Spectroscopic evidence for hydration and dehydration of lipid bilayers upon interaction with metal ions: a new physical insight.

In this manuscript, we investigate the interactions of different metal ions with zwitterionic phospholipid bilayers of different chain lengths using the well-known membrane probe PRODAN and steady state and time resolved fluorescence spectroscopy. We used three zwitterionic lipids that are widely different in their phase transition temperature, namely, dipalmitoylphosphatidylcholine (DPPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine (POPC) and salts of zinc (Zn), calcium (Ca) and magnesium (Mg). The steady state and time resolved studies reveal that the affinity of the metal ions follows the order Zn2+ > Ca2+ > Mg2+. The study further reveals that the lipid membrane with an unsaturated chain exhibits very small affinity towards metal ions. We find that the Zn2+ and Ca2+ metal ions induce significant gelation in the lipid bilayer possibly by dehydrating the lipid bilayer surface. The study also demonstrates that unlike Zn2+ and Ca2+, dehydration does not take place for Mg2+. The extreme hydration induced by Mg2+ is rationalized by the tight hydration of Mg2+ and very high free energy barrier of Mg2+ to bind with lipid oxygen as compared to that of water molecules.