A general method for the preparation of mixed micelles of hydrophobic peptides and sodium dodecyl sulphate.

A new method is reported for the incorporation of hydrophobic peptides into sodium dodecyl sulphate (SDS) micelles. First, a homogeneous solution of peptide and detergent is obtained by adding the peptide in trifluoroethanol to an equal volume of an aqueous solution of SDS. Upon subsequent addition of excess water, mixed peptide-SDS micelles are formed. Next, all solvent is removed by lyophilization and an appropriate amount of water is added to the dry powder. For various hydrophobic peptides this was shown to yield clear and stable solutions that are highly concentrated and suitable for characterization by spectroscopic techniques.

MRI study of pharyngeal airway changes during stimulation of the hypoglossal nerve branches in rats.

The medial branch (Med) of the hypoglossal nerve innervates the tongue protrudor muscles, whereas the lateral branch (Lat) innervates tongue retractor muscles. Our previous finding that pharyngeal airflow increased during either selective Med stimulation or whole hypoglossal nerve (WHL) stimulation (coactivation of protrudor and retractor muscles) led us to examine how WHL, Med, or Lat stimulation affected tongue movements and nasopharyngeal (NP) and oropharyngeal (OP) airway volume. Electrical stimulation of either WHL, Med, or Lat nerves was performed in anesthetized, tracheotomized rats while magnetic resonance images of the NP and OP were acquired (slice thickness 0.5 mm, in-plane resolution 0.25 mm). NP and OP volume was greater during WHL and Med stimulation vs. no stimulation (P < 0.05). Ventral tongue depression (measured in the midsagittal images) and OP volume were greater during Med stimulation than during WHL stimulation (P < 0.05). Lat stimulation did not alter NP volume (P = 0.39). Our finding that either WHL or Med stimulation dilates the NP and OP airways sheds new light on the control of pharyngeal airway caliber by extrinsic tongue muscles and may lead to new treatments for patients with obstructive sleep apnea.

Magnetic alignment and orientational order of dipalmitoylphosphatidylcholine bilayers containing palmitoyllysophosphatidylcholine.

Mixed bilayers of 1-palmitoyl-sn-glycero-3-phosphocholine (palmitoyllysophosphatidylcholine; PaLPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (dipalmitoyl phosphatidylcholine; DPPC) have been investigated by 2H-NMR and 31P-NMR spectroscopy. Binary phospholipid mixtures were studied in which the acyl chains of one or the other component were perdeuterated. At temperatures below the main order-disorder phase transition, the mixed PaLPC/DPPC bilayers appear to coexist with PaLPC micelles. The micelles disappear at temperatures above the phase transition, where mixed bilayers in the liquid-crystalline state are formed. The orientational order of the alkyl chains of the PaLPC component is essentially identical to that of the DPPC component in the mixed bilayers, both in the low temperature and liquid-crystalline phases. However, the presence of PaLPC perturbs the segmental ordering of DPPC as compared to the pure system. The order is increased in the low-temperature phase, where effective diffusion of the chains about their long axes occurs, but is decreased in the liquid-crystalline phase compared to pure DPPC bilayers. The mixed liquid-crystalline bilayers orient preferentially with their director axes perpendicular to the magnetic field. This alignment is easily observed in 31P- and 2H-NMR spectra, where the intensity of the perpendicular edges of the lineshapes is pronounced. One possible explanation of the magnetic alignment involves alteration of the curvature free energy of the DPPC bilayer due to incorporation of PaLPC in the mixed membranes.

Chemical-shift imaging utilizing the positional shifts along the readout gradient direction.

In this work, we describe a method that uses the linear phase acquired during the readout period due to chemical shift to generate individual magnetic resonance (MR) images of chemically shifted species. The method utilizes sets of Fourier (or k-space) data acquired with different directions of the readout gradient and a postprocessing algorithm to generate chemical shift images. The methodology is developed for both Cartesian data acquisition and for radial data acquisition. The method is presented here for two chemically shifted species but it can be extended to more species. In this work, we present the theory, show the results in phantoms and in human images, and discuss the artifacts and signal-to-noise ratio of the images obtained with the technique.

Anti-inflammatory drugs reduce age-related decreases in brain volume in cognitively normal older adults.

Previous studies have indicated a decreased risk for developing Alzheimer's disease in anti-inflammatory (AI) drug users. Yet few studies have determined whether AI drug use provides a protective effect against normal age-related changes in the brains of older adults. Regional volume changes in gray and white matter were assessed cross-sectionally using optimized voxel-based morphometry in 36 females taking AI drugs as arthritis or pain medication and 36 age- and education-matched female controls. Although mean gray and white matter volume differences between AI drug users and the non-AI group were small, AI drug use interacted with age, such that the non-AI group showed significantly greater age-related volume changes in regions of both gray and white matter compared to the AI drug users. These regions included the superior and medial frontal gyri, middle and inferior temporal gyri, fusiform and parahippocampal gyri, and occipital gray matter as well as temporal, parietal, and midbrain white matter. The results are consistent with the notion that AI drugs provide protection against age-related changes in brain volume. It is possible that inflammation plays a role in volume decreases associated with normal aging, and that suppressing the inflammatory response moderates this decrease.

Low-temperature 2H NMR spectroscopy of phospholipid bilayers containing docosahexaenoyl (22:6 omega 3) chains.

Polyunsaturated fatty acids are widely distributed components of biological membranes and are believed to be involved in many biological functions. However, the mechanisms by which they act on a molecular level are not understood. To further investigate the unique properties of omega 3 polyunsaturated phospholipid bilayers, deuterium nuclear magnetic resonance (2H NMR) studies have been made of the liquid-crystalline (L alpha) and gel phases of a homologous series of mixed-chain phosphatidylcholines containing docosahexaenoic acid: (per-2H-n:0)(22:6)PC, where n = 12, 14, 16, and 18. The moments of the 2H NMR lineshapes have been evaluated, and from these the warming and cooling main phase transition temperatures were determined. The transition temperatures of the mixed-chain series were found to be significantly lower than those of the corresponding lipids in the disaturated series, di(per-2H-n:0)PC, with hystereses ranging from 2 to 14 degrees C. Distinct effects of the docosahexaenoyl chain on bilayer order were found, though these effects varied across the mixed-chain series. In evaluating the moment data, an empirical method for normalizing the moments with respect to differences in temperature was applied, in addition to using the reduced temperature method. For the systems studied here, the method of normalization had no significant effect on the interpretation of the moment data.

Two-dimensional 1H-NMR of transmembrane peptides from Escherichia coli phosphatidylglycerophosphate synthase in micelles.

Two 28-residue peptides, PTLLTLFRVILIPFFVLVFYKKKGKKKG [Pgs-(6-25)-peptidyl-KKKGKKKG; Pgs peptide A] and VEYAGIALFFVAAVLTLWSMLQYLSAAR [Pgs-(149-176)-peptide, Pgs peptide E], were synthesized and studied by CD and two-dimensional 1H-NMR spectroscopy. The first 20 amino acid residues of Pgs peptide A are identical to one predicted transmembrane segment (Pro6-Tyr25) of the integral membrane protein phosphatidylglycerophosphate synthase (Pgs) of Escherichia coli. Pgs peptide E is identical to another predicted transmembrane segment (Val149-Arg176), which is located in the C-terminal end of this lipid synthase. Pgs peptides A and E were dissolved in methanol or trifluoroethanol or were incorporated into solvent-free micelles of fully deuterated SDS. In all these systems, CD spectra of both peptides indicated an alpha-helical secondary structure. However, peptides that were solubilized in micelles exhibited the highest content of alpha-helix as judged from comparison of the CD spectra. Thermodynamically stable isotropic solutions at high peptide concentrations (1-3 mM) could only be obtained with the peptide incorporated in micelles; in organic solvents, significant peptide aggregation occurred. Relatively sharp peaks were obtained with 1H-NMR spectroscopy of the peptides in SDS micelles, which indicates rapid tumbling of the peptides in the micellar environment. Translational-diffusion coefficients of the micelles with and without peptide, determined by pulsed-field-gradient NMR, showed that the micellar size was unaffected by the solubilized peptide. The radius of the hydrated micelles was estimated to be about 2.7 nm (i.e. the mass of the aggregate is almost 30 kDa). Two-dimensional NMR spectroscopy of both peptides solubilized in the micelles indicated an alpha-helical conformation. This observation is strengthened by an investigation of the hydrogen exchange of the peptide amide protons, where significantly less exchange of the amide protons was observed in the middle of the peptides compared with the ends.

Analysis and comparison of motion-correction techniques in diffusion-weighted imaging.

Motion continues to be a significant problem in MRI, producing image artifacts that can severely degrade image quality. In diffusion-weighted imaging (DWI), the problem is amplified by the presence of large gradient fields used to produce the diffusion weighting. Three correction methods applicable for correction of specific classes of motion are described and compared. The first is based on a generalised projection onto convex sets (GPOCS) postprocessing algorithm. The second technique uses the collection of navigator echoes to track phase errors. The third technique is based on a radial-scan data acquisition combined with a modified projection-reconstruction algorithm. Although each technique corrects well for translations, the radial-scan method proves to be more robust when more complex motions are present. A detailed description of the causes of MR data errors caused by rigid body motion is included as an appendix.

High-resolution diffusion imaging with DIFRAD-FSE (diffusion-weighted radial acquisition of data with fast spin-echo) MRI.

A novel MRI method, DIFRAD-FSE (diffusion-weighted radial acquisition of data with fast spin-echo), is demonstrated that enables rapid, high-resolution multi-shot diffusion-weighted MRI without significant artifacts due to motion. Following a diffusion-weighting spin-echo preparation period, multiple radial lines of Fourier data are acquired using spin-echo refocusing. Images can be reconstructed from the radial data set using a magnitude-only filtered back-projection reconstruction algorithm that removes phase errors due to motion. Results from human brain imaging demonstrate the ability of DIFRAD-FSE to acquire multiple radial lines of Fourier data each TR period without significant artifacts due to relaxation and to produce high-resolution diffusion-weighted MRI images without significant artifacts from motion.

Thermotropic phase properties of 1,2-di-O-tetradecyl-3-O-(3-O-methyl- beta-D-glucopyranosyl)-sn-glycerol.

The hydration properties and the phase structure of 1,2-di-O-tetradecyl-3-O(3-O-methyl-beta-D-glucopyranosyl)-sn-glycerol (3-O-Me-beta-D-GlcDAIG) in water have been studied via differential scanning calorimetry, 1H-NMR and 2H-NMR spectroscopy, and x-ray diffraction. Results indicate that this lipid forms a crystalline (Lc) phase up to temperatures of 60-70 degrees C, where a transition through a metastable reversed hexagonal (Hll) phase to a reversed micellar solution (L2) phase occurs. Experiments were carried out at water concentrations in a range from 0 to 35 wt%, which indicate that all phases are poorly hydrated, taking up < 5 mol water/mol lipid. The absence of a lamellar liquid crystalline (L alpha) phase and the low levels of hydration measured in the discernible phases suggest that the methylation of the saccharide moiety alters the hydrogen bonding properties of the headgroup in such a way that the 3-O-Me-beta-D-GlcDAIG headgroup cannot achieve the same level of hydration as the unmethylated form. Thus, in spite of the small increase in steric bulk resulting from methylation, there is an increase in the tendency of 3-O-Me-beta-D-GlcDAIG to form nonlamellar structures. A similar phase behavior has previously been observed for the Acholeplasma laidlawii A membrane lipid 1,2-diacyl-3-O-(6-O-acyl-alpha-D-glucopyranosyl)-sn-glycerol in water (Lindblom et al. 1993. J. Biol. Chem. 268:16198-16207). The phase behavior of the two lipids suggests that hydrophobic substitution of a hydroxyl group in the sugar ring of the glucopyranosylglycerols has a very strong effect on their physicochemical properties, i.e., headgroup hydration and the formation of different lipid aggregate structures.

Gadolinium-containing copolymeric chelates--a new potential MR contrast agent.

RATIONALE AND OBJECTIVES: To develop and partially characterize a new class of potential blood pool magnetic resonance (MR) contrast agents. METHODS: Various copolymeric chelates of gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) were prepared with differing molecular weights of polyethylene glycol (PEG) or polypropylene glycol (PPG) as linkers between the monomeric chelate units. Gadolinium content of the polymeric chelates was determined by atomic absorption spectra. Relaxivity of the polymeric chelates was measured at 1.5 Tesla and compared with Gadolinium DTPA. MR angiography (MRA) was performed in rabbits comparing Gd-DTPA with Gd copolymers. RESULTS: The gadolinium content of the copolymeric chelates ranged from 2.95 to 22.2% on weight basis. The molecular weight of the PEG linkers in the copolymers ranged from about 150 to about 3400. The r1 (1/T1, mM(-1) s(-1)) for Gd DTPA = 4.1. The r1 values for the different Gd-containing polymers ranged from 3.8 to 5.8, with the lowest r1 for the polymer prepared with the lowest-molecular-weight complex. The higher-molecular-weight complexes resulted in moderately higher relaxivity. MRA with Gd-copolymers, in rabbits, showed markedly greater vascular enhancement relative to an equivalent dose of Gd-DTPA. Vascular enhancement was much more sustained with the copolymeric agent and confined to vascular space; i.e. no appreciable background tissue enhancement--a reflection of distribution into extravascular fluid space--was observed. CONCLUSIONS: Relative to Gd-DTPA monomers, PEG-containing Gd DTPA polymeric complexes provided moderate increases in relaxivity but markedly greater efficacy during in vivo MRA. In vitro relaxivity studies of Gd-copolymers showed only an approximately 50% increase in r1 relaxivity compared with Gd-DTPA. The PEG-containing complex's lack of rigidity may have diminished the effect of spin diffusion on relaxation, thereby accounting for this modest increase. The greater efficacy of Gd-copolymers during in vivo MRA may reflect compartmentalization within the vascular space and possibly enhanced relaxation of the macromolecular copolymers in the blood. Gd-copolymers are promising agents that merit additional study.

MRI and NMR spectroscopy of the lipids of atherosclerotic plaque in rabbits and humans.

The early stages of atherosclerosis are characterized by the deposition of cholesteryl esters and triglycerides into the arterial wall. In the excised human atherosclerotic plaque these lipids are in a liquid-like state at body temperature and observable via MRI and NMR spectroscopy. To assess the ability of MRI to quantitatively image the lipids of atherosclerotic plaque in vivo, we have investigated eight New Zealand White rabbits fed atherogenic diets (2 weight (wt)% cholesterol, 1 wt% cholesterol + 6 wt% peanut oil, and 1 wt% cholesterol + 6 wt% com oil). Postmortem examination indicated that all rabbits developed atherosclerosis in the aorta. Except for one animal, magnetic resonance angiography showed no noticeable obstruction in the aorta. MRI was carried out in an attempt to image atherosclerotic plaque lipids directly, but no signal was detected in vivo. However, a plaque lipid signal was observed from excised tissue using a small diameter RF coil. 1H NMR spectroscopy of the atherosclerotic plaque from excised aortas indicated that the major fraction of plaque lipids in rabbits is not in a liquid state at physiological temperature and are only marginally MRI-visible compared to human plaque lipid. The differences in the MRI characteristics of rabbit and human plaque are due to differences in the fatty acid profile of the cholesteryl esters, chiefly a decrease of linoleic acid in rabbit lesions.