A systematic study of the molecular and electronic structure of the eight possible members in the trans-matrine series and of two dehydro- derivatives, sophocarpine and sophoramine, has been performed. According to DFT calculations these alkaloids exhibit a variety of form and junction of the four six-membered rings and all but sophocarpine have a strong preference for one conformation. Sophocarpine is predicted to have a marked conformational flexibility at the lactamic nitrogen and exist as a mixture of two nearly isoenergetic conformers in the gas phase or solution. The theoretical predictions are consistent with the available X-ray experimental results as well as IR and NMR evidence. The absolute configuration of the preferred conformer of each compound has been established theoretically and corroborated with the specific optical rotation calculated at the sodium D line. The conformational equilibrium of sophocarpine has also been supported by this physical property.
