Determination of preferred conformations of brassinosteroids by means of NMR investigations and Boltzmann statistical analysis of simulated annealing calculations

. Brassinosteroids are a class of steroidal phytohormones with high growth-promoting properties. The preferred side-chain conformations of 10 brassinosteroids were determined by means of detailed NMR investigations and molecular-modeling studies. Vacuum conformations obtained by simulated annealing calculations and Boltzmann statistical analysis were compared with solution conformations derived from NOE experiments and molecular dynamic simulations, and with X-ray structures. In general, results from simulated annealing calculations and NMR-supported molecular dynamics simulations are in good agreement. For some of the compounds investigated the conformation was less well-defined at the end of the side-chain. It could be shown that the energetically most favorable and most probable conformations also include the conformations obtained by NMR supported molecular-dynamics calculations and by X-ray analysis. For the most bioactive compound brassinolide ( 1 ) the majority of conformations show a side-chain bent towards the β-face of the steroid skeleton, whereas for the less bioactive brassinosteroids, conformations with straight side-chains or side-chains bent towards the α-face are more frequent.