Aluminum binding to phosphatidylcholine lipid bilayer membranes: aluminum exchange lifetimes from 31P NMR spectroscopy

Two-dimensional (2D) 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR) exchange spectroscopy (EXSY) demonstrated that aluminum binds to the phosphate group of phosphatidylcholine (PC) in multilamellar vesicles at pH 3.2, forming preferentially 2/1, in addition to 1/1 (PC/Al) complexes in slow exchange with one another, and with free PC, on the NMR timescale. Exchange rate constants between these three co-existing species were measured as a function of temperature using one-dimensional (1D) selective inversion recovery (SIR) 31P MAS NMR. Over the temperature range from 5 to 35 °C all three exchange rate constants increased by roughly an order of magnitude from k ≈ 1–2 to 10–14 s−1, exhibiting Arrhenius behavior with activation energies on the order of 30–45 kJ mol−1 and correspondingly positive enthalpies of activation. Entropies of activation were uniformly negative, consistent with an ordered transition state. From a biological perspective, the results demonstrate that aluminum binding to PC in biomembranes is transient on a biologically relevant time scale, so that the lipid bilayer portion of biomembranes is unlikely to act as a long term repository for aluminum, but rather should be viewed as a temporary reservoir of biologically available aluminum.