Lanthanide Ion-Induced Isotropic Shifts and Broadening for Nuclear Magnetic Resonance Structural Analysis of Model Membranes

Abstract

The effects of a series of aquated lanthanide ions on the nuclear magnetic resonance spectrum of phospholipid bilayer suspensions are reported. The ability of these ions to provide a spectral distinction between morphologically exterior resonances and their interior counterparts was confirmed. Measurements of the relative shifting and broadening capabilities of these ions are reported, and their correlation with solid-state magnetic susceptibility anisotropies is shown to implicate a dipolar (pseudocontact) mechanism as the source of this effect. The potential for the use of dipolar shifts as structural probes is discussed and an alternative method using dipolar shifting and broadening reagents simultaneously is presented.The use of nuclear magnetic resonance spectroscopy (NMR) as a tool for the study of artificial phospholipid bilayers (a model membrane system), as well as biological membranes themselves, has been of great current interest. Recently, Bystrov et al. (1) reported the 1H NMR spectrum of a phospholipid bilayer in the presence of aquated Eu+3 ions, in which they observed an upfield shift of a portion of the N-methyl choline resonance, which originally had been a strong singlet in the absence of Eu+3. A dependence of the magnitude of the shift on the concentration of europium was also noted. These workers advanced the explanation that this metal ion, by coordination to the exterior phosphate head groups, was capable of lifting the accidental chemical shift equivalence of the protons on the inside and outside of the closed bilayers. The spectral result was thus two singlets, corresponding to the (-NMe3) i and (-NMe3), groups. Presumably, the outer protons experienced an isotropic shift due to the paramagnetic ion and the inner protons remained unshifted. Other reagents that induce spectral modifications of this kind have been reported and include small chemical shifts induced through the agency of anions (2, 3) and elemental iodine (2), and broadening of resonances by the paramagnetic Mn+2 and Gd+3 ions (1, 3, 4).It is evident that a technique capable of deriving quantitative information on the molecular disposition of membrane components would be of great value, and the reagents described above are potentially useful in this context. This apAbbreviations: (-X-)i and (-X-)0, resonance assignments for those portions of an NMR (nuclear magnetic resonance) signal arising from chemically similar but spatially distinct nuclei residing on the inside and outside of a closed bilayer, respectively; Ln +3, a generic, trivalent lanthanide ion; a, isotropic chemical shift in ppm relative to the unshifted portion of the same resonance; vP/2, full observed resonance width at half-height; V1/2', full corrected width at half-height, i.e., V1/2 (observed) -vl/2 (natural).pears to be particularly true of the Eu+3 ion, by virtue of the larger changes involved. Thus, in connection with the problem of site-labeling of membrane constituents, we sought to investigate the utility of lanthanide ions as structural prob...

31 P Nuclear Magnetic Resonance Spectroscopy of Native and Recombined Lipoproteins

“…It has been repeatedly demonstrated that paramagnetic ions (shift reagents) (37) permit differentiation of the internal and external surfaces of membranes without their impairment (38)(39)(40). In aqueous PC dispersions containing paramagnetic ions, the N + (CH3)3 signal of the PC molecules in the lipid bilayer consists of two components.…”

Solid-state NMR spectroscopy of a membrane protein in biphenyl phospholipid bicelles with the bilayer normal parallel to the magnetic field

“…4A and 4B demonstrate that it is possible to ''backflip" the TBBPC bicelles from the parallel to perpendicular alignments with lanthanides, while illustrating perturbations in the form of broad signals in Fig. 4B that result from the significant effects on chemical shift and line width broadening associated with Dy 3+ ions [23].…”

Subnanometer Structure of an Asymmetric Model Membrane: Interleaflet Coupling Influences Domain Properties

“…The choline distribution was determined using solution 1 H NMR, which detects choline but not choline-d 13 , by quantifying the shift of the outer leaflet choline methyl resonances in the presence of externally added paramagnetic lanthanide ion Pr 3+ (Figures S4−S6). 18 Pr 3+ does not permeate into the LUV core on the time scale of days, and the interaction is therefore specific for outer leaflet headgroups. 19 Because bilayer defects at the boundaries between gel and fluid domains may facilitate passive ion transport, 20 we also verified that phase-separated vesicles were impermeable to Pr 3+ during the time required to measure the NMR spectrum (Supporting Information Figure S8).…”