Configurations and conformations of abscisic acid

The likely configurations and conformations of ABA in solution have been determined by a combination of empirical force field calculations and NMR. The rotation of the methyl and alcohol groups, which would not be expected significantly to effect binding to a receptor, was excluded from the analysis. The molecule of ABA has four possible degrees of freedom of movement that can result in significant changes in its overall shape. The four degrees of freedom are rotation around the C-1, C-3 and C-5 bonds, and interconversion of the ring from one pseudo chair form, in which the side chain is axial to the plane of the ring, to a second pseudo chair form where the side chain is equatorial to the plane of the ring. The motions of parts of the molecule with reference to these four degrees of freedom were analysed by empirical force field calculations (molecular dynamics) and the results were correlated with NMR data obtained on the molecule in aqueous solution. Free rotation occurs around the C-1 bond, limited rotation of approximately 180° also occurs around the C-5 bond and very limited motion occurs around the C-3 bond of approximately 20°. No evidence was found for interconversion of the ring with its axial side chain to that with its side chain equatorial. The range of motions was defined and its consequences for the design of analogues and for binding to receptor molecules are discussed. No evidence for the other conformation of the ring (side chain equatorial) could be obtained by ¹H NMR even at 368 K.