Conformational Analysis of 5,4'-Dihydroxy-7,5',3'-trimethoxyisoflavone in Solution Using ¹H NMR: A Density Functional...

Among 20 compounds isolated from the extracts of Ouratea ferruginea the 5,4'-dihydroxy-7,5',3'-trimethoxyisoflavone (9) showed the best inhibitory effect on glutathione S-transferase (GST) and so deserves our attention. In this work we investigated the preferred molecular structure of 9 in chloroform solution using the density functional theory (DFT) and molecular dynamics simulation. Comparison between experimental ¹H NMR data in CDCl₃ solution and calculated chemical shifts enabled us to precisely determine the conformation adopted by 9 in solution, which can be used in further theoretical studies involving interaction with biological targets. Moreover, the experimental NMR data were used as reference to assess the ability of DFT based methods to predict ¹H NMR spectrum in solution for organic compounds. Among various DFT functionals the hybrid B3LYP was the most adequate for the calculation of chemical shifts in what CH protons are concerned. Regarding the OH hydrogen, inclusion of explicit CHCl₃ solvent molecules adequately placed around the solute led to good agreement with the experimental chemical shifts (in CDCl₃). It is a well-known fact that theoretical prediction of chemical shifts for OH hydrogens poses as a challenge and also revealed that the way the solvent effects are included in the DFT calculations is crucial for the right prediction of the whole ¹H NMR spectrum. It was found in this work that a supermolecule solute-solvent calculation with a minimum of four CHCl₃ molecules is enough to correctly reproduce the ¹H NMR experimental profile observed in solution, revealing that the calculated solvated structure used to reproduce the NMR chemical shifts is not unique.