Predicting the structural preferences of luteolin-7-O-β-d-glucoside in the gas phase: An application of the hybrid MCMM/QM approach

The hybrid Monte-Carlo multiple minimum/quantum mechanical approach, which includes fully random conformational searches and high-level quantum mechanical calculations, is employed to predict the low-lying conformers of luteolin-7-O-β- d-glucoside (L7G) in the gas phase. The conformational searches allow a large number of reasonable initial geometries to be generated. In order to save the optimization time, the step-by-step classification is applied to reduce the number of possible conformations. The structural analysis of L7G suggests that hydrogen bonds play an important role in the structural stability. Meanwhile, forming cooperative intra-ring hydrogen bonds could further lower the relative energies. Compared with L4'G, the glycosidic dihedral angles of L7G distribute extensively. Conformer of L7G prefers a loose geometry because of the lack of the constraint from inter-ring hydrogen bonds. However, the glycosidic dihedral angles of L4'G are limited in a small range due to the inter-ring hydrogen bonds between luteolin and glucose rings. A comparison of L7G and L4'G reveals the influence of the glycosidic linkage on the distribution of glycosidic dihedral angles.