Europium III binding and the reorientation of magnetically aligned bicelles: Insights from deuterium NMR spectroscopy

Solid-state deuterium (H-2) NMR spectroscopy was used to study the reorient ation of magnetically ordered bicelles in the presence of the paramagnetic lanthanide Eu3+. Bicelles were composed of 1,2-dimyristoyl-sn-glycero-3-pho sphocholine (DMPC) plus 1,2-dihexanoyl-sn-glycero-3-phosphocholine plus eit her the anionic lipid 1,2-dimyristoyl-sn-3-phosphoglycerol, or the cationic lipid 1,2-dimyristoyl-3-trimethyl ammonium propane. Alignment of the bicel les in the magnetic field produced H-2 NMR spectra consisting of a pair of quadrupole doublets, one from the alpha -deuterons and one from the beta -d euterons of DMPC-alpha,beta -d(4). Eu3+ addition induced the appearance of a second set of quadrupole doublets, having approximately twice the quadrup olar splittings of the originals, and growing progressively in intensity wi th increasing Eu3+ at the expense of the intensity of the originals. The ne w resonances were attributed to bicelles having a parallel alignment with r espect to the magnetic field, as opposed to the perpendicular alignment pre ferred in the absence of Eu3+. Therefore, the equilibrium degree and kineti cs of reorientation could be evaluated from the 2H NMR spectra. For more ca tionic initial surface charges, higher amounts of added Eu3+ 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 Eu3+, regardless of the bicelle composition. The kinetics of reorientation were a function of lipid concentration. At high lipid concentration, a sin gle fast rate of reorientation (minutes) described the approach to the equi librium degree of orientation. At lower lipid concentrations, two rates pro cesses 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.