Configurational Statistics of Acyl Chains in Polyunsaturated Lipid Bilayers from ²H NMR

To gain a better understanding of the biological roles of polyunsaturated phospholipids, deuterium (²H) NMR studies have been conducted of l-perdeuteriopalmitoyl-2-docosahexaenoyl-5rt-glycero-3-phosphocholine, an asymmetric or mixed-chain saturated-polyunsaturated phospholipid, in the liquid crystalline (LJ phase. The palmitoyl (16:0) chain at the glycerol sn-1 position was labeled with²H by perdeuteration, whereas the polyunsaturated, docosahexaenoyl (22:6ω3) chain at the sn-2 position was unlabeled, i.e., protiated. The²H NMR results were compared to studies of 1,2-diperdeuteriopalmitoyl-s/z-glycero-3-phosphocholine, in which both the sn-1 and sn-2 palmitoyl chains were perdeuterated, as well as l-palmitoyl-2-perdeuteriopalmitoyl-wglycero-3-phosphocholine, in which only the sn-2 chain was perdeuterated. Multilamellar phospholipid dispersions containing 50 wt % H₂0 were employed, and²H NMR spectra were obtained using quadrupolar echo methods at a magnetic field strength of 8.5 T. The experimental²H 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|scd(0|, to be derived as a function of chain position. Significant differences in the²H NMR spectra and derived|scd(0I profiles of the per-²H-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.