Thermotropic phase behavior of monoglyceride-dicetylphosphate dispersions and interactions with proteins:: A 2H and 31P NMR study

The phase behavior of a 1-[H-2(35)]-stearoyl-rac-glycerol ([H-2(35)]-MSG)/dicetylphosphate (DCP) mixture and its interaction with beta-lactoglobulin and lysozyme were studied by H-2 and P-31 nuclear magnetic resonance (NMR). The behavior of the lipids was monitored by using deuterium-labeled [H-2(35)]-MSG as a selective probe for H-2 NMR and DCP for P-31 NMR. Both H-2 and P-31 NMR spectra exhibit characteristic features representative of different phases. In the lamellar phases, P-31 NMR spectra of DCP are different from the spectra of natural phospholipids, which is attributable to differences in the intramolecular motions and the orientation of the shielding tensor of DCP compared with phospholipids. The presence of the negatively charged amphiphile DCP has a large effect on the phase behavior of [H-2(35)]-MSG. At low temperature, the presence of DCP inhibits crystallization of the gel phase into the coagel. Upon increasing the temperature, the gel phase of [H-2(35)]-MSG transforms in the liquid-crystalline lamellar phase. In the presence of DCP, the gel phase directly transforms into an isotropic phase. The negatively charged beta-lactoglobulin and the positively charged lysozyme completely neutralize the destabilizing effect of DCP on the monoglyceride liquid-crystalline phase and they even stabilize this phase. Without DCP the proteins do not seem to interact with the monoglyceride. These results suggest that interaction is facilitated by electrostatic interactions between the negatively charged DCP and positively charged residues in the proteins. In addition, the nonbilayer-forming DCP creates insertion sites for proteins in the bilayer.