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

Two-dimensional (2D) P-31 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/A1) 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) P-31 MAS NMR. Over the temperature range from 5 to 35 degrees C all three exchange rate constants increased by roughly an order of magnitude from k approximate to 1-2 to 10-14 s(-1), exhibiting Arrhenius behavior wit 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. (C) 2005 Elsevier Ireland Ltd. All rights reserved.