NMR spectroscopic and quantum mechanical analyses of enhanced solubilization of hesperidin by theasinensin a

Purpose: The use of hesperidin in the pharmaceutical field is limited by its aqueous insolubility. The effects of natural compounds in tea on the solubility of hesperidin were evaluated and the underlying mechanism was investigated by nuclear-magnetic resonance (NMR) and quantum mechanical calculations. Methods: The solubility of hesperidin was measured by liquid chromatography time-of-flight mass spectrometry; the structure of the hesperidin/theasinensin A complex was characterized by ¹H-NMR, diffusion-ordered NMR spectroscopy, and rotating frame NOE spectroscopy, as well as theoretically by quantum mechanical calculations. Results: Among the natural compounds in tea, theasinensin A was the most effective in improving hesperidin solubility. The complexation of hesperidin with theasinensin A led to changes in the chemical shift of protons in hesperidin (Δδ: 0.01-0.27 ppm) and diffusion coefficient (ΔD: 0.66-1.32∈×∈10^-10 m²/s) of hesperidin. ROE correlation signals between hesperidin and theasinensin A and quantum mechanical calculations revealed that two hesperidin molecules formed a stable complex with theasinensin A (2:1 complex) with a ΔG energy of -23.5 kJ/mol. Conclusions: This is the first study that provides insight into the enhanced solubility of hesperidin through interactions with theasinensin A via a 2:1 complex formation between hesperidin and theasinensin A.