Influence of Trivalent Metal Ions on Lipid Vesicles: Gelation and Fusion Phenomena

In this contribution, we report the interaction of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid vesicles with a series of trivalent metal ions of the same group, namely, Al3+, Ga3+, and In3+, to get a distinct view of the effect of size, effective charge, and hydration free energy of these metal ions on lipid vesicles. We employed steady-state and time-resolved spectroscopic techniques including time-resolved anisotropy measurement, confocal imaging, and dynamic light scattering (DLS) measurement to probe the interaction. Our study reveals that all of the three trivalent metal ions induce gelation in lipid vesicles by removing water molecules from the interfacial region. The extent of gelation induced by the metal ions follows the order of In3+ > Ga3+ ≥ Al3+. We explain this observation in light of different free-energy terms. Notably, the degree of interaction for trivalent metal ions is higher as compared to that for divalent metal ions at physiological pH (pH ∼ 7.0). Most importantly, we observe that unlike divalent metal ions, trivalent metal ions dehydrate the lipid vesicles even at lower pH. The DLS measurement and confocal imaging indicate that In3+ causes significant aggregation or fusion of the PC vesicles, while Al3+ and Ga3+ did not induce any aggregation at the experimental concentration. We employ Derjaguin-Landau-Vervey-Overbeek (DLVO) theory to explain the aggregation phenomena induced by In3+.