Structural Determination of Antioxidant and Anticancer Flavonoid Rutin in Solution through DFT Calculations of 1H NMR Chemical Shifts

As the knowledge of the predominant molecular structure of antioxidant and anticancer flavonoid rutin in solution is very important for understanding the mechanism of action, a quantum chemical investigation of plausible rutin structures including solvent effects is of relevance. In this work, DFT calculations were performed to find possible minimum energy structures for the rutin molecule. H-1 NMR chemical shift DFT calculations were carried out in DMSO solution using the polarizable continuum model (PCM) to simulate the solvent effect. Analysis of the experimental and theoretical H-1 NMR chemical shift profiles offers a powerful fingerprint criterion to determine the predominant molecular structure in solution. Therefore, our aim is to find the best match between experimental (in DMSO-d) and theoretical (PCM-DMSO) H-1 NMR spectrum profiles. Among 34 optimized structures located on the potential energy surface, we found that structure 32, with a B-ring deviated 30 degrees from a planar configuration (geometry usually assumed for polyphenols), showed an almost perfect agreement with experimental the H-1 NMR pattern when compared to the corresponding fully optimized planar geometry. This structure is also predicted as the global minimum based on room-temperature Gibbs free energy calculations in solution and, therefore, should be experimentally observed. This is new and valuable structural information regarding structure-activity relationship studies, and such information is hard to obtain by experimentalists without the aid of the X-ray diffraction technique.