SIDE-CHAIN CONFORMATION OF THE GROWTH-PROMOTING PHYTOHORMONES BRASSINOLIDE AND 24-EPIBRASSINOLIDE

Quantitative 2D NOE measurements and restrained molecular dynamics sim ulations (with explicit solvent) were carried out in order to determin e preferential solution side chain conformations of the two most impor tant native brassinosteroids, brassinolide and 24-epibrassinolide. The NOE assignment was assisted by 1D NOE difference spectroscopy and inc luded prochiral assignment of the side chain methyl groups Me-26 and M e-27. 2D NOE intensities were converted into inter-proton distances us ing the 'complete relaxation matrix analysis' methodology. Restrained molecular dynamics simulations in a chloroform solvent box led to a we ll defined solution conformation in the case of brassinolide. The incr eased side chain flexibility in the case of 24-epibrassinolide is disc ussed, considering missing NOE correlations and vicinal proton couplin g constants. (C) 1997 by John Wiley & Sons, Ltd.