Infrared spectra of brassinolide and castasterone steroid phytohormones and their 24-epi derivatives

Abstract

Absorption bands in IR spectra of brassinolide, castasterone, and their 24-epi derivatives in the frequency range 3800-1000 cm -1 have been interpreted. A number of spectral features distinguishing brassinolide from castasterone have been found. The conducted analysis shows that the structural differences manifest themselves in IR spectra of the investigated brassinosteroids in the region of stretching vibrations of CO-H, C=O, C-OH, C-O-C, CH 3 , CH 2 , and CH groups. The main distinctions in IR spectra of brassinolides and castasterones are due to the B ring structure.All hydroxyl groups of brassinosteroids pressed in KBr are included in hydrogen bonds. Broad bands attributed to associated OH groups are observed in the IR spectra. Two possible kinds of hydrogen bonding exist. IR spectra of diluted solutions of the studied brassinosteroids contain two bands caused by vibrations of free hydroxyls and those associated by intramolecular hydrogen bonds.A fraction of the C=O groups of brassinosteroids pressed in KBr remains free. High-frequency bands in solid-state IR spectra are due to vibrations of free C=O groups whereas low-frequency bands are attributed to stretching vibrations of associated groups. The lack of low-frequency bands in solution spectra proves that the C=O⋅⋅⋅H-O hydrogen bond is intermolecular rather than intramolecular. Vibrational frequencies of brassinolide C=O groups are higher than the corresponding frequencies for castasterones because of the influence of oxygen in the seven-membered B ring.