Polylysine-induced H-2 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, wi th phosphatidylserine-containing lipid bilayer membranes was investigated u sing H-2 NMR spectroscopy. Lys(30) and Lys(100) added to multilamellar vesi cles composed of (70:30) (mol:mol) mixtures of choline-deuterated 1-palmito yl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) + 1 -palmitoyl-2-oleoyl-sn-g lycero-3-phosphoserine (POPS) produced two resolvable (2) H NMR spectral co mponents under conditions of low ionic strength and for cases where the glo bal anionic lipid charge was in excess over the global cationic polypeptide charge. The intensities and quadrupolar splittings of the two spectral com ponents were consistent with the existence of polylysine-bound domains enri ched in POPS, in coexistence with polylysine-free domains depleted in POPS. Lys(5) however, yielded no H-2 NMR resolvable domains. Increasing ionic st rength caused domains to become diffuse and eventually dissipate entirely. At physiological salt concentrations, only Lys, no yielded H-2 NMR-resolvab le domains. Therefore, under physiological conditions of ionic strength, pH , and anionic lipid bilayer content, and in the absence of other, e.g., hyd rophobic, contributions to the binding free energy, the minimum number of l ysine residues sufficient to produce spectroscopically resolvable POPS-enri ched domains on the 2 H NMR millisecond timescale may be fewer than 100, bu t is certainly greater than 30.