Phosphorylation of S17 in the N-terminal "lid" of MDM2 (residues 1-24) is proposed to regulate the binding of p53. The lid is composed of an intrinsically disordered peptide motif that is not resolved in the crystal structure of the MDM2 N-terminal domain. Molecular dynamics simulations of MDM2 provide novel insights into how the lid undergoes complex dynamics depending on its phosphorylation state that have not been revealed by NMR analyses. The difference in charges between the phosphate and the phosphomimetic 'Asp' and the change in shape from tetrahedral to planar are manifested in differences in strengths and durations of interactions that appear to modulate access of the binding site to ligands and peptides differentially. These findings unveil the complexities that underlie protein-protein interactions and reconcile some differences between the biochemical and NMR data suggesting that lid mutation or deletion can change the specific activity of MDM2 and provide concepts for future approaches to evaluate the effects of S17 modification on p53 binding.