Vanadyl binding to phospholipid membranes

Abstract

Binding of the paramagnetic VO 2+ ion to charged and to uncharged phospholipid membranes is studied experimentally by ESR spectroscopy and theoretically by model calculations based upon the Gouy-Chapman-Stern model. The comparison between experimental and theoretical data reveals a peculiar binding mechanism. Although the divalent ion binds to the—at pH ∼ 3 monovalent—head group of dipalmitoyl phosphatidic acid (DPPA) in a 1:1 stoichiometry the measurements are best described in a model assuming no charge reversal at the surface. This indicates that subsequently to vanadyl-ion binding a second proton is dissociated, and explains why binding to the phospholipid dipalmitoyl phosphatidyl glycerol (DPPG) obeys a 2:1 (lipid/ion) stoichiometry. The intrinsic proton binding constant was found to be 40 M −1 for both lipids, whereas the VO 2+ binding constant amounts to 10 3 and 10 4 M −1 for DPPA and DPPG, respectively. Being two to three orders of magnitude larger than those obtained for most other divalent ions forming outer-sphere complexes the value demonstrates that VO 2+ forms strong inner-sphere complexes with the phospholipid head groups. Thus, binding also changes the membrane structure which may be partly responsible for the specific effect of VO 2+ in biological systems. A comparative study of Mn 2+ binding shows the expected behavior of this better known ion.