The ionization and binding equilibra considered in the data analyses. I, II and III refer to the DB-67 lactone, DB-67 carboxylate monoanion and dianion, respectively. Their corresponding membrane/water partition coefficients are represented by KI, KIII and KIV. The equilibrium constants Ka and Ka2 refer to the ionization of the E-ring lactone and A-ring hydroxyl groups, respectively, and the subscripts and superscripts m and w refer to the intravesicular and extravesicular compartments.