MOLECULAR MODELING OF THE (S)-ABA BINDING-SITE OF THE RECEPTOR INVOLVED IN THE INDUCTION OF FREEZING TOLERANCE - A HYPOTHETICAL RECEPTOR MODEL

Molecular modeling was used to compare abscisic acid (ABA) analogs to rationalize reported structure-activity relationships based on the ind uction of freezing tolerance in bromegrass (Bromus inermis Leyss.) cel l culture. A modified version of the active analog approach was employ ed with (S)-ABA used as the standard to which other compounds were sup erimposed. Common conformations that present similar three-dimensional steric and electronic patterns were identified through conformational searches. From this analysis, a hypothetical model for the putative ( S)-ABA receptor was constructed with the following features. The ring is the pseudochair conformation with an axial side chain. The C-1' has the same absolute stereochemistry as (S)-ABA. The model suggests that discrimination between (S)-ABA, phaseic acid, and (R)-ABA is due to t he presence of an ether bridge or a methyl group below the C-2' of (S) -ABA, in a region proposed to be occupied by the receptor. The side ch ain is syn with the ring and positioned approximately above the 2'-car bon of the ring. The model serves as a working hypothesis for testing receptor requirements and can be used to direct future analog studies.