Most of the linkers were of one of the types shown in Fig. . First, following one of the series of the previously synthesized 2-aminobenzothiazoles , we focused on amide-based linkers, because these offer the possibility to easily 'branch' the compound with additional substituents (see red arrow in Fig. ). We further considered amine-based linkers, which also offer the possibility of 'branching' the compounds (tertiary amines, see red arrow in Fig. ), since molecular interaction fields calculated previously suggested favorable spots for hydrogen bond donors (but not for acceptors) in the center of the TbPTR1 pocket, where the linker amide bond would be likely located.