Diacylglycerol pyrophosphate (DGPP), a phosphorylated form of phosphatidic acid (PA) found in plants and yeast but not in mammals, is a minor lipid that accumulates transiently under various abiotic stresses and during biotic interactions. DGPP formation may attenuate PA content but DGPP itself might be a signalling lipid.
In this work we studied the interfacial behavior of DGPP and their mixture with palmitoyl oleoyl phosphatidic acid (PA) at the air-water interface at pH 8, where both lipids are negativelly charged (-2 and -1 respectively). DGPP forms expanded monolayers that collapse at 35 mN m-1 and 0.64 nm2 with compressibility values between 20 mN m-1 and 60 mN m-1. At the lift off, the surface potential is 150 mV, corresponding to a molecular dipole moment perpendicular to the interface of 0.14 D and at collapse, is 350 mV (0.22 D).
At pH 5, the isotherm is more condensed and collapses at 45 mN m-1 and 0.50 nm2, in agreement with a charge diminution.
The mixture of DGPP with its precursor PA forms non ideal mixtures, homogeneous at the microscopic level (0.06 mm2); the interaction among these lipids leads to a diminution of the mean molecular area and to an increase of the compressibility at all lateral pressures. The excess free energy of compression are negative (-1500 to -2100 J mol-1) for all the proportions analyzed. The dipole moment perpendicular to the interface of the molecules in the mixtures at high lateral pressures is similar to the value of pure PA.
In summary, the favorable interaction of PA and DGPP, leading to an increase of the film packing, may represent a structural signalling event involving DGPP.
