The Binding of Cu²⁺ to Lipid Membranes Is Not Substantially Influenced by Electrostatic Screening

Gouy-Chapman theory predicts that salt screening and modulating the interfacial charge density should strongly influence the apparent dissociation constant, K_d,app, between Cu²⁺ and negatively charged phosphatidylserine (PS) lipids in supported lipid bilayers (SLBs). Specifically, K_d,app would be expected to increase (weaken binding) by a factor of 40 when 100 mM NaCl is introduced into the solution because of electrostatic screening between the membrane and Cu²⁺ cations. Surprisingly, however, fluorescence quenching measurements demonstrate that K_d,app increases by less than a factor of 2 when increasing the salt concentration in the presence of standard buffers, such as tris(hydroxymethyl)aminomethane (Tris). Moreover, increasing the negative surface charge density by a factor of 4 would be predicted to decrease (strengthen binding) K_d,app by 3 orders of magnitude. Instead, K_d,app increases slightly when 15 mol % phosphatidylglycerol (PG), a negatively charged lipid, is introduced into SLBs already containing 5 mol % PS. Such findings indicate that electrostatic double layer theory is not a useful approach for predicting the binding behavior of transition metal cations to negatively charged interfaces. The problem lies with the fact that standard buffers, such as Tris, form a wide variety of coordination complexes in bulk solution with transition metal cations like Cu²⁺. Typically, dozens of complexes are present simultaneously at any given pH value and the net charge on them ranges from positive to neutral to negative. Such variations in charge on the complexes result in electrostatic screening and interfacial potential effects that are substantially diminished or nonexistent. These results should generally apply to the binding behavior of first row transition metal ions, when the cations predominantly reside in complexes rather than as free ions. This includes in vivo conditions, where the concentration of free transition metal ions is often very low.