In this case, the addition of a phosphate group appears to be the sole factor behind the change in conformational preferences, rather than the charge state of the phosphate group. In contrast, we find that FB18 yields χ 1 distributions that are generally quite distinct between the unmodified residues and their phosphorylated counterparts. Moreover, there are significant differences in side chain conformational preferences between the protonated (S1P, T1P, and Y1P) and deprotonated (SEP, TPO, and PTR) phosphorylated residues. In this sense, FB18 yields χ 1 distributions that are immediately distinguishable from those of ff99SB. We discuss The χ 2 distributions generated by each force field, shown in Figure , also display striking differences, particularly for phosphorylated serine and threonine. In particular, ff99SB predicts nearly all trans conformations for phosphorylated serine (both S1P and SEP), whereas FB18 predicts sampling of all three states -with a preference for the gauche states -in S1P and a strong preference for gauche -state in SEP. Likewise, ff99SB predicts a fairly broad distribution of χ 2 angles in T1P and TPO (between 60 • and 180 • ), while FB18 predicts more strongly peaked distributions (albeit within the same general range). It is likely that the χ 2 distributions for phosphorylated threonine diverge from the usual gauche ± -trans paradigm due to the due to the fact that this dihedral angle is defined by rotation about the central Cβ-Oγ bond between the Cα and P atoms and both the steric effects and intrapeptide hydrogen bonding of the phosphate group. Both force fields are consistent, however, in predicting the ±90 • preference of tyrosine residues, as expected for residues where the Cγ atom is sp 2 -hybridized. 29 reporter of the χ 1 distribution of a side chain is the 3 J Hα,Hβ scalar coupling. To that end, we calculated predicted 3 J Hα,Hβ coupling values for all of the residues we examined in this study to see what, if any, differences we might find. We observe that ff99SB and FB18 differ in their predictions by anywhere between 0.6 and 2.7 Hz for these residues. These differences are approximately an order of magnitude greater than the typical experimental uncertainties of ∼0.05 Hz. Therefore we think that if NMR scalar coupling data could be obtained for the side chains of these dipeptides, it would almost certainly enable force field developers to discriminate between "less accurate" and "more accurate" when it comes to reproducing the intrinsic conformational preferences of phosphorylated residues.