Europium III Binding and the Reorientation of Magnetically Aligned Bicelles: Insights from Deuterium NMR Spectroscopy

Abstract

Solid-state deuterium ( 2H) NMR spectroscopy was used to study the reorientation of magnetically ordered bicelles in the presence of the paramagnetic lanthanide Eu 3+. Bicelles were composed of 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) plus 1,2-dihexanoyl- sn-glycero-3-phosphocholine plus either the anionic lipid 1,2-dimyristoyl- sn-3-phosphoglycerol, or the cationic lipid 1,2-dimyristoyl-3-trimethyl ammonium propane. Alignment of the bicelles in the magnetic field produced 2H NMR spectra consisting of a pair of quadrupole doublets, one from the α-deuterons and one from the β-deuterons of DMPC- α, β- d 4. Eu 3+ addition induced the appearance of a second set of quadrupole doublets, having approximately twice the quadrupolar splittings of the originals, and growing progressively in intensity with increasing Eu 3+, at the expense of the intensity of the originals. The new resonances were attributed to bicelles having a parallel alignment with respect to the magnetic field, as opposed to the perpendicular alignment preferred in the absence of Eu 3+. Therefore, the equilibrium degree and kinetics of reorientation could be evaluated from the 2H NMR spectra. For more cationic initial surface charges, higher amounts of added Eu 3+ were required to induce a given degree of reorientation. However, the equilibrium degree of bicellar reorientation was found to depend solely on the amount of bound Eu 3+, regardless of the bicelle composition. The kinetics of reorientation were a function of lipid concentration. At high lipid concentration, a single fast rate of reorientation (minutes) described the approach to the equilibrium degree of orientation. At lower lipid concentrations, two rates processes were discernible: one fast (minutes) and one slow (hours). The data indicate, therefore, that bicelle reorientation is a phase transition made critical by bicelle-bicelle interactions.