Chemical exchange of hydroxyl protons in quercetin measured by pulsed field gradient NMR

The diffusion behavior of hydroxyl protons (OH) in quercetin in 100% DMSO-d6 (deuterium dimethylsulfoxide) and a 90% DMSO-d6 solution containing 10% H2O was investigated with 600 MHz 1H pulsed field gradient (PFG) nuclear magnetic resonance (NMR). Only resonances of the 5-hydroxyl protons (OH5) were well resolved in NMR spectra of quercetin for all solutions under study. This phenomenon is explained by the intramolecular hydrogen bonding between OH5 protons and the 4-carbonyl oxygen (CO4). During diffusion experiments, the OH5 protons showed a biexponential diffusion decay, indicating an exchange process with water. As water content in the solvents increased, the lifetime (τq) of the OH5 protons decreased from 96.7±10.0 ms in 100% DMSO-d6 to 14.3±1.4 ms in the 90% DMSO-d6 solution containing 10% H2O, indicating an increase in the exchange rate (kq = l/τq) of the OH5 protons. This study demonstrates that the diffusion approach with PFG-NMR is much faster and easier for estimating the short lifetime or fast exchange rate of hydroxyl protons in quercetin.