To provide insight into the interaction energy between the mimosine rings, we performed Density Functional Theory (DFT) calculations (see Methods). First, we analyze the interaction energies for the sidechain models of Phe, Tyr, and Mms (See Figure ). We started the optimization from two different ring orientations, in which the rings lie parallel or antiparallel to each other; namely, the methyl substituents, representing the beta carbon, lie at the same or opposite sides. Departing from the parallel orientation, we found significant differences among the three dimers. In the case of Phe-Phe dimer, we found both parallel and antiparallel structures with small differences in DE int -5.5 kcal/mol versus -5.1 kcal/mol, respectively. However, in the case of Tyr, the stacking between the rings in the parallel conformation is disturbed to form a hydrogen bond, with a DE int of -6.3 kcal/mol, whereas the antiparallel conformation maintains the ring stacking although the interaction is slightly weaker -5.7 kcal/mol. In the case of mimosine, the optimization that started from the parallel conformation led to the relative rotation of the rings yielding the antiparallel structure, with a significant interaction energy of -9.1 kcal/mol. Therefore, the stability order of the dimers is Mms-Mms > Tyr-Tyr > Phe-Phe. These differences in energy are even greater when gas-phase calculations are considered with DE int Mms-Mms (-17.0 kcal/mo) > Tyr-Tyr (-8.4 kcal/mol) > Phe-Phe (-6.2 kcal/mol).