Disclosure the Relationship ”Structure Biology Activity” on the Basis of Conformational Analysis of the Brassinosteroid’s Stereoisomers by the Molecular Modeling Methods

The conformational analysis of one of the most biologically active brassinosteroids, natural brassinolide, as well as of the less active natural 24-epibrassinolide, synthetics (22S,23S)-24-epibrassinolide and (22S,23S)-homobrassinolide was carried out by molecular mechanics and the DFT quantum chemical calculations followed by the comparison of their side chain structures. The 22R,23R,24S configuration of two hydroxyls and the methyl group of the brassinolide side chain was shown to support the structures in which its diol system makes the intramolecular O6…H(O5) hydrogen bond. At the same time, the O6H hydroxyl group is unbounded and may participate in forming the intermolecular hydrogen bond with a receptor. As opposed to this observation, the 22S,23S,24R-configuration of (22S,23S)-24-epibrassinolide is in line with the structures where the O6H hydroxyl group is screened by the 21-methyl group, causing the less biological activity of this hormone. It was shown that important factor of high brassinosteroid’s bioactivity is the curvature of their side chain in the line of the $\beta$-side of the steroid’s backbone, as well.