Study of the effect of poly(L-lysine) on phosphatidic acid and phosphatidylcholine/phosphatidic acid bilayers by Raman spectroscopy

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Abstract
The effect of polylysine (PLL) on dimyristoylphosphatidic acid (DMPA), on dimyristoylphosphatidylcholine (DMPC), and on mixtures of these lipids was investigated by Raman spectroscopy. These results show that long polylysine. (Mr .apprxeq. 200000) increases the stability of the acyl chain matrix of DMPA to form a more closely packed structure with a stoichiometry of one lysine residue per PA molecule. On the other hand, short PLL (Mr 4000) destablilizes the PA bilayer, and the complex formed undergoes a gel to liquid-crystalline transition at a lower temperature than of the pure lipid. For both cases, we have observed that bound polylysine adopts a .beta.-sheet conformation as opposed to the .alpha.-helical structure previously found for dipalmitoylphosphatidylglycerol/long PLL complexes [Carrier, D., and Pezolet, M. (1984) Biophys. J. 46, 497-506]. The difference in the thermal behavior of complexes of DMPA with long and short polylysines is believed to be associated with the fact that in the complex the long polypeptide adopts the .beta.-sheet conformation over the whole range of temperatures investigated while the short one undergoes a change of conformation from .beta.-sheet of random coil upon heating. Therefore, the conformation of the lipid-bound polypeptides depends on the nature of the polar head group of the lipid, not only on its net charge, and it affects considerably the thermotropism of the lipid. On the other hand, both long and short polylysines show no affinity for phosphatidylcholine since the temperature profiles of DMPC and of DMPC/PLL complexes exhibit exactly the same behavior. When mixed with PA/PC mixtures, long PLL induces a partial lateral phase separation. One phase consits mainly of DMPC but contains some DMPA, and the second phase is richer in the PA component and is perturbed by PLL. On the contrary, short polylysine does not induce phase separation as both lipids of the mixture exhibit the same thermotropic behavior.

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