Theoretical investigation of the structural and electronic properties of luteolin, apigenin and their deprotonated species

The structural properties of luteolin, apigenin and of all their possible mono-deprotonated species are studied with ab initio methods in vacuo and in solution. Molecular structures have been optimized by MP2 and DFT using different functionals and basis sets. Harmonic vibrational frequency calculations have been performed on the optimized structures to ensure geometries obtained were real minima. All calculations resulted in non-planar structures except for deprotonation in 4′ position at the DFT level. An interesting behavior for the torsional potential between phenyl B ring and the 1,4-benzopyrone skeleton has been detected. Therefore, potential energy profiles along this coordinate have been characterized and analyzed in terms of conjugative and steric interactions through the evaluation of the coefficients fitted with an eight-term Fourier expansion. The frontier molecular orbitals and the HOMO-LUMO gap at the different geometries along this torsion curve have been computed and their changes have been thoroughly analyzed. Finally, structural and electronic changes on deprotonation and frontier molecular orbitals for all mono-deprotonated species have been studied.