The effect of solution electrolytes on the uptake of photosensitizers by liposomal membranes: a salting-out effect

In this study we investigated, spectroscopically, the effect of electrolytes on the partitioning of hematoporphyrin IX (HP) and hypericin (Hy) into non-charged lipid vesicles. Our aim was to assess the salting-out effect of electrolytes on membrane-partitioning. We titrated aqueous solutions of HP and Hy with lecithin liposomes, at different concentrations of several monovalent and divalent electrolytes in the suspension. The partitioning constant of HP to lecithin liposomes increased from 3.3 (mL/mg) in water containing only 5 mM buffer to 8.7 (mL/mg) at 0.36 M KCl. KF had a similar effect. NaCl caused a 3-fold increase in the partitioning of Hy to liposomes. MgSO₄ and MgCl₂ also increased the partitioning of HP, by a factor of more than 4 and this occurred already at 0.03 M concentration. We analyze the comparative effects of the electrolytes in relation to the Hofmeister series. The salting-out effect could be utilized to enhance the uptake of HP and Hy, and possibly other photosensitizers as well, by artificial and natural membranes.