For the phosphorylated serine dipeptides (in our nomenclature: S1P and SEP), the experimental data suggest that there is a substantial increase in hydrogen bond formation to the proton of N self upon deprotonation of the phosphate group (i.e., when S1P becomes SEP), while there is no change in the hydrogen bond formation propensity for the proton of N cap . MD simulations performed with ff99SB yield nearly zero phosphate-to-backbone hydrogen bonding in either S1P or SEP (Table ). Conversely, simulations performed with FB18 display hydrogen bonding to both amide protons for both S1P and SEP, with increases in hydrogen bonding propensities for both upon deprotonation of the phosphate group. Here it seems that neither force field offers an entirely satisfactory reproduction of the qualitative trends present in the experimental data, as ff99SB does not form these phosphate group-tobackbone hydrogen bonds enough and FB18 perhaps forms them too readily (at least to the proton of N cap ).