Kinetic and equilibrium studies of incorporation of di-sulfonated aluminumphthalocyanine into unilamellar vesicles

The interactions of cis-di-sulfonated aluminum phthalocyanine (PcS2Al) with dimyristoylphosphatidylcholine (DMPC) unilamellar vesicles have been inves tigated by fluorescence spectroscopy. At pH 7.0, PcS2Al incorporates into t he vesicles with a high affinity constant (2.7 x 10(6) M-1, in terms of pho spholipid concentration). The fluorescence changes following rapid mixing o f PcS2Al with vesicles are biphasic. The first phase is attributed to the e ntry of PcS2Al into the vesicles, as deduced from the linear dependence of the rate upon lipid concentration. More surprisingly, this rate is strongly pH dependent with a marked maximum around pH 7.3, a result interpreted in terms of the coordination state of the aluminum ion in aqueous solutions. A t this pH, a hydroxide ion neutralizes the residual positive charge of the metal ion that remains unbalanced after coordination by the phthalocyanine cycle. A water molecule is likely to complete the metal coordination sphere . Only this form, PcAl+(OH-)(OH2), with an uncharged core is quickly incorp orated into the vesicles, The protonation of OH- or the deprotonation of th e coordinated H2O leading to a positively or negatively charged core, respe ctively, account for the observed pH effect. Studies on the effect of chole sterol addition and exchange of PcS2Al between vesicles and albumin all ind icate the absence of transfer of the phthalocyanine between the vesicle hem ileaflets, a result expected from the presence of the two negatively charge d sulfonated groups at the ring periphery. Instead, the slower kinetic phas e is likely due to the movement of the phthalocyanine becoming more buried within the outer leaflet upon the loss of the water molecule coordinated to the aluminum ion. (C) 1999 Elsevier Science B.V. All rights reserved.