Magnetic Resonance in Chemistry

Günther, H.

doi: 10.1002/mrc.1361pmid: 14745785

Ulrich, Anne S.; Ramamoorthy, Ayyalusamy

doi: 10.1002/mrc.1362pmid: N/A

Watts, Anthony

doi: 10.1002/mrc.1363pmid: N/A

Semchyschyn, Darlene J.; Macdonald, Peter M.

doi: 10.1002/mrc.1333pmid: 14745788

The effects of bilayer surface charge on the conformation of the phosphocholine group of phosphatidylcholine were investigated using a torsion angle analysis of quadrupolar and dipolar splittings in, respectively, 2H and 13C NMR spectra of 1‐palmitoyl‐2‐oleoyl‐sn‐glycero‐3‐phosphocholine (POPC) labelled in the phosphocholine group with either deuterons (POPC‐α‐d2, POPC‐β‐d2 and POPC‐γ‐d9) or carbon‐13 (POPC‐α‐13C and POPC‐αβ‐13C2) and incorporated into magnetically aligned bicelles containing various amounts of either the cationic amphiphile 1,2‐dimyristoyl‐3‐trimethylammoniumpropane (DMTAP) or the anionic amphiphile 1,2‐dimyristoyl‐sn‐glycero‐3‐phosphoglycerol (DMPG). Three sets of quadrupolar splittings, one from each of the three deuteron labelling positions, and three sets of dipolar splittings (13Cα–31P, 13Cα–13Cβ, 13Cβ–14N), were measured at each surface charge, along with the 31P residual chemical shift anisotropy. The torsion angle analysis assumed fast anisotropic rotation of POPC about its long molecular axis, thus projecting all NMR interactions onto that director axis of motion. Dipolar, quadrupolar and chemical shift anisotropies were calculated as a function of the phosphocholine internal torsion angles by first transforming into a common reference frame affixed to the phosphocholine group prior to motional averaging about the director axis. A comparison of experiment and calculation provided the two order parameters specifying the director orientation relative to the molecule, plus the torsion angles α3, α4 and α5. Surface charge was found to have little effect on the torsion angle α5 (rotations about Cα–Cβ), but to have large and inverse effects on torsion angles α3 [rotations about P–O(11)] and α4 [rotations about O(11)–Cα], yielding a net upwards tilt of the P–N vector in the presence of cationic surface charge, and a downwards tilt in the presence of anionic surface charge, relative to neutrality. Copyright © 2004 John Wiley & Sons, Ltd.

Santos, Jose S.; Lee, Dong‐Kuk; Ramamoorthy, Ayyalusamy

doi: 10.1002/mrc.1327pmid: 14745789

The effect of tricyclic antidepressants (TCA) on phospholipid bilayer structure and dynamics was studied to provide insight into the mechanism of TCA‐induced intracellular accumulation of lipids (known as lipidosis). Specifically we asked if the lipid–TCA interaction was TCA or lipid specific and if such physical interactions could contribute to lipidosis. These interactions were probed in multilamellar vesicles and mechanically oriented bilayers of mixed phosphatidylcholine–phosphatidylglycerol (PC–PG) phospholipids using 31P and 14N solid‐state NMR techniques. Changes in bilayer architecture in the presence of TCAs were observed to be dependent on the TCA's effective charge and steric constraints. The results further show that desipramine and imipramine evoke distinguishable changes on the membrane surface, particularly on the headgroup order, conformation and dynamics of phospholipids. Desipramine increases the disorder of the choline site at the phosphatidylcholine headgroup while leaving the conformation and dynamics of the phosphate region largely unchanged. Incorporation of imipramine changes both lipid headgroup conformation and dynamics. Our results suggest that a correlation between TCA‐induced changes in bilayer architecture and the ability of these compounds to induce lipidosis is, however, not straightforward as imipramine was shown to induce more dramatic changes in bilayer conformation and dynamics than desipramine. The use of 14N as a probe was instrumental in arriving at the presented conclusions. Copyright © 2004 John Wiley & Sons, Ltd.

Gaede, Holly C.; Gawrisch, Klaus

doi: 10.1002/mrc.1329pmid: 14745790

The benefits of gradient techniques in the study of lipid membranes are demonstrated on a sample of 1‐palmitoyl‐2‐oleoyl‐sn‐glycero‐3 phosphocholine (POPC) liposomes embedded with ibuprofen. Most techniques from gradient NMR spectroscopy on solution samples are directly applicable to membrane samples subjected to magic angle spinning (MAS). Gradient‐enhanced homo‐ and heteronuclear chemical shift correlation techniques were used to make resonance assignments. Gradient NOESY experiments provide insight into the location and dynamics of lipids, ibuprofen and water. Application of gradients not only reduces experiment time but also the t1 noise in the multi‐dimensional spectra. Diffusion measurements with pulsed field gradients characterize lateral movements of lipid and drug molecules in membranes. The theoretical framework for data analysis of MAS diffusion experiments on randomly oriented multilamellar liposomes is presented. Published in 2004 by John Wiley & Sons, Ltd.

Orädd, Greger; Lindblom, Göran

doi: 10.1002/mrc.1338pmid: 14745791

This mini‐review focuses on the utilization of pulsed magnetic field gradients to measure diffusional motion in systems of macroscopically oriented lipid bilayers. The NMR diffusion technique is proposed to have appreciable potential for future biophysical investigations in the field of membrane biology. Topics such as transport of molecules both across and in the plane of the membrane can be successfully studied, and the formation of lipid domains and their intrinsic dynamics can also be scrutinized. First, a short introduction to the NMR technique is given together with a brief discussion on methods of obtaining a good bilayer orientation. Then, a number of recent results on biophysical/biological membrane systems of great interest is presented, in which some unique conclusions on so‐called ‘raft membranes’ are reached. It is shown for systems with large two‐phase areas of liquid disordered and liquid ordered phases that lipid lateral diffusion is faster in the former phase and has a smaller apparent activation energy. Further, on the time‐scale of the experiments (50–250 ms), exchange between the two phases is fast in the phospholipid–cholesterol–water ternary system, whereas it is slow in the sphingomyelin–dioleoylphosphatidylcholine–cholesterol–water quaternary system. Copyright © 2004 John Wiley & Sons, Ltd.

Tiburu, Elvis K.; Dave, Paresh C.; Lorigan, Gary A.

doi: 10.1002/mrc.1324pmid: 14745792

We report the utilization of magnetically aligned phospholipid bilayers (bicelles) to study the effects of cholesterol in phospholipid bilayers for both chain perdeuterated DMPC and partially deuterated α‐[2,2,3,4,4,6‐d6]‐cholesterol using 2H solid‐state NMR spectroscopy. The quadrupolar splittings at 40°C were 25.5 and 37.7 kHz, respectively, for the 2,4‐2Heq and 2,4‐2Hax deuterons when the bilayer normal of the discs was aligned perpendicular to the static magnetic field. The quadrupolar splittings were doubled when Yb3+ ions were added to flip the bicelles 90° such that the bilayer normal was colinear with the magnetic field. The results suggest that cholesterol is incorporated into the bicelle discs. For chain perdeuterated DMPC‐d54, incorporated into DMPC–DHPC bicelle discs, the individual quadrupolar splittings of the methylene and methyl groups doubled on going from the perpendicular to the parallel alignment. Also, the presence of cholesterol increased the overall ordering of the acyl chains of the phospholipids. SCDi calculations were extracted directly from the 2H quadrupolar splittings of the chain perdeuterated DMPC. The order parameter, SCDi, calculations clearly indicated an overall degree of ordering of the acyl chains in the presence of cholesterol. We also noted a disordering effect at higher temperatures. This study demonstrates the ease with which 2H order parameters can be calculated utilizing magnetically aligned phospholipid bilayers when compared with randomly dispersed membrane samples. Copyright © 2004 John Wiley & Sons, Ltd.

Epand, Richard M.; Epand, Raquel F.; Bain, Alex D.; Sayer, Brian G.; Hughes, Donald W.

doi: 10.1002/mrc.1335pmid: 14745793

Mixtures of cholesterol with phosphatidylcholine species containing the polyunsaturated acyl chains arachidonoyl or docosahexaenoyl were studied by 13C magic angle spinning (MAS) NMR using both cross‐polarization and direct polarization, by 31P NMR and by differential scanning calorimetry. Several unique features of these systems were observed. The separation of cholesterol in crystalline form occurred at much lower molar fractions than with other forms of phosphatidylcholine. The crystals that were formed were sensitive to the history of the sample. At cholesterol molar fractions below 0.5, they dissolved into the membrane by sequential heating and cooling scans. With higher molar fractions of cholesterol, larger amounts of anhydrous crystals were formed after the first heating. This was accompanied by the formation of non‐lamellar phases. The cholesterol crystals that were formed generally were not observed by direct polarization 13C MAS NMR, even with delay times of 100 s. This suggests that the cholesterol crystals are in a more rigid state in mixtures with these lipids. This is in contrast with the terminal methyl group of the acyl chains that is too mobile to allow cross‐polarization using 1 ms contact times. Copyright © 2004 John Wiley & Sons, Ltd.

de Planque, Maurits R. R.; Rijkers, Dirk T. S.; Liskamp, Rob M. J.; Separovic, Frances

doi: 10.1002/mrc.1326pmid: 14745794

The transmembrane domain of the nicotinic acetylcholine receptor (nAChR) plays a role in the regulation of the activity of this important ligand‐gated ion channel. The lipid composition of the host membrane affects conformational equilibria of the nAChR and several classes of inhibitors, most notably anaesthetics, interact directly or indirectly with the four transmembrane M‐segments, M1–M4, of the nAChR subunits. It has proven difficult to gain insight into structure–function relationships of the M‐segments in the context of the entire receptor and the biomembrane environment. However, model membrane systems are well suited to obtain detailed information about protein–lipid interactions. In this solid‐state NMR study, we characterized interactions between a synthetic αM1 segment of the T. californica nAChR and model membranes of different phosphatidylcholine (PC) lipids. The results indicate that αM1 interacts strongly with PC bilayers: the peptide orders the lipid acyl chains and induces the formation of small vesicles, possibly through modification of the lateral pressure profile in the bilayer. The multilamellar vesicle morphology was stabilized by the presence of cholesterol, implying that either the rigidity or the bilayer thickness is a relevant parameter for αM1–membrane interactions, which also has been suggested for the entire nAChR. Our results suggest that the model systems are to a certain extent sensitive to peptide–bilayer hydrophobic matching requirements, but that the lipid response to hydrophobic mismatch alone is not the explanation. The effect of αM1 on different PC bilayers may indicate that the peptide is conformationally flexible, which in turn would support a membrane‐mediated modulation of the conformation of transmembrane segments of the nAChR. Copyright © 2004 John Wiley & Sons, Ltd.