A 31P-NMR spin-lattice relaxation and 31P[1H] nuclear Overhauser effect study of sonicated small unilamellar phosphatidylcholine vesicles

Design and evaluation of a 1H/31P double-resonant helmet coil for 3T MRI of the brain

Proton magnetic resonance imaging (MRI) can be combined with signals from non-proton nuclei (X-nuclei) to provide metabolic information. Double-resonant coils are often used for X-nuclei MR studies where the proton element is employed for scout imaging and B ₀ shimming. This work describes the development of a new double-resonant coil capable of operating at both proton and X-nuclei frequencies. The proposed design extends the wheel-and-spoke coil, which allows for quadrature drive, by adding an extra ring outside the coil to achieve double-resonance. Furthermore, in order to maximise SNR by increasing the filling factor, the shape of the coil has been modified to a helmet style making it suitable for brain applications. The performance of the double-resonant helmet coil was evaluated by simulation and MR measurements. The helmet coil was successfully tuned to the ¹H/³¹P resonance frequencies of a 3T MR scanner, with high isolation between the two quadrature ports. MR measurements of a phantom were carried out, and the averaged sensitivity of the double-resonant helmet coil over the whole phantom was found to be higher than that of the conventional double-tuned birdcage coil at both frequencies.

Unique backbone-water interaction detected in sphingomyelin bilayers with 1H/31P and 1H/13C HETCOR MAS NMR spectroscopy

Two-dimensional (1)H/(31)P dipolar heteronuclear correlation (HETCOR) magic-angle spinning nuclear magnetic resonance (NMR) is used to investigate the correlation of the lipid headgroup with various intra- and intermolecular proton environments. Cross-polarization NMR techniques involving (31)P have not been previously pursued to a great extent in lipid bilayers due to the long (1)H-(31)P distances and high degree of headgroup mobility that averages the dipolar coupling in the liquid crystalline phase. The results presented herein show that this approach is very promising and yields information not readily available with other experimental methods. Of particular interest is the detection of a unique lipid backbone-water intermolecular interaction in egg sphingomyelin (SM) that is not observed in lipids with glycerol backbones like phosphatidylcholines. This backbone-water interaction in SM is probed when a mixing period allowing magnetization exchange between different (1)H environments via the nuclear Overhauser effect (NOE) is included in the NMR pulse sequence. The molecular information provided by these (1)H/(31)P dipolar HETCOR experiments with NOE mixing differ from those previously obtained by conventional NOE spectroscopy and heteronuclear NOE spectroscopy NMR experiments. In addition, two-dimensional (1)H/(13)C INEPT HETCOR experiments with NOE mixing support the (1)H/(31)P dipolar HETCOR results and confirm the presence of a H(2)O environment that has nonvanishing dipolar interactions with the SM backbone.

An improved NMR study of liposomes using 1-palmitoyl-2-oleoyl-sn-glycero-3-phospatidylcholine as model

In this paper we report a comparative characterization of Small Unilamellar Vesicles (SUVs), Large Unilamellar Vesicles (LUVs) and Multilamellar Vesicles (MLVs) prepared from 1-palmitoyl-2-oleoyl-sn-glycero-3-phospatidylcholine (POPC), carried out using two NMR techniques, namely High Resolution NMR in solution and High Resolution-Magic Angle Spinning (HR-MAS). The size and size distributions of these vesicles were investigated using the dynamic light scattering technique. An improved assignment of the (1)H-NMR spectrum of MLVs is also reported.

Multilamellar liposomes formed by phosphatidyl nucleosides: an NMR-HR-MAS characterization

We present an NMR investigation of multilamellar vesicles (MLVs) obtained from phosphatidyl nucleosides, 5'-(1-palmitoyl-2-oleoyl-sn-glycero(3)phospho)cytidine (1), 5'-(1-palmitoyl-2-oleoyl-sn-glycero(3)phospho)inosine (2), and their mixtures. Because of the lower stability of liposomes obtained from 2, studies have been preferentially performed in this case with mixed liposomes 2/POPC (4:1). The investigation is conducted mostly via the HR-MAS technique and the general observation is that the resolution achieved in this way is superior to that obtained in the past with small unilamellar vesicles (SUVs). A full assignment is now possible, which includes the spectral region of the ribose ring and part of the glycerol moiety. Also in the case of MLVs, both for 1 and 2, a stacking between the aromatic bases of the same liposome layer seems to be ruled out, although in both cases the nucleobases appear to be exposed to the aqueous phase. The splitting of both aromatic H-5cyt and H-6cyt is ascribed to the presence of two aggregate populations that may correspond to the two syn and anti conformations observed for cytidine monophosphate in aqueous solution. On the basis of NOESY cross-peaks, it is not always possible to discriminate between inter- and intramolecular interactions; however, the distances found for 1 appear to be compatible with the intramolecular contacts in the anti conformation of the cytidine and also with intermolecular interactions between neighboring molecules of 1. We also find that the glycerol moiety does not seem to interact with the cytidine; however, part of the ribose ring seems to be close to the glycerol moiety. More generally, the interaction of one base with the sugar moiety of a neighboring base, previously observed for SUVs, still appears to be true for MLVs. Studies have been performed also for mixed liposomes obtained from the mixture of 1 and 2, where it is observed that the HR-MAS spectra of the corresponding MLVs are not simply the sum of the spectra of the two isolated components. In particular, there is the presence of a NOESY cross-peak between the aromatic protons H-6cyt and H-2ino, and this permits us to rule out large patchwork domains containing only one nucleoside components in the mixed liposomes. Finally, a study is performed on the time evolution of the system obtained by mixing the previously prepared liposomes of 1 and 2. No interaction is obtained in this case, i.e., the spectra are constitutive, which is consistent with the general picture of liposomes as kinetic traps that are not fusing with each other.

Parameters influencing the determination of liposome lamellarity by 31P-NMR

The lamellarity of liposomes influences to a great extent the encapsulation efficiency, the efflux rate of liposomally encapsulated material, and the fate of a drug after cellular uptake. 31P-NMR in combination with the use of chemical shift reagents has been described for the determination of lamellarity of liposomes and this study was performed to evaluate the applicability of 31P-NMR analysis as published in the past. To date, very few details about the required conditions throughout the measurements are known. In this study the influence of various parameters on the measurement, such as different buffers with changing ion concentrations, varying pH and different shift reagents at increasing concentrations was investigated. Results were discussed by using cryo-electronmicroscopy as a reference method. The data of this study show that 31P-NMR might not result in the correct determination of liposome lamellarity, depending on the experimental settings and the shape of the liposomes.

Interactions of basic amphiphilic peptides with dimyristoylphosphatidylcholine small unilamellar vesicles: optical, NMR, and electron microscopy studies and conformational calculations

The interactions of DMPC small unilamellar vesicles with four amphiphilic polypeptides [(LKKL)n, (LRRL)n, (LKKL)4, and (YKKY)n] have been investigated by circular and infrared dichroism, turbidimetry, electron microscopy, and fluorescence, 1H, and 31P nuclear magnetic resonance spectroscopy. The main results obtained are the following: (1) Well-defined complexes are formed by the association of one amino acid residue with approximately two lipid molecules. (2) In the presence of polypeptides fusions are observed between SUVs when the molar ratio p is less than 0.05, and a clearance effect is observed when p is higher than 0.05. (3) The anchoring sites of the polypeptides on DMPC molecules are the negative phosphate groups through electrostatic interactions with the terminal NH3+ of lysine residues. (4) The polypeptides adopt an alpha-helical conformation with their axis parallel to the membrane surface. The hydrophobic part of the amphiphilic alpha helix can penetrate the outer lipid leaflet down to the C5 position. (5) Choline methyl groups are not involved in the interactions between lipid molecules and amino acid residues. (6) Phosphorus atom mobility around the P-O-glycerol bond is strongly reduced whereas that of methylene groups is progressively weakened when going up from C13 to C1. Finally, using modeling and energy calculations a model of possible Ac(LKKL)4NHEt-DMPC SUV complexes is presented.

Acyl chain orientational order in large unilamellar vesicles: comparison with multilamellar liposomes: a 2H and 31P nuclear magnetic resonance study

Large unilamellar vesicles (LUVs) composed of 1-[2H31]palmitoyl-2-oleoyl phosphatidylcholine (POPC-d31), with diameters of approximately 117 +/- 31 and 180 +/- 44 nm, were prepared by extrusion through polycarbonate filters with pore sizes of 0.1 and 0.2 microns, respectively. The 2H nuclear magnetic resonance (NMR) spectra obtained at 21 degrees C contain two components: a broad component (approximately 17 kHz linewidth) corresponding to the methylene groups and a narrower component originating from the methyl groups. Spectra with increasing powder pattern characteristics were obtained by reducing the rate of phospholipid reorientations by addition of glycerol (to increase the solvent viscosity) and by lowering the temperature. Full powder spectra, characteristic of liquid-crystalline bilayers, were obtained for both LUV samples at 0 degrees C in the presence of 50 wt% glycerol. Individual quadrupolar splittings were not resolved in these spectra, due to broader linewidths in the LUVs, which have significantly shorter values for spin-spin relaxation time T2 measured from the decay of the quadrupolar echo (90 microseconds) than the multilmellar vesicles (MLVs; 540 microseconds). Smoothed order parameter profiles (OPPs) were obtained for these samples by integration of the dePaked spectra. The OPPs were very similar to the OPP of POPC-d31 MLVs in 50 wt% glycerol at the same temperature, indicating that orientational order in MLVs and LUVs with a diameter of > or = 100 nm is essentially the same. The presence of 80 wt% glycerol was found to have a disordering effect on the vesicles.