The next important question was which length of the linker would ensure optimal interactions of both linked fragments with TbPTR1 in subpocket C or D. The 3,4-dichlorophenyl moiety was already used as a substituent in the previously published 2-aminobenzothiazoles series (e.g., compounds II, III, and V in Tab. 1). Overall, the crystallographic data show that the substituents connected by 3-atom or shorter linkers result in positioning of the 3,4-dichlorophenyl moiety in subpocket D of the TbPTR1 binding site, so that the moiety is exposed to solvent (e.g., see the crystallographic complex of V and TbPTR1 in Fig. ).