Headgroup interactions in mixed phospholipid bilayers

Abstract

2H NMR methods have been used to study how bilayer-forming phospholipids interact with each other at the membrane surface. Aqueous dispersions of dimyristoyl-sn-phosphatidylcholine (Myr2-PtdCho), dimyristoyl-sn-phosphatidylethanolamine (Myr2-PtdEtn), and dimyristoyl-sn-phosphatidyl-3-glycerol, specifically deuterated at different positions in their headgroups, give well-resolved 2H NMR powder spectra. These spectra are characteristic of a lipid bilayer with quadrupole splittings that are sensitive to the amplitude of headgroup motion. In binary mixed bilayers of deuterated lipids with an unlabeled component, all parts of the deuterated headgroup monitor the presence of the second lipid from changes in the measured quadrupole splittings. The headgroups of the charged lipids, dimyristoyl-sn-phosphatidylserine and dimyristoyl-mn-phosphatidyl-3-glycerol, interact to the largest extent with the choline moiety of Myr2-PtdCho and the ethanolamine moiety of Myr2-PtdEtn, whereas a somewhat smaller but still marked alteration in headgroup motion was observed in Myr2-PtdCho/Myr2-PtdEtn mixtures. The large changes in the deuterium quadrupole splittings for the zwitterionic lipids after addition of a charged lipid suggest that either a strong perturbation in the hydrogen bonding occurs or changes take place in the water structure at the membrane surface, or possibly both.The lipid component of biological membranes is usually composed of a limited number of phospholipid types, which are identified by the chemical nature of their polar headgroup. Both the conformational and motional properties of lipid headgroups are well characterized for model membranes containing single phospholipid species, mainly on the basis of proton, deuterium, and phosphorus NMR and neutron diffraction studies (1)(2)(3)(4)(5). In particular, 2H NMR has been used to investigate conformational changes induced by ions, cholesterol, and anesthetics (6-8). However, limited information is available on lipid structure at membrane surfaces if more than one naturally occurring phospholipid type is present (9-12).Here we present results from 2H;NMR studies of fully hydrated bilayers of phospholipids deuterated in their headgroups, which suggest that strong interactions occur at the membrane surface between different lipids in binary mixed bilayers. In particular, phospholipids that carry a residual negative charge significantly change the measured quadrupole splittings for zwitterionic lipids. These observations imply that headgroup reorientations and motional amplitude changes can be induced in mixed lipid bilayers by alterations in the structured water layer at the membrane surface. MATERIALS AND METHODSMaterials. The following specifically deuterated dimyristoyl derivatives of sn-phosphatidylcholine (Myr2-PtdCho), phosphatidyl-3-glycerol (Myr2-PtdGro), and phosphatidylethanolamine (Myr2-PtdEtn) were synthesized as described (9): R-O-C2H2-C2Hr-N+(CH3)3 = Myr2-PtdCho-d4; R-O-CH2-CH2-N+(C2H3)3 = Myr2-PtdCho-d9; R-O-C2H2-C2Hg-N+H3 = Myr2-PtdEtn-d4; and ...

Effects of diacylglycerols on conformation of phosphatidylcholine headgroups in phosphatidylcholine/phosphatidylserine bilayers

“…A change in the average order parameters of the PC headgroup would be expected to shift both a-and ,B-splittings in the same direction and thus cannot account for the counterdirectional change (Scherer and Seelig, 1989). A similar change in PC headgroup quadrupole splittings has been reported by others investigating effects on membrane surface charge density of various charge carriers including metal ions (Brown and Seelig, 1977;Akutsu and Seelig, 1981;Altenbach and Seelig, 1984;Macdonald and Seelig, 1987), hydrophobic ions (Altenbach and Seelig, 1985), the membrane-bound anion SCN- (Macdonald and Seelig, 1988), charged amphiphiles (Seelig et al, 1987; Scherer and ), the zwitterionic lipophilic molecule phloretin (Bechinger and Seelig, 1991), charged phospholipids (Sixl and Watts, 1983;Scherer and Seelig, 1987), and peptides (Dempsey and Watts, 1987;Dempsey et al, 1989; Kuchinka and Roux et al, 1989; Beschiaschvili and Seelig, 1990a,b). These results have been used to develop a molecular voltmeter model (Scherer and Seelig, 1989) in which membrane-bound ions assert a Coulombic force on the P--N+ dipole of the choline moiety, repelling or attracting the positive nitrogen with respect to the membrane surface, which can cause changes in the conformation of the headgroup and alterations of average C-D bond angles with the molecular axis.…”

Interaction of Pulmonary Surfactant Protein SP-A with DPPC/Egg-PG Bilayers

“…In the phosphatidylcholine headgroup, the methylene bonded to the oxygen of the phosphate and the adjacent methylene are labeled as a and b, respectively. Counterdirectional shifts in the quadrupole splittings of the aand b-deuterons have been attributed to changes in the orientation of the phosphatidlycholine headgroup with respect to the bilayer normal (Sixl and Watts, 1983;Seelig et al, 1987;Scherer and Seelig, 1989). Incorporation of charged amphiphiles into a bilayer can influence the torque applied to the phosphatidylcholine headgroup electric dipole without substantially perturbing bilayer packing (Seelig et al, 1987;Scherer and Seelig, 1989).…”

Effects of pH and cholesterol on DMPA membranes: a solid state 2H- and 31P-NMR study

“…Because a linear correlation exists between the principal values of the 31p chemical shift tensor and the P-O bond length (Un and Klein, 1989), we believe that the small variations in the P-O bond length on protonation or deprotonation will cause small modifications, if any, in the principle tensor values of the H2PO4, H2PO , and pO2groups of DMPA in its three different ionization states. Regarding a change in orientation, it has already been demonstrated that the membrane surface charge density induces conformational changes of the PC headgroup (Sixl and Watts, 1983;Seelig et al, 1987;Scherer and Seelig, 1989). As a result, PCs are already described as behaving like "molecular voltmeters."…”