1H, 13C MAS NMR and DFT GIAO study of quercetin and its complex with Al(III) in solid state

Quercetin (Q) as a pentahydroxy flavonoid, has three possible chelating sites competing in complexation processes. 1H and 13C MAS NMR spectra were recorded for solid quercetin and its Al(III) complex (AlQ). 1H MAS spectrum of quercetin shows a broad resonance at ca. 12 ppm that confirms the existence of intramolecular C5OH … OC4 hydrogen bond. Such a signal is absent in the spectrum of AlQ, which is in accordance with other spectroscopic data and the suggested model for the solid-state structure of the complex. DFT GIAO calculations were used to verify the experimental 13C CPMAS NMR data and to suggest the best model structure for the complex AlQ. The calculated shielding constants for different conformers of isolated quercetin molecules, quercetin trimer as taken from the X-ray data, and different model structures for possible Al(III) complexes were compared with the 13C CPMAS NMR experimental values. The results demonstrate the importance of intermolecular interactions when dealing with structures in solid state and the successful application of the combined DFT GIAO and 13C CPMAS NMR approach. All data confirm that the chelating site of Q in the solid complex AlQ involves the deprotonated C5OH and the C4O groups at ring C, in contrast to the available studies performed in solution.

Graphical abstractUncharged, solid Al(III) complex of quercetin was obtained and the binding sites of quercetin were identified using solid-state NMR. The best model structure of the quercetin-Al complex, optimized with B3LYP/6-31+G**, was selected by comparing the calculated and experimental differences in the 13C chemical shifts resulting from coordination, i.e. Δcalc' = δcomplcalc − δfreeLcalc and Δexp' = δcomplexp − δfreeLexp.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The structure of Al(III) complex of quercetin was studied in solid state. ► Experimental, solid-state NMR, and theoretical data were used. ► Quantum chemical calculations were performed for all possible quercetin conformers. ► For the first time the intermolecular interactions in a quercetin trimer were studied. ► Coordination with the carbonyl group and deprotonated OH at C5 best reproduce the data.