The parametrization of the mimosine amino acid was performed using the AmberTools14 package. Both the ff99SB and ff03 force fields include atom types that can be used to define the topology of every atom in the mimosine residue. Therefore, the original parameter sets of the force fields contain all necessary bonded and van der Waals parameters. Nonetheless, atomic charges for the residue had to be derived for both force fields. We followed the methods described by the force field developers as closely as possible. A minor change is that we replaced the geometry optimizations, originally done with prior force fields, by quantum mechanical optimizations (see Supporting Information). This small modification was first validated by deriving the charges of selected amino acids for both force fields. Specifically, we used our protocol for phenylalanine and tyrosine, which were chosen due to their structural similarity to mimosine, and asparagine and tryptophan, since both contain an sp 2 nitrogen like that in the mimosine ring.