Nuclear magnetic resonance studies of mobile and immobilized phospholipid membranes

Xiaoxing Qiu, Purdue University

Abstract

Model membranes formed by mobile and immobilized phospholipids were studied by $\sp{31}$P and $\sp1$H NMR. Four main research projects as described in the following four paragraphs were completed during this work. One of the main finding was that the magnetic field strength of modern NMR spectrometers (11.7 T) is sufficient to quantitatively orient phospholipid bilayers. Systematic studies of several phospholipid dispersions demonstrated that the magnetically induced orientation of phospholipid membranes is a general phenomenon. In addition, magnetic orientation effect depends on temperature, field strength, and liposome size. $\sp{31}$P NMR was also used to probe the interfacial properties of immobilized phospholipids on IAMs. The correlation times for internal rotation ($\tau\sb{\parallel}$) and wobbling ($\tau\sb{\perp}$) of the phospholipid headgroup were calculated from $\sp{31}$P NMR spectra. Immobilized phospholipid headgroups on IAM surfaces undergo rapid reorientation similar to the mobile phospholipids forming liposome membranes with $\tau\sb{\parallel}\sim 1$ ns. However, the immobilized phospholipids wobble with correlation times $\tau\sb{\perp}$ that depend on the solvent bathing the IAM surface. Based on these dynamic results, different interfacial structures of solvated IAMs are proposed. The obtained structural and motional properties will give insight in the surface recognition of IAMs. With the aim of synthesizing chemically stable IAM surfaces, a general synthetic route for preparing ether phospholipids (PL) suitable for immobilization was developed. This is the first report demonstrating that o-chlorophenyl dichlorophosphate (CPDCP) can be used as a mild phosphorylation reagent for the preparation of PL analogs. Phosphorylation with CPDCP is 50-90% efficient, but more important is that the protecting groups are stable during this critical synthetic step. Another important finding from this work was that long chain diacylated phosphocholine analogs can form micelle instead of bilayer if a heteroatom in its acyl chain. A antiviral phospholipid, AC2, was used for these studies. $\sp1$H NMR studies of sonicated dispersions of AC2 indicate that AC2 sonicated dispersions form micelles. Furthermore, $\sp{31}$P NMR lineshapes showed that the AC2 micelles extensively fuse into giant bilayer liposomes. These unusual membrane properties of sonicated AC2 dispersions may be important for the antiviral activity and metabolism of the phospholipids.

Degree

Ph.D.

Advisors

Pidgeon, Purdue University.

Subject Area

Analytical chemistry

COinS