation to environmental stress such as water deficiency, embryonic development, and so on. ABA has electrophilicity derived from the conjugated dienecarboxyl group in the side chain. Terem and Utley reported that ABA methyl ester (1-Me) gave a bicyclic compound (3) by cathodic reduction in electrolysis. An electron from a cathode could localize at C-2 of ABA, and C-2 may attack C-2' to give compound 3. The electrophilicity of ABA may be important for binding to a receptor, and anion radical of ABA could be used to label receptor proteins. We have studied the electrochemical property of ABA by analysis of its cathodic reaction. First, we chased the experiment of Terem and Utley with 100% CH_3CN solution of ABA methyl ester. However, no significant product formed under their condition. When the solvent was changed to an aqueous CH_3CN solution without pH control, a major product, 3-methyl-5-(1,3,3-trimethyl-2,5-dioxo-cyclohexyl)-pent-3-enoic cid (4), formed. The spectral data of its methyl ester (4-Me) was almost coincident with those of 3. ABA methyl ester in buffer solutions was reduced at a cathode at pH 3, 7, and 10. Cyclic voltammetry showed irreversible reduction with a peak potential at -1.6V. At pH 3 and 7, the ester gave 4-(1-hydroxy-2,2,6-trimethyl-4-oxo-bicyclo[4.1.0]hept-7-yl)-3-methyl-but-3-enoic acid methyl ester (5-Me) as a major product, and, at pH 10, 4-Me formed in addition to compound 5-Me. Compound 4-Me, an isomer of 5-Me, formed by isomerization under an alkaline condition. This finding showed that an electron localizes not at C-2 but at C-5, and C-5 attacks C-2'. Non-empirical calculation proved that Muliken negative charge and spin density at C-5 is the highest in anion radical of ABA methyl ester. Compound 3 was not detected under these conditions, suggesting that Terem and Utley incorrectly identified compound 4-Me as compound 3. Cyclic voltammetry of ABA was similar to its methyl ester. Cathodic reduction of ABA at pH 3 and 7 gave compound 5 as a major product. At pH 10, compounds 5 and 4 were not formed, and 1-deoxy-ABA (6), a dimer (7) with epoxy group and other compounds formed. The difference of the product at pH 10 between ABA and its methyl ester would be caused by dissociation of the carboxyl group. Cathodic reduction of phaseic acid (2), a metabolite of ABA, gave compound 8, showing that C-5 directly attacks C-4'. New compounds 4, 5, 7, and 8 were inactive in rice seedling test and lettuce seed germination test. This result suggests that the electrophilic side chain of ABA at C-1' is important for the biological activity.