Although Corona et al. found improved selectivity for PTR1 over hDHFR by hydrophilic N10 substitutions, our data for reference compound 1d with a hydroxyethyl substituent did not support this observation (Figure ). Docking simulations indicated that interactions with a highly conserved structural water might induce an unfavorable conformation of the substituent's aliphatic chain (Figure , SI). To relax the geometry while allowing interactions between the substituent and water, we elongated the aliphatic linkage to a hydroxypropyl in 2e.