β-Ionone cyclodextrins inclusion complexes: 1H NMR study and photolysis

Complexes of β-ionone (BI) with β-cyclodextrin (β-CD), 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) and 2-hydroxypropyl-γ-cyclodextrin (HP-γ-CD) were studied by 1H NMR spectroscopy in aqueous and methanolic solutions. From the dependence of chemical shifts of the BI protons on CD concentration (Job’s plot), it was concluded that BI forms 2:1 inclusion complexes (CD:BI) with β-CD and HP-β-CD, but a 1:1 complex with HP-γ-CD. The stability constants in water are K21 (BI/β-CD) ≥ 2×106 M−2; K21 (BI/HP-β-CD) = 5×106 M−2; K11 (BI/HP-γ-CD) = 1×104 M−1. In 1:1 water/methanol solution the stability constants decrease by about two orders of magnitude compared to those in water. For example, for BI/HP-β-CD complex, K21=8×104 M−2 in this case. Photolysis of trans-β-ionone in water and aqueous CD solutions leads to only one final product, retro-γ-ionone. Other isomers, cis-β-ionone and racemic α-pyran, are formed at earlier stages of the photolysis. The role of cyclodextrin in this process is discussed.