One hypothesis is that, with shorter linkers, the 3,4-dichlorophenyl tail does not reach the TbPTR1 subpocket C to form favorable halogen bonds, and therefore instead occupies subpocket D. Indeed, in docking simulations of V, for the top-ranked pose (by docking score, Fig. ), in which the 3,4-dichlorophenyl tail is positioned in subpocket C, no halogen bonds are formed, and typical hydrogen bonds of the 2-aminobenzothiazole core are not formed. In contrast, the second best pose (Fig. ) has the 3,4-dichlorophenyl tail in subpocket D , and corresponds to the binding mode in the crystallographic complex with TbPTR1 (Fig. ). So far, none of the 2-aminobenzothiazole derivatives crystallized with TbPTR1 have extended into subpocket C, except compound X 22 , which occupies both subpockets C and D with a branched substituent (Fig. , chemical structure in Fig. , SI). Therefore, to evaluate the possibility of targeting subpocket C, we tested 4-atom-long linkers, as present in the amidebased scaffold of 1a (Tab. 2). For 1a, in the top-ranked pose from docking simulations (Fig. ), one halogen bond forms together with the expected hydrogen bonding pattern of 2-aminobenzothiazole. However, the tail of 1a occupies subpocket D in the second best pose (Fig. ), suggesting that such a binding mode is also possible. On one hand, there may be an unfavorable entropic contribution to binding free energy for the compounds containing longer, more rotatable, linkers, if their flexibility is restrained upon binding. On the other hand, such linkers are more adaptable, which may be advantageous for targeting multiple PTR1 variants (and potentially also parasite DHFR). There is, however, the risk of binding to off-targets. Therefore, we also tried to make the initial scaffolds more conformationally restrained by substituting the scaffolds at the amide or amine nitrogen.