No abstract

To gain a better understanding of the biological roles of polyunsaturated phospholipids, deuterium (2H) NMR studies have been conducted of 1 -perdeuteriopalmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine, an asymmetric or mixed-chain saturated-polyunsaturated phospholipid, in the liquid crystalline (La) phase. The palmitoyl (16:O) chain at the glycerol sn-1 position was labeled with 2H by perdeuteration, whereas the polyunsaturated, docosahexaenoyl (22:6w3) chain at the sn-2 position was unlabeled, Le., protiated. The 2H NMR results were compared to studies of 1,2-diperdeuteriopalmitoyl-snglycero-3-phosphocholine, in which both the sn-1 and sn-2 palmitoyl chains were perdeuterated, as well as l-palmitoyl-2perdeuteriopalmitoyl-sn-glycero-3-phosphocholine, in which only the sn-2 chain was perdeuterated. Multilamellar phospholipid dispersions containing 50 wt % H 2 0 were employed, and 2H NMR spectra were obtained using quadrupolar echo methods at a magnetic field strength of 8.5 T. The experimental 2H NMR spectra were numerically deconvoluted (de-Paked) to yield subspectra corresponding to the parallel bilayer orientation with respect to the main applied magnetic field. The increased resolution of the de-Paked subspectra enabled profiles of the segmental order parameters of the individual C-2H bonds, denoted by lScD(i)1, to be derived as a function of chain position. Significant differences in the 2H N M R spectra and derived JScD!i)l profiles of the per-*H-16:0 chains of the polyunsaturated and saturated bilayers were found. Based on simplified statistical mechanical theories, the differences can be interpreted in terms of an increase in the configurational freedom of the palmitoyl chains in the polyunsaturated bilayer, relative to bilayers of phosphatidylcholines with two identical saturated chains. The increased configurational freedom may correspond to an increase in the equilibrium area per chain in the case of the polyunsaturated bilayer. Possible further interpretations of the results in terms of the thickness of the hydrocarbon region and the presence or lack of interdigitation of the polyunsaturated and saturated acyl chains are also briefly discussed. We conclude that the configurational properties of the acyl chains of polyunsaturated bilayers are significantly different from those of saturated phospholipid bilayers.

Development of high magnetic field MRI techniques is hampered by the significant artifacts produced by B0 field inhomogeneities in the excited slices. A technique, gradient-echo slice excitation profile imaging (GESEPI), is presented for recovering the signal lost caused by intravoxel phase dispersion in T2*-weighted images. This technique superimposes an incremental gradient offset on the slice refocusing gradient to sample k-space over the full range of spatial frequencies of the excitation profile. A third Fourier transform of the initial two-dimensional image set generates an image set in which the artifacts produced by the low-order B0 inhomogeneity field gradients in the sample are separated and removed from the high-order microscopic field gradients responsible for T2* contrast. Application to high field brain imaging, at 3.0 T for human and at 9.4 T for immature rat imaging demonstrates the significant improvement in quality of the T2*-weighted contrast images.

Natural abundance 13C spin-lattice (TI) relaxation time measurements are reported for unilamellar vesicles of 1,2-dipalmitoylphosphatidylcholine (1,2-dipalmitoyl-sn-glycero-3-phosphocholine), in the liquid crystalline phase, at magnetic field strengths of 1. 40, 1.87, 2.35, 4.23, 7.05, 8.45, and 11.7 tesla (resonance frequencies of 15.0, 20.0, 25.1, 45.3, 75.5, 90.5, and 126 MHz, respectively), and the results are compared to previous 2H T, studies of multilamellar dispersions. For both the 13C and 2H T, studies, a dramatic frequency dependence of the relaxation was observed. At superconducting magnetic field strengths (4.23-11.7 tesla), plots of the 13C TF1 relaxation rates as a function of acyl chain segment position clearly reveal the characteristic "plateau" signature of the liquid crystalline phase, as found previously from 2H NMR studies. The dependence of Tj ' on ordering, determined previously from 2H NMR, and the Tj' dependence on frequency, determined from both '3C and 2H NMR studies, suggest that a unified picture of the bilayer molecular dynamics can be provided by a simple relaxation law of the form T' l ATf + BS2 H W-'1/2. In the above expression, A and B are constants, SCH (=SC-D) is the bond segmental order parameter, and of is the nuclear Larmor frequency. The first (A) term includes contributions from fast, local segmental motions characterized by the effective correlation time Tf, whereas the second (B) term describes slower, collective fluctuations in the local ordering. The value of Tf 10-11 sec, obtained by extrapolating Tj' to infinite frequency, suggests that the segmental microviscosity of the bilayer hydrocarbon region does not differ appreciably from that of the equivalent n-paraffinic liquids of similar chain length.NMR techniques were among the first biophysical methods to be applied to lipid bilayers and biological membranes (1, 2). Perhaps the foremost progress in recent years has been made in applications of NMR lineshape analysis to studies of the molecular ordering and conformations of membranous lipids (3-7). Yet, in spite of early promise (8-11), the interpretation of nuclear spin relaxation experiments has not progressed similarly and, in fact, has remained an outstanding problem in membrane biophysics for more than a decade (cf. ref. 12). The bulk of previous work has involved spin-lattice (TI) relaxation time measurements, which are sensitive to details of the molecular motions in the MHz region. Perhaps the major justification for T, relaxation studies of membranes is their dual character as both solid-like and liquid-like materials. The analogies to simpler liquid crystals point to the necessity of obtaining both static and dynamic information in defining these systems and of distinguishing membranes from other classes of biological macromolecules, such as the globular and fibrous proteins and the nucleic acids. These latter biopolymers, while also rich in dvnamic behavior (13,14), appear to have fairly well-defined average structures that can be directly re...