In this study, we have produced force field parameters for the non-protein amino acid mimosine, with specific emphasis in the generation of new sets of charges for both Amber force fields ff99SB and ff03 using validated methods. We have first run metadynamics simulations on terminally blocked amino acids to estimate the Ramachandran free energy surfaces using the new parameters, and compared them with similar calculations for Phe and Tyr. When performed on the Mms dipeptide these simulations revealed a similar free energy landscape to that of the Phe and Tyr for both ff99SB and ff03. While all three dipeptides had the b and a as their most stable conformations, Mms showed remarkably lower barriers, which may lead to faster conformational transitions. Although Mms showed a relatively more stable a L conformation it was not stable enough to play a role in the behavior of the peptide unless an external effect significantly stabilizes it.