Comparison of metastable phase behavior of the complexes of dipalmitoyl phosphatidylglycerol with Mg+2 and myelin basic protein.

Both divalent cations and myelin basic protein (BP) induce metastable phase behavior in phosphatidylglycerol (PG). The stable phase of the divalent cation--PG complex is dehydrated cylinders or sheets of lipid in which adjacent bilayers are probably bridged by the divalent cation. The structure of the stable phase of the BP-PG complex is unknown. To help understand the metastable phase behavior in the BP-PG complex, it was compared with that of the MG+2-PG complex using fatty-acid spin labels and differential scanning calorimetry. The stable state of the BP-PG complex melts at a similar temperature and with a similar enthalpy as the Na+-salt form of the lipid, while the stable state of the Mg+2-PG complex melts 35 degrees higher than and with an enthalpy 63% greater than the Na+-salt form. BP causes pronounced motional restriction of a fatty acid spin labeled near the terminal methyl group in the metastable and stable states of its complex with PG. Computer resolution of spectra of fatty-acid spin labels in the Mg+2-PG complex revealed that the spectra characteristic of the stable state were exchange broadened, showing that the probes are not soluble in the stable dehydrated state of this complex. The much different behavior of the BP-PG complex suggests that its stable phase is not dehydrated, that the probes are not displaced from the complex, and that their behavior reflects the ability of hydrophobic residues of basic protein to penetrate into the bilayer while its basic residues bind electrostatically to the lipid polar head groups.