Orientation of the headgroup of phosphatidylinositol in a model biomembrane as determined by neutron diffraction.

Derivatives of the sodium salt of dimyristoylphosphatidylinositol (DMPI) have been synthesized specifically deuterated in the headgroup. A 50:50 (molar) mixture of DMPI with dimyristoylphosphatidylcholine (DMPC) hydrated to the level of 16 waters/lipid gives a biomembrane-like Lalpha phase at 50 degrees C. Comparison of the neutron diffraction scattering profiles for deuterated and undeuterated membranes allowed the depth of each deuterium (hydrogen) within the bilayer to be determined to +/-0.5 A. This gave the orientation of the inositol ring which lies more-or-less along the bilayer normal projecting directly out into the water. This orientation is similar to that of the sugar residue in glycolipids and confirms previous models for PI. On the assumption that the (P)O-DAG bond is more-or-less parallel to the bilayer normal, it is consistent with a roughly trans, trans, trans, gauche- conformation for the glyceryl-phosphate-inositol link. In the case of DMPI, it is the C4-hydroxy group which is most fully extended into the water layer, but when this is phosphorylated, the inositol ring turns over and tilts so that the C5-hydroxy group is now the one furthest extended into the water layer. Hence, at each stage in the pathway PI --> PI-4P --> PI-4,5-P2, it is the hydroxy position most exposed to the water which undergoes phosphorylation. Whereas the orientation of the inositol ring in DMPI can be seen simply as maximizing its hydration, the tilt of the ring in DMPI-4P cannot be explained in this way. It is suggested that it is due to an electrostatic interaction.

The headgroup orientation of dimyristoylphosphatidylinositol-4-phosphate in mixed lipid bilayers: a neutron diffraction study.

The trisodium salt of dimyristoylphosphatidylinositol-4-phosphate (DMPI-4P) has been synthesised specifically deuterated at particular sites in the headgroup. These materials have been used in neutron diffraction experiments, which successfully located the position (depth) of each of these deuterated sites to within +/- 0.5 A in a mixed model membrane (a 1:1 molar mixture of DMPI-4P with dimyristoyl-phosphatidylcholine, DMPC, in the L alpha phase, hydrated to the level of 28 water molecules per lipid molecule). The diffracted intensities were measured at four different D2O/H2O ratios and six orders of diffraction were obtained. These data sets, in conjunction with computer modelling, have been used to determine the orientation of the inositol ring of DMPI-4P, localising each vertical H-H distance to within approximately +/- 0.03 A. The orientation of the inositol ring is found to be one in which the C5 hydroxyl is extended out into the aqueous medium. This is, therefore, the most accessible site for water-borne reagents. This may be significant for the important pathway leading from PI-4P to PI-4,5P2. On the assumption that the P/ODAG bond is orientated parallel to the bilayer normal, these results are consistent with two possible conformations for the portion of the headgroup connecting the diacylglycerol to the inositol ring. Distinction between these two is difficult, but one may be favoured since the other involves close atom-atom contacts.

Neutron diffraction reveals the orientation of the headgroup of inositol lipids in model membranes.

Neutron diffraction studies show that the inositol ring in the headgroup of phosphatidylinositol extends perpendicular to the membrane surface but that phosphorylation of the 4-position causes the ring to tilt over.