| Sodium cholate (NaC) is a bio-surfactant that forms very small primary micelles in aqueous media. When phospholipids (PLs) are hosted within these micelles, their 31P NMR resonances are sharp but spectral overlaps do occur. To alleviate this problem, the impact of temperature and pH on chemical shifts (delta) was investigated. Not only is it possible to remove overlaps by controlling temperature and pH but the temperature dependence offers additional information for accurate assignments. The largest positive temperature coefficients (TCs) were observed for PLs with headgroups containing OH or NH3 + groups capable of forming water-bridged H-bonds with the phosphate group. The deprotonation of the NH3+ group in phosphatidylserine (PS) and phosphatidylethanolamine (PE) or of the phosphate group in phosphatidic acid resulted in significant increases in delta. Diacyl, alkyl-acyl, and alkenyl-acyl PLs with the same headgroup had comparable coefficients, but differed slightly in delta. Compared to their parent PLs, all lyso analogs exhibited smaller TCs; perhaps due to the presence of an intramolecular H-bond between the OH at the sn-2 position and the phosphate oxygen(s). The information acquired with PL standards was used to re-evaluate the PL composition of human lens membranes. Previous corrections based on the PL spectral analysis in an organic solvent were confirmed with the use of the NaC in aqueous media. The new assignments reveal that the PLs present in adult human lenses are, besides sphingolipids, ether-linked lyso analogs of PE and other glycero-PLs.;From experimental and theoretical studies of NMR shifts, a new and more detailed model has been formulated for the micellization of NaC. In the first micellization, the hydrophobic sides of more than two monomers come together to form a barrel-like micelle in which a "belt" of cooperative H-bonds involving water molecules and the OH groups in C7 and C12 surrounds the central region of the outer surface. In addition, the COO- in the 'tail' of one monomer may interact with the OH on C3 to form an anti-parallel sequence of possibly four or more monomers. This model provides more details than previous ones and should be tested with complementary biophysical methods. |
