Force field parameters were assigned to the protein, ligands, and buffer molecules with the LEaP program. The ff12SB force field was used for the protein, GAFF forcefield parameters were used for HEPES, and forcefield parameters for the phosphotyrosine (PTY) residue were taken from the set determined by Steinbrecher et al , as available in the AMBER frcmod.phosaa10 file. The cyclized phosphotyrosine (CPY) parameters were the same as PTY with the exception of the cyclopropyl moiety, which used the GAFF parameters for sp 3 carbons in triangle systems (cx ). Partial charges for both the HEPES molecules and the modified phosphotyrosine residues were determined using the RESP method, as available through the R.E.D. Server using Gaussian09 C.01 . The full set of parameters used is available in the Supporting Information.