Brassinolide-like activity of castasterone analogs with varied side chains against rice lamina inclination

Brassinolide (BL) and castasterone (CS) are the representative members of brassinosteroid class of plant steroid hormone having plant growth promoting activity. In this study, eleven CS analogs bearing a variety of side chains were synthesized to determine the effect of the side chain structures on the BL-like activity. The plant hormonal activity was evaluated in a dwarf rice lamina inclination assay, and the potency was determined as the reciprocal logarithm of the 50% effective dose (ED₅₀) from each dose-response curve. The reciprocal logarithm of ED₅₀ (pED₅₀) was decreased dramatically upon deletion of the C-28 methyl group of CS. The introduction of oxygen-containing groups such as hydroxy, methoxy, and ethoxycarbonyl was also unfavorable to the activity. The pED₅₀ was influenced by the geometry of carbon-carbon double bond between C-24 and C-25 (cis and trans), but the introduction of a fluorine atom at the C-25 position of the double bond did not significantly change the activity. The binding free energy (ΔG) was calculated for all ligand-receptor binding interactions using molecular dynamics, resulting that ΔG is linearly correlated with the pED₅₀.