DIHEMATOPORPHYRIN ETHER-PHOSPHOLIPID INTERACTIONS. THE ROLES OF SURFACE CHARGE and LATERAL PHASE SEPARATIONS

Abstract Fluorescence spectroscopy was utilized to investigate the equilibrium interaction of dihem-atoporphyrin ether(s) (DHE) with binary and ternary phospholipid mixtures of defined composition in order to define the roles of net negative surface charge and lateral phase separations in DHE-membrane partitioning. Binary phospholipid mixtures employed were composed of dimyristoyl-phosphatidylcholine (DMPC) mixed with increasing weight percentages of dimyristoylphosphatidylgly-cerol (DMPG) providing controlled variation of net membrane surface charge. Two types of ternary phospholipid mixtures were utilized. Ternary acid mixtures contained various percentages of palmitoyl-lysophosphatidyl choline (LPC) + palmitic acid (PA) dispersed in DMPC. Ternary alcohol mixtures contained various percentages of LPC + hexadecanol (OL) dispersed in DMPC. The ternary phospholipid mixtures are known to be phase separated. At total DHE concentrations of 0.33 μA/ and using 100% DMPC, the DHE partition coefficient (P) is 250 000. This partition coefficient is to some extent dependent on the DHE concentration. The observed partition coefficients show little dependence on surface charge in DMPC-DMPG mixtures. However, P decreases markedly with increasing phase separation in the ternary lipid mixtures. The fluorescence of membrane-bound DHE is dependent on the composition of the ternary mixtures in a manner suggesting micropartitioning of DHE into the phospholipid bulk phase as well as into the disordered regions between laterally phase separated phospholipid domains.