Lipid Protons Research Articles

Gradient reversal technique and its applications to chemical-shift-related NMR imaging.

The chemical-shift effect often degrades slice selectivity thereby degrading the contrast and spatial resolutions in a high-field NMR imaging. We propose a new pulse sequence which can compensate for this chemical-shift effect during slice selection. This technique provides correct slice definition as well as clear separation between water and lipid protons in high-field proton NMR imaging. Particular applications of this technique are 2D imaging with a thin-slice selection and chunk 3D imaging, where correction of the chemical-shift effect in slice selection is important. Another interesting application of the technique is chemical-shift-selective (CHESS) imaging. This technique does not require spectral-selective rf pulses and it takes advantage of the chemical-shift effect during slice selection. The latter is found to be useful for human in vivo chemical-shift imaging in high-field NMR. In this paper, the theoretical derivation of the proposed gradient reversal technique, its applications, and related experimental results are presented.

In vivo proton spin-lattice relaxation times of normal and dystrophic muscles.

In vivo spin-lattice relaxation times (T1) of water and lipid protons were measured in normal and dystrophic chicken pectoralis muscles at different ages. Values were obtained with a surface coil used as both a receiver and a transmitter. A 2 theta-T1-theta-Acquisition sequence was used for these measurements. Accuracy was verified with an inversion-recovery method using a slotted tube resonator as the transmitter and a surface coil as the receiver. It was observed that the T1 values of water protons in normal muscles decrease with age, the T1 values of water protons do not change with age in dystrophic muscles, and the T1 values of lipid protons increase with age in normal and dystrophic muscles. These results indicate a failure of the normal maturation of dystrophic muscles.

Relaxation in bile: Evidence and analysis of cross-relaxation between water and lipid protons

Relaxation in bile: Evidence and analysis of cross-relaxation between water and lipid protons

Gallbladder bile: an experimental study in dogs using MR imaging and proton MR spectroscopy.

Magnetic resonance (MR) imaging, proton MR spectroscopy, and biochemical analysis were performed to investigate MR signal intensity (SI) differences between concentrated and dilute gallbladder bile of seven fasting and five sincalide-treated dogs. MR images revealed high SI from bile of fasting dogs and low to medium SI in sincalide-treated dogs when spin-echo (SE) pulse sequences with repetition rates of 0.5 and 2.0 sec were used. Proton MR spectra were similar for fasting and sincalide-treated dogs. In fasting dogs, water content in the bile was slightly lower, and cholesterol, phospholipid, and bile acid concentrations were higher. More than 90% of proton signals in all Fourier transform free induction decay spectra emanated from water molecules, and no lipid proton resonances were detected in Fourier transform SE spectra after tau delays of 7 msec. These results indicate that the differences in SI are caused by alterations in relaxation times of water protons, possibly resulting from the interactions of water protons and macromolecules.

High resolution proton magnetic resonance of sonicated phospholipids

We have recorded high resolution proton magnetic resonance spectra of sonicated phospholipid vesicles. The following lipids were used in separate experiments: phosphatidylglycerol, phosphatidylserine, phosphatidylethanolamine, and phosphatidylcholine from egg yolk as well as dimyristoyl phosphatidylcholine. Mixed lipid vesicles were also investigated. Assignments of the peaks associated with the various protons of the different lipids are presented. It is shown that in favorable cases, it is possible to resolve the different phospholipid head groups of mixed lipid samples. Spin lattice relaxation times (T1) of each peak were collected at 500 MHz and 90 MHz. The influence of the addition of a small concentration of spin labeled phospholipid on i) the linewidths ii) the spin lattice relaxation times, was determined. It is shown that nitroxide radicals selectively broaden the peaks associated with the protons localized at a comparable depth of the bilayer. On the other hand, T1 are less selectively perturbed. Potential applicability of 1H-NMR for the investigation of lipid-proton specificity in membranes is discussed.

Selective Saturation NMR Imaging

The application of selective saturation (or solvent suppression) techniques in nuclear magnetic resonance (NMR) imaging offers the opportunity to significantly expand the range of NMR studies. Data acquired at 1.44 T are presented using a two-dimensional spin-echo sequence preceded by a selective (saturating) radiofrequency pulse. Individual water or lipid proton resonances were eliminated (greater than 90% reduction in signal intensity) resulting in images of H2O or -CH2- distribution with resolution and imaging time equivalent to conventional proton images. Data are also presented demonstrating the feasibility of using selective saturation to image proton metabolites at low concentrations with a three-dimensional chemical shift imaging approach. Lactate was investigated because of its importance in the pathophysiology of ischemic insult. Phantom studies without solvent suppression failed to detect lactate at 80 mM; however, with solvent suppression, lactate at 40 mM was imaged in a reasonable time (approximately 50 min). With the favorable NMR characteristics of the methyl protons of lactate and with improvements in imaging systems, this technique may play an important role in the noninvasive evaluation of tissue ischemia using 1H NMR.

Proton spin relaxation studies of fatty tissue and cerebral white matter

Proton spin longitudinal ( T 1) and transverse ( T 2) relaxation and proton density studies were carried out on human fatty tissue and bovine white matter, both in the native state and after immersion in D 2O. It is concluded that nuclear magnetic resonance signals from fatty tissue result mainly from methyl and methylene protons of hydrocarbons. No contribution from lipid protons could be detected for white matter, although it contains a high percentage of lipids. Imaging experiments, resulting in T 1, T 2, and proton density maps, support the results obtained with spectroscopic relaxation studies.

Nuclear magnetic resonance: in vivo proton chemical shift imaging. Work in progress.

Using a three-dimensional Fourier transform approach, proton nuclear magnetic resonance (NMR) chemical shift images have been obtained in vivo for the first time. At a proton resonance frequency of 61.5 MHz, chemical shift-resolved images of simple phantoms indicate that a spectral resolution of 0.7 parts per million (ppm) is readily achievable at all locations within the image matrix, even when using a magnet not specifically designed for chemical shift spectroscopy. In vivo images of the human forearm and of a cat head yield separable signals from water and lipid protons. However, using simple radiofrequency pulse sequences, our data show that relatively little signal originates from membrane lipids (e.g., myelin) in the brain. The measurement of magnetic susceptibility using this technique is also demonstrated. While helping to elucidate the genesis of the NMR response in complex biological systems, this methodology also has potential applications in medical diagnosis. The technique is also applicable to the chemical shift imaging of other nuclei; for example, phosphorus (P-31).

Re-examination of rhodopsin structure by hydrogen exchange.

The hydrogen exchange behavior of rhodopsin was re-examined by studies of the protein in the disc membrane and after solubilization in octyl glucoside. The methods used measure either the peptide hydrogens alone (hydrogen-deuterium exchange by infrared spectroscopy) or all slowly exchanging hydrogens (hydrogen-tritium exchange by hel filtration). Under mild exchange conditions, disc membranes and solubilized lipid-free proteins show very similar exchange behavior, indicating the absence of slowly exchanging lipid protons. At high temperature, exchange of an additional large group of very slow peptide NH can be detected. The total number of slow hydrogens significantly exceeds the amide content, and apparently includes slowly exchanging protons from perhaps 40% of the protein's non-amide side chains. This is thought to require the involvement of many polar side chains in internal H-bonding. The exchange rates of the non-amide side chains sites have not been determined. However, to the extent that these contribute to the fast time region of the measured kinetic H-exchange curve, previously identified with exposed, non-H-bonded peptides, the estimate of freely exposed rhodopsin peptides must be reduced. The fraction of free peptides could range from a remarkably high value of 70% down to about 45%.

Proton relaxation study of molecular motions in low density lipoproteins

Molecular motion and molecular organization of human serum low-density lipoprotein (LDL) has been studied in the temperature range − 30 to 30°C by proton magnetic relaxation. LDL in deuterated Tris-HCl buffer exhibit two mobile phases. The slow-relaxing phase ( T 1 ⋍ 1.5 s ) is assigned to the incompletely deuterated water of the buffer, and the fast-relaxing phase ( T 1 ⋍ 60 ms ) to the fatty acid chains of the lipoprotein core. It has been established that there is a correlation between the state of the outer surface and the interior of the LDL particle: the number of fast-relaxing protons is significantly altered by cooling the system through the freezing point of the buffer or by selecting buffers of different ionic strengths. At room temperature, ∼ 30% of the lipid protons of LDL in the 0.1 m buffer and ∼ 40% of the lipid protons of LDL in the 0.01 m buffer relax quickly within the time-scale of n.m.r. frequency (24 MHz).

Diffusion in rigid bilayer membranes. Use of combined multiple pulse and multiple pulse gradient techniques in nuclear magnetic resonance

Combined NMR multiple pulse homonuclear decoupling and multiple pulse gradient techniques have been used to determine the self-diffusion coefficients at 25/sup 0/C of the phospholipids in the L/sub ..beta..'/(gel) phase of L-..cap alpha..-dipalmitoylphosphatidylcholine in a 15% (w/w) D/sub 2/O model membrane and of potassium oleate in a 30% (w/w) D/sub 2/O lamellar phase (above the phase transition). The values found, 1.6 x 10/sup -10/ and 1.3 x 10/sup -8/cm/sup 2/S/sup -1/, respectively, are considered in reasonable agreement with values obtained by other investigators using fluorescence photobleaching recovery. The technique has the advantage that it monitors lipid protons and hence avoids possible complications of added probes. The use and limitations of the method are discussed, and the values found for diffusion are compared with those determined or estimated by other methods.

Hydrogen bond among the ionic groups of ampholytic phospholipid

The nature of interaction among the ionic groups of lipids was studied by proton NMR measurements for ampholytic analogues of phospholipids, each differing in structural separation between PO 4 − and NH 3 + groups. Single resonance lines observed for the ionic protons of lipids in deuterochloroform indicate the rapid exchange of protons between PO 4H and NH 3 +, and showed that the two groups interact intramolecularly and/or intermolecularly by forming a hydrogen bond. The hydrogen bond is found to be weak for dipalmitoylphosphatidyl-propanolamine and -butanolamine but strong for both the -ethanolamine and -pentanolamine. The hydrogen bond counteracts with calcium binding and is almost broken for calcium complexes of the lipids.

Nuclear magnetic resonance studies of the polypeptide alamethicin and its interaction with phospholipids

High resolution nuclear magnetic resonance spectroscopy is used to study alamethicin (a cyclic polypeptide antibiotic which induces interesting ion transport properties in both natural and artificial membranes) and the interaction of alamethicin with phospholipids. The aggregation of alamethicin in aqueous solution at neutral pH is largely unaffected by Na +, K + or Ca 2+ ions, and the nuclear magnetic resonance spectrum of the polypeptide is not changed upon formation of complexes with these ions either in deuterium oxide or in organic solvent. Large changes are induced in the nuclear magnetic resonance spectra of sonicated aqueous dispersions of phosphatidylcholine and phosphatidylserine on the addition of small quantities of alamethicin. The peaks from the lipid protons are broadened, indicating a loss of freedom of motion of the lipid molecules. The effect on phosphatidylserine dispersions is the larger, and it is estimated that each alamethicin molecule induces a transition of about 600 lipid molecules from the normal multilamellar state to some new form of aggregate. Particle size measurements, using analytical column chromatography and ultracentrifugation, show that the reduction in motion of the lipid molecules is not due to a decrease in tumbling rate, but is probably primarily due to hydrophobic interactions. This is confirmed by electron spin resonance spin label experiments. X-ray diffraction and electron microscopy experiments reveal that the phospholipid multilamellar structure is broken down by the alamethicin. Electrophoresis shows that, in the new structure, the alamethicin molecules are oriented with their polar side-chains at the lipid-water interface. These results are significant for an understanding of the general field of lipid-peptide interactions, and may help to explain the antibiotic effect of alamethicin and similar molecules and its effects on the ion transport properties of membranes.