High resolution proton relaxation studies of lecithins

Molecular-Level Structural Analysis of siRNA-Loaded Lipid Nanoparticles by 1H NMR Relaxometry: Impact of Lipid Composition on Their Structural Properties

1H NMR relaxometry was applied for molecular-level structural analysis of siRNA-loaded lipid nanoparticles (LNPs) to clarify the impact of the neutral lipids, 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and cholesterol, on the physicochemical properties of LNP. Incorporating DSPC and cholesterol in ionizable lipid-based LNP decreased the molecular mobility of ionizable lipids. DSPC reduced the overall molecular mobility of ionizable lipids, while cholesterol specifically decreased the mobility of the hydrophobic tails of ionizable lipids, suggesting that cholesterol filled the gap between the hydrophobic tails of ionizable lipids. The decrease in molecular mobility and change in orientation of lipid mixtures contributed to the maintenance of the stacked bilayer structure of siRNA and ionizable lipids, thereby increasing the siRNA encapsulation efficiency. Furthermore, NMR relaxometry revealed that incorporating those neutral lipids enhanced PEG chain flexibility at the LNP interface. Notably, a small amount of DSPC effectively increased PEG chain flexibility, possibly contributing to the improved dispersion stability and narrower size distribution of LNPs. However, cryogenic transmission electron microscopy represented that adding excess amounts of DSPC and cholesterol into LNP resulted in the formation of deformed particles and demixing cholesterol within the LNP, respectively. The optimal lipid composition of ionizable lipid-based LNPs in terms of siRNA encapsulation efficiency and PEG chain flexibility was rationalized based on the molecular-level characterization of LNPs. Moreover, the NMR relaxation rate of tertiary amine protons of ionizable lipids, which are the interaction site with siRNA, can be a valuable indicator of the encapsulated amount of siRNA within LNPs. Thus, NMR-based analysis can be a powerful tool for efficiently designing LNP formulations and their quality control based on the molecular-level elucidation of the physicochemical properties of LNPs.

B0-field dependence of MRI T1 relaxation in human brain

Tissue longitudinal relaxation characterized by recovery time T1 or rate R1 is a fundamental MRI contrast mechanism that is increasingly being used to study the brain's myelination patterns in both health and disease. Nevertheless, the quantitative relationship between T1 and myelination, and its dependence on B0 field strength, is still not well known. It has been theorized that in much of brain tissue, T1 field-dependence is driven by that of macromolecular protons (MP) through a mechanism called magnetization transfer (MT). Despite the explanatory power of this theory and substantial support from in-vitro experiments at low fields (<3 ​T), in-vivo evidence across clinically relevant field strengths is lacking. In this study, T1-weighted MRI was acquired in a group of eight healthy volunteers at four clinically relevant field strengths (0.55, 1.5, 3 and 7 ​T) using the same pulse sequence at a single site, and jointly analyzed based on the two-pool model of MT. MP fraction and free-water pool T1 were obtained in several brain structures at 3 and 7 ​T, which allowed distinguishing between contributions from macromolecular content and iron to tissue T1. Based on this, the T1 of MP in white matter, indirectly determined by assuming a field independent T1 of free water, was shown to increase approximately linearly with B0. This study advances our understanding of the T1 contrast mechanism and its relation to brain myelin content across the wide range of currently available MRI strengths, and it has the potential to inform design of T1 mapping methods for improved reproducibility in the human brain.

An NMR database for simulations of membrane dynamics

Computational methods are powerful in capturing the results of experimental studies in terms of force fields that both explain and predict biological structures. Validation of molecular simulations requires comparison with experimental data to test and confirm computational predictions. Here we report a comprehensive database of NMR results for membrane phospholipids with interpretations intended to be accessible by non-NMR specialists. Experimental ¹³C-¹H and ²H NMR segmental order parameters (S(CH) or S(CD)) and spin-lattice (Zeeman) relaxation times (T(1Z)) are summarized in convenient tabular form for various saturated, unsaturated, and biological membrane phospholipids. Segmental order parameters give direct information about bilayer structural properties, including the area per lipid and volumetric hydrocarbon thickness. In addition, relaxation rates provide complementary information about molecular dynamics. Particular attention is paid to the magnetic field dependence (frequency dispersion) of the NMR relaxation rates in terms of various simplified power laws. Model-free reduction of the T(1Z) studies in terms of a power-law formalism shows that the relaxation rates for saturated phosphatidylcholines follow a single frequency-dispersive trend within the MHz regime. We show how analytical models can guide the continued development of atomistic and coarse-grained force fields. Our interpretation suggests that lipid diffusion and collective order fluctuations are implicitly governed by the viscoelastic nature of the liquid-crystalline ensemble. Collective bilayer excitations are emergent over mesoscopic length scales that fall between the molecular and bilayer dimensions, and are important for lipid organization and lipid-protein interactions. Future conceptual advances and theoretical reductions will foster understanding of biomembrane structural dynamics through a synergy of NMR measurements and molecular simulations.

Differential effects of uncharged aminoamide local anesthetics on phospholipid bilayers, as monitored by 1H-NMR measurements

We have collected evidences of a "transient site" for the local anesthetics (LA) lidocaine, etidocaine, bupivacaine and mepivacaine in sonicated egg phosphatidylcholine (EPC) vesicles. The effects of the uncharged anesthetic species at a fixed LA/EPC ratio inside the bilayer were measured by chemical shifts (C.S.) and longitudinal relaxation times (T(1)) of the lipid hydrogens. Two sort of changes were detected: (I) decrease, indicating specific orientation of the LA aromatic ring (measured as up-field C.S. changes by the short-range ring-current effect) and less rotational freedom (smaller T(1) values) for EPC hydrogens such as the two glycerol-CH(2) and the choline-CH(2) bound to the PO(4-) group, probably due to the nearby presence of the LA; (II) increase, indicating the aromatic ring is now perpendicular to the orientation observed before (causing down-field changes in C.S.) and larger T(1) values for all the choline and glycerol hydrogens, as a result of LA insertion behind these well-organized bilayer regions. The less hydrophobic, linear and nonlinear (lidocaine and mepivacaine, respectively) aminoamide analogs provide similar effects-described in I; their hydrophobic counterparts (etidocaine and bupivacaine) also produced comparable effects (depicted in II). The preferential positioning and orientation of each LA inside the bilayer is then determined by its hydrophobic and steric properties. We propose that this "transient site" in the lipid milieu exists also in biological membranes, where it can modulates the access of the uncharged LA species to its site(s) of action in the voltage-gated sodium channel.

Conformational studies of sphingolipids by NMR spectroscopy. I. Dihydrosphingomyelin

The conformational features of dihydrosphingomyelin (DHSM), the major phospholipid of human lens membranes, were investigated by 1H and 31P nuclear magnetic resonance spectroscopy. Several postulates emerge from the observed trends: (a) in partially hydrated samples of DHSM in CDCl3 above 13 mM, at which lipid-lipid interactions prevail, the amide proton is mostly involved in intermolecular H-bonds that link neighboring phospholipids through bridging water molecules. In the absence of water, the NH group is involved in an intramolecular H-bond that restricts the mobility of the phosphate group. (b) In the monomeric form of the lipid molecule, the amide proton of the major conformer is bound intramolecularly with one of the anionic and/or ester oxygens of the phosphate group. A minor conformer may also be present in which the NH proton participates in an intramolecular H-bond linking to the OH group of the sphingoid base. (c) Complete hydration leads to an extension of the head group as water molecules bind to the phosphate and NH groups via H-bonds, thus disrupting the intramolecular H-bonds prevalent at low concentrations.

Carbazole as an excited state proton transfer fluorescent probe for lipid bilayers in alkaline medium

In our effort to look for novel excited state proton transfer (ESPT) fluorescent probes in alkaline pH range, we have examined carbazole as a possible candidate because of its high extinction coefficient, high quantum yield and a larger difference in ionization constant between the ground and excited state (pKa - pKa*). The photodissociation of carbazole was studied in liposome membrane by steady state fluorescence measurements at alkaline pH ranges. The neutral form and the anionic form of carbazole emit at 362 and 417 nm, respectively. This large shift in emission makes it convenient to monitor the physical properties of liposomes. The neutral form fluorescence intensity of carbazole is sensitive to phase changes in the membrane and also shows a maximum at phase transition temperature. This variation of intensity can be explained in terms of redistribution of probe between the surface and interior of the liposomes. Cholesterol induced phase changes of liposomes were also sensed by the ESPT of carbazole.

Magic-angle spinning NMR studies of molecular organization in multibilayers formed by 1-octadecanoyl-2-decanoyl-sn-glycero-3-phosphocholine

Magic-angle spinning 1H and 13C nuclear magnetic resonance (NMR) have been employed to study 50%-by-weight aqueous dispersions of 1-octadecanoyl-2-decanoyl-sn-glycero-3-phosphocholine (C[18]:C[10]PC) and 1-octadecanoyl-2-d19-decanoyl-PC (C[18]:C[10]PC-d19), mixed-chain phospholipids which can form interdigitated multibilayers. The 1H NMR linewidth for methyl protons of the choline headgroup has been used to monitor the liquid crystalline-to-gel (LC-to-G) phase transition and confirm variations between freezing and melting temperatures. Both 1H and 13C spin-lattice relaxation times indicate unusual restrictions on segmental reorientation at megahertz frequencies for C(18):C(10)PC as compared with symmetric-chain species in the LC state; nevertheless each chemical moiety of the mixed-chain phospholipid exhibits motional behavior that may be classified as liquidlike. Two-dimensional nuclear Overhauser spectroscopy (NOESY) on C(18):C(10)PC and C(18):C(10)PC-d19 reveals cross-peaks between the omega-methyl protons of the C18 chain and the N-methyl protons of the phosphocholine headgroup, and several experimental and theoretical considerations argue against an interpretation based on spin diffusion. Using NMR relaxation times and NOESY connectivities along with a computational formalism for four-spin systems (Keepers, J. W., and T. L. James. 1984. J. Magn. Reson. 57:404-426), an estimate of 3.5 A is obtained for the average distance between the omega-methyl protons of the C18 chain and the N-methyl protons of the phosphocholine headgroup. This finding is consistent with a degree of interdigitation similar to that proposed for organized assemblies of gel-state phosphatidylcholine molecules with widely disparate acyl-chain lengths (Hui, S. W., and C.-H. Huang. 1986. Biochemistry. 25:1330-1335); however, acyl-chain bendback or other intermolecular interactions may also contribute to the NOESY results. For multibilayers of C(18):C(10)PC in the gel phase, 13C chemical-shift measurements indicate that trans conformers predominate along both acyl chains. 13C Spin-lattice relaxation times confirm the unusual motional restrictions noted in the LC state; nevertheless, 13C and 1H rotating-frame relaxation times indicate that the interdigitated arrangement enhances chain or bilayer motions which occur at mid-kilohertz frequencies.

Dynamics of the phosphate group in phospholipid bilayers. A 31P-1H transient Overhauser effect study

Two recent studies have addressed the question of the dynamics of the phosphate in egg phosphatidylcholine multilayers by measurement and interpretation of 31P NMR spin-lattice relaxation. In the first (Milburn, M. P., and K. R. Jeffrey. 1987. Biophys. J. 52:791-799), the temperature dependences of the two contributions to the 31P relaxation rate, a dipolar interaction of the phosphorus with neighboring protons and a time-dependent anisotropic chemical shielding interaction were separately measured. A further study (Milburn, M. P., and K. R. Jeffrey. 1989. Biophys. J. 56:543-549) incorporated the anisotropic nature of phospholipid motions into the dynamic model of the headgroup motion by measuring the 31P spin-lattice relaxation time in oriented samples as a function of angle between the bilayer normal and the magnetic field. These angular dependent measurements were made at high field so that analysis could by made using the chemical shielding interaction because the 31P-1H dipolar interaction in phospholipid systems is complex and as such poorly understood. Nuclear Overhauser effect (NOE) studies have attempted to identify the important proton species contributing to the 31P-1H dipolar interaction (Yeagle, P. L., W. C. Hutton, C. Huang, and R. B. Martin. 1975. Biochemistry. 15:2121-2124) and despite some controversy in interpretation (Burns, R. A., R. E. Stark, D. A. Vidusek, and M. F. Roberts. 1983. Biochemistry. 22:5084-5090), it was generally agreed that the choline methyl and methylene protons are the major contributors to the 31P-1H NOE. To further understand the nature of the 31P-1H dipolar interaction, we carried out 31P-1H Transient Overhauser effect (TOE) measurements on egg phosphatidylcholine multilayers. Protons from both the lipids and water are important in understanding the TOE measurements in both D20 dispersions and H20 dispersions of egg PC. A quantitative analysis of the TOE has enabled the cross-relaxation rate between the phosphorus and the two proton types to be determined. It is suggested that these TOE experiments are a direct observation of the interaction between the phospholipid phosphate and surrounding water protons. The correlation time describing the relative motion of the phosphate group and the water molecules is on the order of 10- 11 s. The TOE measurements in phospholipid dispersions can be easily understood in terms of a straight forward model of the dipolar interaction and provide complementary information to NOE and T1 measurements.

Spontaneous formation of small unilamellar vesicles by pH jump: a pH gradient across the bilayer membrane as the driving force

31P NMR and infrared spectroscopic methods have been used to study the formation of small unilamellar vesicles by the pH-jump method. It is shown that increasing the pH of different lamellar phospholipid dispersions (phosphatidic acids and phosphatidylserines) induces a pH gradient. This pH gradient is estimated to be 4 +/- 1 pH units, and its direction is such that the inner monolayer of the vesicles is at lower pH. There is spectroscopic evidence for tighter packing of the lipid hydrocarbon chains in the inner monolayer, probably due to the constraints imposed by the high curvature of the small vesicles formed. These results are discussed in terms of the driving force of the spontaneous vesiculation.

Effect of triglyceride levels on methyl and methylene envelope line widths in proton nuclear magnetic resonance spectroscopy of human plasma

Recent studies have described a relation between the line widths of the methyl and methylene resonance envelopes in the proton nuclear magnetic resonance spectrum of human plasma and the occurrence of cancer. An average line width of less than 33 Hz has been reported to correlate with the presence of cancer, whereas greater line widths have not. In 26 normal volunteers, we found a significant inverse correlation between fasting triglyceride level and plasma spectral line width. We also observed that dietary lipids have measurable effects on spectral line widths. In another sample of seven normal persons (three of whom had elevated plasma triglyceride levels), the line widths of whole plasma varied widely (mean, 35.6 +/- 8.8 Hz); however, the mean line widths of the lipoprotein fractions isolated from those samples differed greatly, but the variance within each fraction was small (very-low-density lipoprotein, 22.0 +/- 1.9 Hz; low-density lipoprotein, 35.0 +/- 2.8; high-density lipoprotein, 28.8 +/- 1.9). The results of this study indicate that the plasma triglyceride level has a profound effect on the average spectral line width of plasma. This effect can be explained by the relative amounts of lipoprotein fractions in whole plasma. Plasma triglyceride concentrations of more than 1.24 mmol per liter (greater than 110 mg per deciliter), whatever the source, produce average plasma methyl and methylene line widths of less than 33 Hz.

1H-NMR study of the effect of synthetic polymers on the fluidity, transition temperature and fusion of dipalmitoyl phosphatidylcholine small vesicles

The interaction of water-soluble polymers with dipalmitoyl phosphatidylcholine small vesicles and the effect on vesicle fusion were studied by means of 1H-NMR spectrometry. The motion of dipalmitoyl phosphatidylcholine molecules decreased on interaction with the polymers and was detected as a change in the signal intensity. The interaction behavior of polymers is very sensitive to the chemical structure of the applied polymers. Poly(styrene sulfonic acid) and poly(ethylene glycol) decreased the motion of the choline methyl group, predominantly through coulombic and hydrophobic interaction forces, respectively. For example, in the case of the poly(styrene sulfonic acid)-containing system, the signal intensity of the choline methyl group was decreased about 15% while those of the hydrophobic methylene and terminal methyl groups were scarcely decreased by the addition of polymer to a final concentration of 4.0 x 10(-2) unit mol/l. These polymers are considered to interact with the surface of the vesicle membrane. On the other hand, poly(L-glutamic acid) and poly(N-vinyl-2-pyrrolidone) decreased the signal intensities of not only the choline methyl group, but also those of the hydrophobic methylene and terminal methyl groups. This result suggests that part of these polymers might be incorporated into the hydrophobic region of the vesicle membrane. Addition of the non-ionic polymers inhibited vesicle fusion considerably. This effect was explained by the stabilization of dipalmitoyl phosphatidylcholine vesicles by complexation with these polymers.

The proliferation of hepatocytes and the lipid composition of the endoplasmic reticulum after induction of drug-metabolizing enzymes with trans-stilbene oxide

Three aspects of the induction of drug-metabolizing enzymes brought about by trans-stilbene oxide have been investigated. (1) The liver hypertrophy in rats treated with trans-stilbene oxide was found to result solely from an increase in the number of cells in this organ, without any increase in the size of each individual cell. (2) Administration of trans-stilbene oxide also produces a 27% increase in the phospholipid content of the hepatic endoplasmic reticulum, i.e., a limited proliferation of this organelle occurs. (3) Furthermore, induction causes changes in the lipid composition of the endoplasmic reticulum. The cholesterol content is decreased, the relative content of sphingo-myelin is also lowered, and a number of changes in the fatty-acid composition occur as well. All of these effects would tend to increase the fluidity of the phospholipid bilayer of the endoplasmic-reticulum membrane and may thus affect drug metabolism.

Erythrocyte water permeability. The effects of anesthetic alcohols and alterations in the level of membrane cholesterol

1. Treatment of human erythrocytes with anesthetic n-alkanols (pentanol, hexanol and hepatanol) results in a decrease in the osmotic water permeability of the red cell membrane. 2. The alcohol-induced changes in osmotic water permeability are proportional to the alcohol concentration and roughly parallel diphenylhexatriene that are induced by the alcohols. 3. Enrichment of the red cell membrane in cholesterol also results in a decrease in the osmotic water permeability. 4. Moderate depletion (9% or 40%) of membrane cholesterol is without effect on the osmotic water permeability, even though this treatment results in a significant increase in the rotational mobility of diphenylhexatriene in the membrane lipids.

Evidence for stereospecific phospholipid-cholesterol interaction in lipid bilayers

The CH2 proton NMR linewidths of sn-3 and sn-1 dipalmitoyl phosphatidylcholine respond differently to the addition of cholesterol to the lipid vesicles. This result points to a stereospecific phospholipid-cholesterol interaction in the "hydrogen belt" region.

Nuclear magnetic resonance studies in liposomes: effects of steroids on lecithin fatty acyl chain mobility

The effects of fourteen sterols on the NMR spectra of liposomes derived from egg yolk phosphatidylcholines were studied by continuous-wave and Fourier-transform measurements at 60 MHz. Sterols were compared for their ability to broaden the acyl methylene resonances of phosphatidylcholine, when incorporated into liposomes at 25% molar ratio. The ratio of the phosphatidylcholine peake heights (acyl methylene: choline N-methyl) was used as a criterion of the relative condensing activity for the different sterols. This ratio was inversely proportional to the molar volume of the incorporated sterol, as measured by the parachor of the compound. Small sterols had little condensing effect, and the larger sterols such as cholesterol and ergosterol had maximum condensing effects. The study confirmed the importance of the sterol side-chain at C-17 as a requirement for sterol-phospholipid interaction.

Calorimetric, 13C NMR, and 31P NMR studies on the interaction of some phenothiazine derivatives with dipalmitoyl phosphatidylcholine model membranes

1. The dipalmitoyl phosphatidylcholine/water system was employed to study the interaction of phenothiazines with model membranes. In particular the effects of the drugs upon the lipid phase transition were examined using differential scanning calorimetry and NMR spectroscopy. The studied phenothiazines have peripheral (diethazine) or central (chlorpromazine) properties. 2. Both drugs were observed to lower the phase transition temperature of dipalmitoyl phosphatidylcholine. The molar activity of chlorpromazine is somewhat higher than of diethazine. At low concentrations the drugs affect the dipalmitoyl phosphatidylcholine pretransition endotherm. 3. In the 13C NMR spectra of the drug-containing samples the signal of the trimethylammonium group of dipalmitoyl phosphatidylcholine is broadened, whereas a narrowing of the signal of the fatty-acid chain methylene groups is observed. Further, addition of the phenothiazines causes higher values of the effective chemical shift anisotropy of the 31P in the phosphate group, in comparison to the pure dipalmitoyl phosphatidylcholine sample. 4. The results obtained by three different techniques indicate a higher fluidity in the fatty-acid chain region and a mobility reduction of the polar headgroup of the dipalmitoyl phosphatidylcholine molecules in the presence of the phenothiazines. These phenomena can be well accounted for by a model for the incorporation of the phenothiazines in the dipalmitoyl phosphatidyl-choline bilayer, in which the dialkylaminoalkyl chains are located near the polar headgroups and the ring system does not penetrate far beyond the glycerol backbone into the hydrocarbon phase.