Mode of molecular interaction of triterpenoid saponin ginsenoside Rh2 with membrane lipids in liquid-disordered phases.

Ginsenoside Rh2 (Rh2) is a ginseng saponin comprising a triterpene core and one unit of glucose and has attracted much attention due to its diverse biological activities. In the present study, we used small-angle X-ray diffraction, solid-state NMR, fluorescence microscopy, and MD simulations to investigate the molecular interaction of Rh2 with membrane lipids in the liquid-disordered (Ld) phase mainly composed of palmitoyloleoylphosphatidylcholine compared with those in liquid-ordered (Lo) phase mainly composed of sphingomyelin and cholesterol. The electron density profiles determined by X-ray diffraction patterns indicated that Rh2 tends to be present in the shallow interior of the bilayer in the Ld phase, while Rh2 accumulation was significantly smaller in the Lo phase. Order parameters at intermediate depths in the bilayer leaflet obtained from 2H NMR spectra and MD simulations indicated that Rh2 reduces the order of the acyl chains of lipids in the Ld phase. The dihydroxy group and glucose moiety at both ends of the hydrophobic triterpene core of Rh2 cause tilting of the molecular axis relative to the membrane normal, which may enhance membrane permeability by loosening the packing of lipid acyl chains. These features of Rh2 are distinct from steroidal saponins such as digitonin and dioscin, which exert strong membrane-disrupting activity.

Protein-lipid acyl chain interactions: Depth-dependent changes of segmental mobility of phospholipid in contact with bacteriorhodopsin.

Boundary lipids surrounding membrane proteins play an essential role in protein function and structure. These protein-lipid interactions are mainly divided into electrostatic interactions between the polar amino acids of proteins and polar heads of phospholipids, and hydrophobic interactions between protein transmembrane sites and phospholipid acyl chains. Our previous report (Kawatake et al., Biochim. Biophys. Acta 1858 [2016] 2106-2115) covered a method for selectively analyzing boundary lipid interactions and showed differences in membrane protein-peripheral lipid interactions due to differences in their head group. Interactions in the hydrophobic acyl chains of phospholipids are relatively consistent among proteins, but the details of these interactions have not been elucidated. In this study, we reconstituted bacteriorhodopsin as a model protein into phospholipid membranes labeled with 2H and 13C for solid-state NMR measurement to investigate the depth-dependent effect of the head group structure on the lipid bilayer. The results showed that the position of the phospholipid near the carbonyl carbon was affected by the head group in terms of selectivity for protein surfaces, whereas in the deep interior of the bilayer near the leaflet interface, there was little difference between the head groups, indicating that the dependence of their interactions on the head group was much reduced.

Effect of the number of sugar units on the interaction between diosgenyl saponin and membrane lipids.

Saponin is the main bioactive component of the Dioscorea species, which are traditionally used for treating chronic diseases. An understanding of the interaction process of bioactive saponins with biomembranes provides insights into their development as therapeutic agents. The biological effects of saponins have been thought to be associated with membrane cholesterol (Chol). To shed light on the exact mechanisms of their interactions, we investigated the effects of diosgenyl saponins trillin (TRL) and dioscin (DSN) on the dynamic behavior of lipids and membrane properties in palmitoyloleolylphosphatidylcholine (POPC) bilayers using solid-state NMR and fluorescence spectroscopy. The membrane effects of diosgenin, a sapogenin of TRL and DSN, are similar to those of Chol, suggesting that diosgenin plays a major role in membrane binding and POPC chain ordering. The amphiphilicity of TRL and DSN enabled them to interact with POPC bilayers, regardless of Chol. In the presence of Chol, the sugar residues more prominently influenced the membrane-disrupting effects of saponins. The activity of DSN, which bears three sugar units, led to perturbation and further disruption of the membrane in the presence of Chol. However, TRL, which bears one sugar residue, increased the ordering of POPC chains while maintaining the integrity of the bilayer. This effect on the phospholipid bilayers is similar to that of cholesteryl glucoside. The influence of the number of sugars in saponin is discussed in more detail.

Combined effect of the head groups and alkyl chains of archaea lipids when interacting with bacteriorhodopsin.

Bacteriorhodopsin (bR), a transmembrane protein with seven α-helices, is highly expressed in the purple membrane (PM) of archaea such as Halobacterium salinarum. It is well known that bR forms two-dimensional crystals with acidic lipids such as phosphatidylglycerol phosphate methyl ester (PGP-Me)-a major component of PM lipids bearing unique chemical structures-methyl-branched alkyl chains, ether linkages, and divalent anionic head groups with two phosphodiester groups. Therefore, we aimed to determine which functional groups of PGP-Me are essential for the boundary lipids of bR and how these functionalities interact with bR. To this end, we compared various well-known phospholipids (PLs) that carry one of the structural features of PGP-Me, and evaluated the affinity of PLs to bR using the centerband-only analysis of rotor-unsynchronized spin echo (COARSE) method in solid-state NMR measurements and thermal shift assays. The results clearly showed that the branched methyl groups of alkyl chains and double negative charges in the head groups are important for PL interactions with bR. We then examined the effect of phospholipids on the monomer-trimer exchange of bR using circular dichroism (CD) spectra. The results indicated that the divalent negative charge in a head group stabilizes the trimer structure, while the branched methyl chains significantly enhance the PLs' affinity for bR, thus dispersing bR trimers in the PM even at high concentrations. Finally, we investigated the effects of PL on the proton-pumping activity of bR based on the decay rate constant of the M intermediate of a bR photocycle. The findings showed that bR activities decreased to 20% in 1,2-dimyristoyl-sn-glycero-3-phosphate (DMPA), and in 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) bilayers as compared to that in PM. Meanwhile, 1,2-Diphytanoyl-sn-glycero-3-phosphate (DPhPA) bilayers bearing both negative charges and branched methyl groups preserved over 80% of the activity. These results strongly suggest that the head groups and alkyl chains of phospholipids are essential for boundary lipids and greatly influence the biological function of bR.

LnDOTA-d8 , a versatile chemical-shift thermometer for 2 H solid-state NMR.

2 H solid-state nuclear magnetic resonance (NMR) is a method for examining the mobility and orientation of molecules in the field of biophysics. In studies on lipid bilayer membranes, 2 H NMR is often adopted to detect a phase transition from the gel to the liquid-crystal phase, which is observed as a change in spectral shape, and to evaluate the ordering of lipid alkyl chains using quadrupole coupling values. Because the mobility of membrane lipids is highly temperature dependent, precise temperature control is a prerequisite for evaluating the physical properties of membranes. Generally, NMR instruments monitor the temperature of the variable temperature (VT) gas. The temperature inside the sample tube and the VT gas match only when the heat generated by the radio frequency (rf) pulse emitted from the coil or magic angle spinning is significantly lower than the cooling capacity of the VT gas. In other words, the sample temperature inside the tube depends on the measurement method. Therefore, in this study, we took advantage of temperature-dependent changes in the chemical shift of a paramagnetic metal-ligand complex. We designed and synthesized a deuterated ligand complex and evaluated its temperature dependence as a thermometer for 2 H solid-state NMR spectroscopy. We chose Tb, Dy, Ho, and Er as the paramagnetic central metals. We then measured the 2 H NMR spectrum of each metal complex and confirmed the 2 H chemical shift to be temperature dependent. Furthermore, with the use of the thermometer molecule with Er, we succeeded in accurately evaluating the segmental melting of an alkyl chain in lipid bilayers with 0.1°C accuracy.

Behavior of Triterpenoid Saponin Ginsenoside Rh2 in Ordered and Disordered Phases in Model Membranes Consisting of Sphingomyelin, Phosphatidylcholine, and Cholesterol.

The ginsenoside Rh2 (Rh2) is a saponin of medicinal ginseng, and it has attracted much attention for its pharmacological activities. In this study, we investigated the interaction of Rh2 with biological membranes using model membranes. We examined the effects of various lipids on the membrane-disrupting activity of Rh2 and found that cholesterol and sphingomyelin (SM) had no significant effect. Furthermore, the effects of Rh2 on acyl chain packing (DPH anisotropy) and water molecule permeability (GP340 values) did not differ significantly between bilayers containing SM and saturated phosphatidylcholine. These results suggest that the formation of the liquid-ordered (Lo) phase affects the behavior of Rh2 in the membrane rather than a specific interaction of Rh2 with a particular lipid. We investigated the effects of Rh2 on the Lo and liquid-disordered (Ld) phases using surface tension measurements and fluorescence experiments. In the surface tension-area isotherms, we compared the monolayers of the Ld and Lo lipid compositions and found that Rh2 is abundantly bound to both monolayers, with the amount being greater in the Ld phase than in the Lo phase. In addition, the hydration state of the bilayers, mainly consisting of the Lo or Ld phase, showed that Rh2 tends to bind to the surface of the bilayer in both phases. At higher concentrations, Rh2 tends to bind more abundantly to the relatively shallow interior of the Ld phase than the Lo phase. The phase-dependent membrane behavior of Rh2 is probably due to the phase-selective affinity and binding mode of Rh2.

Lipid chain-driven interaction of a lipidated Src-family kinase Lyn with the bilayer membrane.

N-Myristoylation is a process of ubiquitous protein modification, which promotes the interaction of lipidated proteins on cell surfaces, in conjunction with reversible S-palmitoylation. We report the cooperative lipid-lipid interaction of two acyl chains of proteins, which increases the protein-membrane interaction and facilitates selective targeting of membranes containing anionic lipids. Lyn is a member of the Src family kinases distributed on the membrane surface by N-myristoyl and neighbouring S-palmitoyl chain anchors at the unique N-terminus domain. We prepared N-terminal short segments of lipidated Lyn to investigate the behaviour of each acyl chain in the lipid composition-dependent membrane interaction by solid-state nuclear magnetic resonance (NMR) analysis. Solid-state 31P-NMR studies revealed that S-palmitoylation of N-myristoylated Lyn peptides increased the interaction between peptides and phospholipid head groups, particularly with the anionic phosphatidylserine-containing bilayers. The solid-state 2H-NMR of Lyn peptides with a perdeutero N-myristoyl chain indicated an increase (0.6-0.8 Å) in the extent of the N-myristoyl chain in the presence of nearby S-palmitoyl chains, probably through the interaction via the acyl chains. The cooperative hydrocarbon chain interaction of the two acyl chains of Lyn increased membrane binding by extending the hydrocarbon chains deeper into the membrane interior, thereby promoting the peptide-membrane surface interaction between the cationic peptide side chains and the anionic lipid head groups. This lipid-driven mechanism by S-palmitoylation promotes the partition of the lipidated proteins to the cytoplasmic surface of the cell membranes and may be involved in recruiting Lyn at the signalling domains rich in anionic lipids.

Amphotericin B assembles into seven-molecule ion channels: An NMR and molecular dynamics study.

Amphotericin B, an antifungal drug with a long history of use, forms fungicidal ion-permeable channels across cell membranes. Using solid-state nuclear magnetic resonance spectroscopy and molecular dynamics simulations, we experimentally elucidated the three-dimensional structure of the molecular assemblies formed by this drug in membranes in the presence of the fungal sterol ergosterol. A stable assembly consisting of seven drug molecules was observed to form an ion conductive channel. The structure is somewhat similar to the upper half of the barrel-stave model proposed in the 1970s but substantially different in the number of molecules and in their arrangement. The present structure explains many previous findings, including structure-activity relationships of the drug, which will be useful for improving drug efficacy and reducing adverse effects.

Depth-Dependent Segmental Melting of the Sphingomyelin Alkyl Chain in Lipid Bilayers.

The chain melting of lipid bilayers has often been investigated in detail using calorimetric methods, such as differential scanning calorimetry (DSC), and the resultant main transition temperature is regarded as one of the most important parameters in model membrane experiments. However, it is not always clear whether the hydrocarbon chains of lipids are gradually melting along the depth of the lipid bilayer or whether they all melt concurrently in a very narrow temperature range, as implied by DSC. In this study, we focused on stearoyl-d-sphingomyelin (SSM) as an example of raft-forming lipids. We synthesized deuterium-labeled SSMs at the 4', 10', and 16' positions, and their depth-dependent melting was measured using solid-state deuterium NMR by changing the temperature by 1.0 °C, and comparing with that observed from a saturated lipid, palmitoylstearoylphosphatidylcholine (PSPC). The results showed that SSM exhibited a characteristic depth-dependent melting, which was not observed for PSPC. The strong intermolecular hydrogen bonds between the sphingomyelin amide moiety probably caused the chain melting to start from the chain terminus through the middle part and end in the upper part. This depth-dependent melting implies that the small gel-like domains of SSM remain at temperatures slightly above the main transition temperature. These sphingomyelin features may be responsible for the biological properties of SM-based lipid rafts.

Diosgenin-induced physicochemical effects on phospholipid bilayers in comparison with cholesterol.

Diosgenin (DGN), which is a sterol occurring in plants of the Dioscorea family, has attracted increasing attention for its various pharmacological activities. DGN has a structural similarity to cholesterol (Cho). In this study we investigated the effects of the common tetracyclic cores and the different side chains on the physicochemical properties of lipid bilayer membranes. Differential scanning calorimetry showed that DGN and Cho reduce the phase transition enthalpy to a similar extent. In 2H NMR, deuterated-DGN/Cho and POPC showed similar ordering in POPC bilayers, which revealed that DGN is oriented parallel to the membrane normal like Cho. It was suggested that the affinity of DGN-Cho in membrane is stronger than that of DGN-DGN or Cho-Cho interaction. 31P NMR of POPC in bilayers revealed that, unlike Cho, DGN altered the interactions of POPC headgroups at 30 mol%. These results suggest that DGN below 30 mol% has similar effects with Cho on basic biomembrane properties.

Conformation and Orientation of Branched Acyl Chains Responsible for the Physical Stability of Diphytanoylphosphatidylcholine.

Diphytanoylphosphatidylcholine (DPhPC) is a synthetic phospholipid in which two methyl-branched acyl chains are introduced into the glycerol moiety, mimicking phospholipids of eukaryotic and eubacterial origins. The lipid bilayers of DPhPC reproduce the outstanding physical properties of methyl-branched lipids that occur in archaeal membranes. DPhPC is commonly used as the base lipid in biophysical experiments, particularly for recording ion-channel currents. However, the dynamics of lipid molecules that induces their useful physical properties is still unclear. In this study, we examined the conformation and orientation of the methyl-branched acyl chain of DPhPC in a membrane using 2H nuclear magnetic resonance (NMR) measurements of the synthetic lipid with a high stereochemical purity and molecular dynamics (MD) simulations. Deuterium-labeled 3',3'-CD3,D-DPhPC (2) and 7',7'-CD3,D-DPhPC (3) showed the characteristic quadrupole splitting width in the 2H NMR spectra, which corresponded to the bent orientation reported for the archaeal lipid PGP-Me [Yamagami, M., et al. (2019) Biochemistry 58, 3869-3879]. However, MD simulations, which reproduced the 2H NMR results well, unveiled the unknown features of DPhPC in the membrane; DPhPC has a chain-specific average orientation, where two bent orientations with upward and downward methyl groups occur at positions C3 and C7 of the sn-1 and sn-2 chains of DPhPC, respectively. These MD and NMR results reveal that these two bent orientations define the average orientation of DPhPC for the shallow part of the acyl chains, which is considered to be an important factor in the stability of DPhPC membranes.

The Amphotericin B-Ergosterol Complex Spans a Lipid Bilayer as a Single-Length Assembly.

Amphotericin B (AmB) is a polyene macrolide antibiotic clinically used as an antifungal drug. Its preferential complexation with ergosterol (Erg), the major sterol of fungal membranes, leads to the formation of a barrel-stave-like ion channel across a lipid bilayer. To gain a better understanding of the mechanism of action, the mode of lipid bilayer spanning provides essential information. However, because of the lack of methodologies to observe it directly, it has not been revealed for the Erg-containing channel assembly for many years. In this study, we disclosed that the AmB-Erg complex spans a lipid bilayer with a single-molecule length, using solid-state nuclear magnetic resonance (NMR) experiments. Paramagnetic relaxation enhancement by Mn2+ residing near the surface of lipid bilayers induced the depth-dependent decay of 13C NMR signals for individual carbon atoms of AmB. We found that both terminal segments, the 41-COOH group and C38-C40 methyl groups, come close to the lipid bilayer surfaces, suggesting that the AmB-Erg complex spans a palmitoyloleoylphosphatidylcholine (POPC) bilayer with a single-molecule length. Molecular dynamics simulation experiments further confirmed the stabilization of the AmB-Erg complex as a single-length spanning complex. These results provide experimental evidence of the single-length complex incorporated in the membrane by making thinner a POPC-Erg bilayer that mimics fungal membranes.

Average Conformation of Branched Chain Lipid PGP-Me That Accounts for the Thermal Stability and High-Salinity Resistance of Archaeal Membranes.

The average conformation of the methyl-branched chains of archaeal lipid phosphatidyl glycerophosphate methyl ester (PGP-Me) was examined in a hydrated bilayer membrane based on the 2H nuclear magnetic resonance (NMR) of enantioselectively 2H-labeled compounds that were totally synthesized for the first time in this study. The NMR results in combination with molecular dynamics simulations revealed that the PGP-Me chain appeared to exhibit behavior different from that of typical membrane lipids such as dimyristoylphosphatidylcholine (DMPC). The C-C bonds of the PGP-Me chain adopt alternative parallel and tilted orientations to the membrane normal as opposed to a DMPC chain where all of the C-C bonds tilt in the same way on average. This characteristic orientation causes the intertwining of PGP-Me chains, which plays an important role in the excellent thermal and high-salinity stabilities of archaeal lipid bilayers and membrane proteins.

Cholesterol-Induced Conformational Change in the Sphingomyelin Headgroup.

Sphingomyelin (SM) and cholesterol (Cho) are the important lipids for the formation of biologically functional membrane domains, lipid rafts. However, the interaction between Cho and the headgroup of SM remains unclear. In this study, we performed solid-state NMR experiments to reveal the Cho effects on the headgroup conformation using 2H-labeled stearoyl-SM (SSM). Deuterated SSMs at the Cα, Cß, and Cγ positions of a choline moiety were separately prepared and subjected to NMR measurements to determine the quadrupolar splitting of 2H signals in hydrated SSM unitary and SSM/Cho (1:1) bilayers. Using 2H NMR and 13C-31P REDOR data, the conformation and orientation of the choline moiety were deduced and compared with those derived from molecular dynamics simulations. In SSM unitary bilayers, three torsional angles in the phosphocholine moiety, P-O-Cα-Cß, were found to be consecutive +gauche(g)/+g/+g or -g/-g/-g. The orientation and conformation of the SSM headgroup were consistent with the results of our molecular dynamics simulations and the previous results on phosphatidylcholines. The quadrupolar coupling at the α methylene group slightly increased in the presence of Cho, and those at the Cß and Cγ decreased more significantly, thus suggesting that Cho reduced the gauche conformation at the Cα-Cß torsion. The conformational ensemble in the presence of Cho may enhance the so-called umbrella effect of the SSM headgroup, resulting in the stabilization of Cho near the SM molecules by concealing the hydrophobic Cho core from interfacial water. We also examined the effect of the chiral centers at the sphingosine chain to the headgroup conformation by determining the enantiomeric excess between the diastereomeric +g/+g/+g and -g/-g/-g conformers using (S)-Cα-deuterated and (R)-Cα-deuterated SSMs. Their 2H NMR measurements showed that the chiral centers induced the slight diastereomeric excess in the SM headgroup conformation.

The Perpendicular Orientation of Amphotericin B Methyl Ester in Hydrated Lipid Bilayers Supports the Barrel-Stave Model.

The clinically important antibiotic amphotericin B (AmB) is a membrane-active natural product that targets membrane sterol. The antimicrobial activity of AmB is generally attributed to its membrane permeabilization, which occurs when a pore is formed across a lipid bilayer. In this study, the molecular orientation of AmB was investigated using solid-state nuclear magnetic resonance (NMR) to better understand the mechanism of antifungal activity. The methyl ester of AmB (AME) labeled with NMR isotopes, d3-AME, and its fluorinated and/or 13C-labeled derivatives were prepared. All of the AmB derivatives showed similar membrane-disrupting activities and ultraviolet spectra in phospholipid liposomes, suggesting that their molecular assemblies in membranes closely mimic those of AmB. Solid-state 2H NMR measurements of d3-AME in a hydrated membrane showed that the mobility of AME molecules depends on concentration and temperature. At a 1:5:45 AME:Erg:dimyristoylphosphatidylcholine ratio, AME became sufficiently mobilized to observe the motional averaging of quadrupole coupling. On the basis of the rotational averaging effect of 19F chemical shift anisotropy, 2H quadrupolar splitting, and 13C-19F dipolar coupling of 14ß-F-AMEs, we deduced that the molecular axis of AME is predominantly parallel to the normal of a lipid bilayer. This result supports the barrel-stave model as a molecular assembly of AmB in membranes.

Small structural alterations greatly influence the membrane affinity of lipophilic ligands: Membrane interactions of bafilomycin A1 and its desmethyl derivative bearing 19F-labeling.

Molecular behavior under bilayer membrane environments is one of the important research topics concerning how organic molecules exert their biological activities when interacting with cellular membranes. However, chemistry-based approaches to this property have not been successful when compared with the structural biological strategy on ligand-receptor interactions. Here, we investigated the molecular behavior of the lipophilic ATPase inhibitor bafilomycin A1 and its derivatives under a lipid environment from a chemical point of view. Our results revealed significant differences in membrane affinity and dynamics among ligands having different inhibitory potencies, suggesting the specific contribution of ligand-membrane interactions to their biological activity.

Evaluation of diacylphospholipids as boundary lipids for bacteriorhodopsin from structural and functional aspects.

Reconstituted membranes with diverse diacylphospholipids were prepared by using bacteriorhodopsin (bR) in which the intrinsic lipid content was decreased to 24% of the original while the trimeric structure and photocycle of bR were retained. Four phospholipids with a different headgroup, phosphatidic acid (PA), phosphatidylcholine (PC), phosphatidylglycerol (PG), and phosphatidylserine (PS), were adopted for reconstitution. By varying the lipid-protein ratios, the interactions of these phospholipids with bR, as a boundary lipid, were evaluated by solid state (2)H/(31)P NMR, circular dichroism (CD), and laser-flash photolysis. The (31)P NMR results revealed that the headgroup of acidic phosphatidylglycerol (PG) interacts more strongly with bR than that of phosphatidylcholine (PC). CD analysis indicated that the trimetric structure of bR was retained in all the phospholipid-bR preparations at low and medium lipid contents. Acidic lipids PA, PG and PS restored the photocycle activity of bR to an extent comparable to (or slightly lower than) that of the purple membrane while PC caused a marked reduction of the bR photocycle efficiency. Among PGs with different fatty acyl groups, those with mono- and di-unsaturated lipids tended to preserve the photocycle efficiency, whereas the fully saturated lipid did not. These results show that acidic unsaturated phospholipids, particularly dioleoylphosphatidylglycerol (DOPG), have higher affinity for bR and efficiently restore its trimetric structure. The present study suggests that bR reconstituted in DOPG bilayers may possibly be used as a model system for spectroscopic investigations of the lipid-bR interactions with the membrane-integral α-helices, and potentially for a similar type of membrane proteins.

The Structure of the Bimolecular Complex between Amphotericin B and Ergosterol in Membranes Is Stabilized by Face-to-Face van der Waals Interaction with Their Rigid Cyclic Cores.

Amphotericin B (AmB) is a polyene macrolide antibiotic isolated from Streptomyces nodosus. The antifungal activity of AmB can be attributed to the formation of an ion-channel assembly in the presence of ergosterol (Erg), in which there are two different AmB-Erg orientations, parallel and antiparallel, as reported previously. In this study, to elucidate the structures of those AmB-Erg complexes based on solid-state nuclear magnetic resonance, a (19)F-labeled AmB derivative was newly prepared by a hybrid synthesis that utilized degradation products from the drug. Using the 2-(trimethylsilyl)ethoxymethyl (SEM) group as the protecting group for the carboxylic acid moiety of AmB, the fully deprotected labeled AmB compounds were obtained successfully. Then, these labeled AmBs were subjected to (13)C{(19)F} rotational-echo double-resonance (REDOR) experiments in hydrated lipid bilayers. The results indicated the coexistence of parallel and antiparallel orientations for AmB and Erg pairing, at a ratio of 7:3. A total of six distances between AmB and Erg were successfully obtained. Geometry analysis using the distance constraints derived from the REDOR experiments provided the plausible AmB-Erg complex structure for both the parallel and antiparallel interactions. The flat macrolide of AmB and the tetracyclic core of Erg closely contacted in a face-to-face manner, thus maximizing the van der Waals interaction between the two molecules. This interaction can be attributed to the coexistence of both the parallel and antiparallel orientations.

(13) C-TmDOTA as versatile thermometer compound for solid-state NMR of hydrated lipid bilayer membranes.

Recent advances in solid-state nuclear magnetic resonance (NMR) techniques, such as magic angle spinning and high-power decoupling, have dramatically increased the sensitivity and resolution of NMR. However, these NMR techniques generate extra heat, causing a temperature difference between the sample in the rotor and the variable temperature gas. This extra heating is a particularly crucial problem for hydrated lipid membrane samples. Thus, to develop an NMR thermometer that is suitable for hydrated lipid samples, thulium-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (TmDOTA) was synthesized and labeled with (13) C (i.e., (13) C-TmDOTA) to increase the NMR sensitivity. The complex was mixed with a hydrated lipid membrane, and the system was subjected to solid-state NMR and differential scanning calorimetric analyses. The physical properties of the lipid bilayer and the quality of the NMR spectra of the membrane were negligibly affected by the presence of (13) C-TmDOTA, and the (13) C chemical shift of the complex exhibited a large-temperature dependence. The results demonstrated that (13) C-TmDOTA could be successfully used as a thermometer to accurately monitor temperature changes induced by (1) H decoupling pulses and/or by magic angle spinning and the temperature distribution of the sample inside the rotor. Thus, (13) C-TmDOTA was shown to be a versatile thermometer for hydrated lipid assemblies. Copyright © 2015 John Wiley & Sons, Ltd.

Correction: Stereoselective synthesis of the head group of archaeal phospholipid PGP-Me to investigate bacteriorhodopsin-lipid interactions.

Correction for 'Stereoselective synthesis of the head group of archaeal phospholipid PGP-Me to investigate bacteriorhodopsin-lipid interactions' by Jin Cui, et al., Org. Biomol. Chem., 2015, DOI: 10.1039/c5ob01252j.