Figure 1.

(a) ¹H NMR titration with compound 5 in DMSO-d₆/0.5% water with tetrabutylammonium dihydrogen phosphate; (i) free receptor; (ii) one equivalent of tetrabutylammonium dihydrogen phosphate; (iii) 2 equivalents of tetrabutylammonium dihydrogen phosphate; (iv) 3 equivalents of tetrabutylammonium dihydrogen phosphate. (b) ¹H NMR titration with compound 5 in DMSO-d₆/0.5% water with tetrabutylammonium dihydrogen phosphate and TBA hydroxide; (i) free receptor; (ii) one equivalent of tetrabutylammonium dihydrogen phosphate; (iii) one equivalent of tetrabutylammonium dihydrogen phosphate + one equivalent of tetrabutylammonium OH; (iv) one equivalent of tetrabutylammonium dihydrogen phosphate +1.5 equiv of tetrabutylammonium OH.