Role of electrostatics in the binding of charged neutral and charged phospholipid metallophthalocyanines to membranes

Binding of the cationic tetra(tributylammoniomethyl)-substituted hydroxoaluminum phthalocyanine (A1PcN(4)) to bilayer lipid membranes was studied by fluorescence correlation spectroscopy (FCS) and intramembrane field compensation (IFC) methods. With neutral phosphatidylcholine membranes, A1PcN(4) appeared to bind more effectively than the negatively charged tetrasulfonated aluminum phthalocyanine (A1PcS(4)), which was attributed to the enhancement of the coordination interaction of aluminum with the phosphate moiety of phosphatidylcholine by the electric field created by positively charged groups of A1PcN(4). The inhibitory effect of fluoride ions on the membrane binding of both A1PcN(4) and A1PcS(4) supported the essential role of aluminum-phosphate coordination in the interaction of these phthalocyanines with phospholipids. The presence of negative or positive charges on the surface of lipid membranes modulated the binding of A1PcN(4) and A1PcS(4) in accord with the character (attraction or repulsion) of the electrostatic interaction, thus showing the significant contribution of the latter to the phthalocyanine adsorption on lipid bilayers. The data on the photodynamic activity of A1PcN(4) and AlPCS4 as measured by sensitized photoinactivation of gramicidin channels in bilayer lipid membranes correlated well with the binding data obtained by FCS and IFC techniques. The reduced photodynamic activity of A1PcN(4) with neutral membranes violating this correlation was attributed to the concentration quenching of singlet excited states as proved by the data on the A1PcN(4) fluorescence quenching. (c) 2007 Elsevier B.V All rights reserved.