Validation of performances of some semiempirical Hamiltonians for predicting molecular structure calculation of natural brassinosteroids: Towards understanding their biological activity by electron exchange effects

Calculations on the molecular structure of three natural brassinosteroids using several semiempirical Hamiltonians: MNDO, AM1, PM3, PDDG/MNDO, PDDG/PM3 have been done to test their accuracy for predicting structural features of these molecules when compared with available crystallographic data. The best molecular structures were obtained by AM1 and PDDG/PM3 semiempirical Hamiltonians. The worst performances were observed for MNDO and PDDG/MNDO. Molecular orbitals (i.e. highest occupied molecular orbital (HOMO) and lower unoccupied molecular orbital (LUMO)), electrostatic potential maps, and atomic charges with all semiempirical methods were also calculated. Results showed that the HOMO was always located in the lateral chain and the LUMO in the lactone group of ring B for the three systems analyzed. Lactone group elicited higher electrostatic potentials than hydroxyl groups. Analysis of charges also revealed an accumulation of charges in the lactone group and in the lateral chain. These modelling results explain how the biological activity process of these molecules occurs when there is electron exchange. The results also indicate that the interaction of brassinosteroids with receptors could involve specific areas of the molecule.