Tetraphosphonates are biomimetic hosts for bisamidinium cations in drugs such as pentamidine and DAPI. Similar to their insertion into DNA's minor groove, these drugs are often sandwiched by two tetraphosphonate hosts (2:1). The alternative binding mode (1:2) produces extremely high assocn. consts. in water of .apprx.108 M-2 (.apprx.12 kcal/mol), which can compete with the biol. process.