A complete picture of the effect of the I19G mutation on the energy landscape of MDM2 (6-125) was sought to complement the calorimetric and NMR measurements. Free Energy Surfaces (FES) for WT in the absence and presence of AM-7209 were computed using our previously described aMD/US/vFEP methodology (see Figures and SI Methods paragraph). The calculations suggests that the lid region of unliganded WT predominantly adopts a partially disordered conformation that occludes the ligand-binding site (label C1 Figure ). They further suggest that the disordered lid in unliganded MDM2 can also transiently adopt an alternative position that gives partial access to the ligand-binding site (label C2, Figure ). Full "opening" of the lid is energetically disfavoured (label C3, Figure ). After AM-7209 binding, the dominant lid conformational state appears to shift to an ordered helical and partially extended motif (label C4, Figure ). An ordered helical and fully extended motif (label C5, Figure ), consistent with X-ray crystallography data, is slightly less energetically favoured. Such a fully extended lid conformation can be stabilised in X-ray diffracted structures via crystal packing. Motif formation requires repositioning of the lid away from core helix a2 to avoid steric clashes with the ligand. A closed disordered motif (label C6, Figure ) with the lid positioned above the ligand is also energetically accessible according to these calculations.