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Surface chemical and nuclear magnetic resonance (NMR) techniques have been used to study the interaction of Ca2+ and lanthanides with lecithins. With both methods positive reactions were detected at metal concentrations > 0.1 mM. ¹H and 31P high-resolution NMR spectra obtained with single bilayer vesicles of lecithin were invariant up to Ca2+ concentrations of 0.1 M indicating that there is only a loose association between Ca2+ and the phospholipid. The weak interaction between Ca2+ and lecithin is confirmed by both surface chemical and NMR techniques showing that the packing of egg lecithin molecules present in bilayers does not change up to Ca2+ concentrations of about 0.1 M. The packing was also independent of pH between 1-10. Contradictory results have been reported in the literature concerning the question of Ca2+ binding to lecithins. The conflicting results are shown to have arisen from differences in the experimental conditions and differences in the sensitivity of the physical methods used by various authors to study Ca2+-lecithin interactions. An estimate of the strength of binding and molecular details of the interaction were derived using paramagnetic lanthanides as isomorphous replacements for Ca2+. From the changes in chemical shifts induced in the presence of lanthanides an apparent binding constant KA ≈ 30 l/mol was calculated at lanthanide concentrations > 10 mM. Using surface chemical methods it was shown that this KA is up to 10 times larger than that for Ca2+ binding. The complete assignment of the ¹H NMR spectrum of lecithin, including the resonances from the relatively immobilized glycerol group, was determined to derive molecular details of the cationlecithin interaction. From spin-lattice relaxation-time measurements and line broadening in the presence of GdCl3 it is concluded that the cations are bound to the phosphate group and that this is the only binding site. The absolute proton shifts induced by paramagnetic lanthanides depended on the nature of the ion, but the shift ratios standardised to the shift of the O3POCH2 (choline) signal were invariant throughout the lanthanide series indicating that the shifts are purely pseudocontact. In contrast the 31P shifts were found to contain significant contact contributions. These findings are consistent with a weak interaction and with the phosphate group being the binding site. The absolute shifts but not the shift ratios depended on the anion present indicating that the cation binding may be accompanied by binding of anions. Contrary to negatively charged phospholipids the interaction of lanthanides with lecithins was enhanced as the ionic strength was increased by adding NaCl. This was explained in terms of stearic hindrance due to the extended conformation of the lecithin polar group. [ABSTRACT FROM AUTHOR]

The morphology and structure of lipoprotein complexes reconstituted from apoprotein from porcine high-density lipoprotein and dimyristoyl lecithin have been studied by a range of physical techniques. At pH 5.6 and ionic strength I ... 0.1. disc-shaped particles are observed with a radius of 4.0-4.5 nm and a thickness of 4.36 ± 0.07 nm. the latter value being in good agreement with that of the average lipid bilayer thickness. The values for the number. weight and Z-average particle weights are Mn = 227000 daltons < Mw = 325000 daltons < Mz = 410000 daltons, respectively. and indicate the polydispersity of these complexes. The number-average particle weight is in good agreement with that of 236000 daltons determined from sedimentation and diffusion experiments. The latter particle weight is also consistent with the particle volume. determined from the dimensions of the disc. and the experimental partial specific volume of the complex. V25 = 0.917 ml/g. At the point inhere all the apoprotein is complexed the average lipid/protein molar ratio is 100 ± 6. Particle size and weight analysis indicate that each lipoprotein disc of radius 4.0-4.5 nm consists of 200 lipid and 2 apoprotein molecules. If lecithin is present in excess of the complex described above, all the lipid may interact with apoprotein up to molar lipid 'protein ratios of 200: 1 depending on the sonication conditions used. The lipoprotein discs undergo an ionic-strength-dependent monomer-dimer equilibrium with low ionic strength leading to the side-by-side aggregation of discs. At an ionic strength I ≈ 0.01 the disc radius is about twice that observed at I ≥ 0.1. The results with dimyristoyl lecithin — apoprotein complexes arc compared with those obtained with dilauroyl and dipalmitoyl lecithin — apoprotein complexes. Lecithin and apoprotein interact to form a loose. disc-shaped complex in which the apoprotein is oriented at the lipid-water (hydrophobic-hydrophilic) interface such that all the polar groups of both lipid and protein are readily accessible to the aqueous medium. Most of the lipid is present as a bilayer, however the cooperative motion of the lipid is significantly reduced due to the interaction with apoprotein. The latter shows a significant increase in α helical content from 50% to about 60-65% when complexed with lecithin. The dispersing (solubilizing) effect which the apoprotein has on unsonicated multilayers of lecithin is explained in terms of its surface activity and conformational properties which magi allow for the regional separation (accumulation) of opposite charges. [ABSTRACT FROM AUTHOR]