Europium III binding and the reorientation of magnetically aligned bicelles: insights from deuterium NMR spectroscopy.
Solid-state deuterium ((2)H) 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 (2)H NMR spectra consisting of a pair of quadrupole doublets, one from the alpha-deuterons and one from the beta-deuterons of DMPC-alpha,beta-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 (2)H 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.
Crowell
K J
KJ
Department of Chemistry, University of Toronto at Mississauga, Mississauga, Ontario, Canada L5L 1C6.
Macdonald
P M
PM
D003903:Deuterium / Q000378:metabolism*; D004134:Dimyristoylphosphatidylcholine / Q000737:chemistry / Q000378:metabolism; D005063:Europium / Q000737:chemistry* / Q000378:metabolism*; D005453:Fluorescence; D007554:Isotopes / Q000737:chemistry / Q000378:metabolism; D007700:Kinetics; D008051:Lipid Bilayers / Q000737:chemistry / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D008280:Magnetics; D008823:Micelles
D007554:Isotopes; D008051:Lipid Bilayers; D008823:Micelles; D005063:Europium; D003903:Deuterium; D004134:Dimyristoylphosphatidylcholine