title: Stereochemical aspects of the formation of double bonds in abscisic acid.
abstract: The stereochemistry of the hydrogen elimination that occurs during the formation of the Delta(4)- and Delta(2)'-double bonds of abscisic acid has been determined from the (14)C/(3)H ratios in abscisic acid biosynthesized by avocado fruit from [2-(14)C,(2R)-2-(3)H(1)]-, [2-(14)C,(2S)-2-(3)H(1)]- and [2-(14)C,(5S)-5-(3)H(1)]-mevalonate. Setting the (14)C/(3)H ratio at 3:3 for [2-(14)C,(2R)-2-(3)H(1)]mevalonate, the corresponding ratio in derived methyl abscisate was 3:2.28; the analogous ratio for methyl abscisate from [2-(14)C,(2S)-2-(3)H(1)]mevalonate was 3:1.63. Removal of the 3'-hydrogen atom of abscisic acid by base-catalysed exchange altered the ratios to 3:1.55 and 3:1.44 respectively. It was concluded that this 3'-hydrogen atom is derived from the pro-2R-hydrogen atom of mevalonate. Removal of the 4-hydrogen atom from methyl abscisate by formation of a derivative, a lactone, lacking this hydrogen atom changed the ratio to 3:1.04 for material derived from [2-(14)C,(2R)-2-(3)H(1)]-mevalonate and to 3:1.05 for [2-(14)C,(2S)-2-(3)H(1)]mevalonate, showing that this hydrogen atom also is derived from the pro-2R-hydrogen atom of mevalonate. These ratios of the lactones are consistent with their retaining one (3)H atom at the 6'-methyl position of abscisic acid from the [(2R)-2-(3)H(1)]- and [(2S)-2-(3)H(1)]-mevalonate. The presence of some label at positions 3' and 4 when [(2S)-2-(3)H(1)]mevalonate was the precursor is attributed to the action of isopentenyl pyrophosphate isomerase. The hydrogen atom at C-5 of abscisic acid is derived from the pro-5S-hydrogen atom of mevalonate.
mesh_terms: D000577:Amides / Q000494:pharmacology; D002247:Carbon Isotopes; D002338:Carotenoids; D002855:Chromatography, Thin Layer; D005638:Fruit; D006859:Hydrogen; D007461:Iodoacetates / Q000494:pharmacology; D007535:Isomerases / Q000037:antagonists & inhibitors; D008798:Mevalonic Acid; D008968:Molecular Conformation; D010937:Plant Growth Regulators / Q000096:biosynthesis; D013329:Structure-Activity Relationship; D014316:Tritium
chemical_list: D000577:Amides; D002247:Carbon Isotopes; D007461:Iodoacetates; D010937:Plant Growth Regulators; D014316:Tritium; D002338:Carotenoids; D006859:Hydrogen; D007535:Isomerases; D008798:Mevalonic Acid