THE CONFORMATIONAL BEHAVIOR OF PHOSPHATIDYLINOSITOL IN MODEL MEMBRANES - H-2-NMR STUDIES

Dimyristoylphosphatidylinositol (DMPI) has been synthesised with the appropriate natural stereochemistry and labelled with deuterium at specific sites in the D-myo-inositol headgroup. H-2-NMR spectroscopy of DMPI in its lamellar phase at a molar ratio of water-to-lipid R(W/L) of 129 and at 70-degrees-C reveals quadrupolar splittings DELTAnu of 3.83 and 2.17 kHz, respectively, for the five axially oriented C-D bonds and the single equatorially oriented C-D bond of the D-myo-inositol headgroup. Between R(W/L) ratios of 129 and 210 and between 30-degrees-C and 80-degrees-C the value of the ratio of these splittings DELTAnu(ax)/DELTAnu(eq) varies significantly (between 1.17 and 4.38). If it is assumed that, at a particular temperature, there is a single preferred orientation of the inositol headgroup, and that motion of the DPMI molecule establishes axial symmetry with respect to the bilayer normal then the ratio of these quadrupolar splittings can be used to impose constraints on that orientation. For example, the data are inconsistent with a situation in which the inositol ring lies parallel to the membrane surface and are difficult to reconcile with an arrangement where the inositol ring lies perpendicular to the surface. Computational modelling identifies four possible 'tilted' orientations, ail of which are consistent with the data, and two of these allow good intramolecular hydrogen bonds to be formed. In one there is hydrogen bonding between the inositol C2-OH and the phosphate pro-R oxygen. This is close to the conformation previously identified as being dominant in DMSO solution (Bushby, R.J., Byard, S.J., Hansbro, P.M. and Reid, D.G. (1990) Biochim. Biophys. Acta 1044, 231-236).