In order to understand the differences between the mimosine residue and the most similar protein amino acids, we used the new set of parameters in molecular simulations. Specifically, we performed metadynamics simulations on the terminally blocked peptides (often termed dipeptides) of Mms, Phe and Tyr to estimate their free energy surfaces as a function of the backbone dihedral angles, F and Y (see Figure ). For the three dipeptides and for both the ff99SB and ff03 force fields we find very similar free energy surfaces suggesting only subtle differences in the backbone conformational preferences. In all cases, the PPII/b conformation is the most stable one, except for the tyrosine dipeptide using ff03, where the a R minimum is 0.7 kJ/mol lower in free energy (see Table ). As expected, the a L conformation is significantly less stable than the b and a R ones, with notably lower energetic differences in the case of mimosine. Additionally, for both force fields we find that the free energy barrier between the PPII/b and a L wells is notably lower than for both Phe and Tyr (see Table ), which should result in more frequent transitions to sample this free energy basin for this non-protein amino acid .