Polylysine-induced 2H NMR-observable domains in phosphatidylserine/phosphatidylcholine lipid bilayers

The interaction of three polylysines, LyS(5) (N = 5), LyS(30) (N = 30), and Lys(100) (N = 100), where N is the number of lysine residues per chain, with phosphatidylserine-containing lipid bilayer membranes was investigated using H-2 NMR spectroscopy. Lys(30) and Lys(100) added to multilamellar vesicles composed of (70:30) (mol:mol) mixtures of choline-deuterated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) + 1 -palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) produced two resolvable (2) H NMR spectral components under conditions of low ionic strength and for cases where the global anionic lipid charge was in excess over the global cationic polypeptide charge. The intensities and quadrupolar splittings of the two spectral components were consistent with the existence of polylysine-bound domains enriched in POPS, in coexistence with polylysine-free domains depleted in POPS. Lys(5) however, yielded no H-2 NMR resolvable domains. Increasing ionic strength caused domains to become diffuse and eventually dissipate entirely. At physiological salt concentrations, only Lys, no yielded H-2 NMR-resolvable domains. Therefore, under physiological conditions of ionic strength, pH, and anionic lipid bilayer content, and in the absence of other, e.g., hydrophobic, contributions to the binding free energy, the minimum number of lysine residues sufficient to produce spectroscopically resolvable POPS-enriched domains on the 2 H NMR millisecond timescale may be fewer than 100, but is certainly greater than 30.