Nuclear magnetic resonance and calorimetric study of the structure, dynamics, and phase behavior of uranyl ion/dipalmitoylphosphatidylcholine complexes

Abstract

The interaction of UO2(2+) with dipalmitoylphosphatidylcholine (DPPC) has been studied as a function of temperature and composition using nuclear magnetic resonance (NMR) spectroscopy, differential scanning calorimetry (DSC), and monolayer studies. Computer simulations of the 31P-NMR powder spectra of DPPC dispersions in the presence of various concentrations of UO2(2+) are consistent with the binding stoichiometry of [UO2(2+)]/[DPPC] = 1:4 at [UO2(2+)]/[DPPC] less than 0.3. This complex undergoes a phase transition to the liquid crystalline phase at T'm = 50 +/- 3 degrees C with a breadth delta T'm = 7 +/- 3 degrees C. This broad transition gradually disappears at higher UO2(2+) concentrations, suggesting the presence of yet another UO2(2+)/DPPC complex (or complexes) whose NMR spectra are indistinguishable from those of the 1:4 UO2(2+)/DPPC species. The temperature-dependent 13C powder spectra of 2(1-13C) DPPC dispersions in the presence of 1.2 mol ratio of UO2(2+) show that this higher order complex (complexes) also undergoes a phase transition to the liquid crystalline state at T'm +/- = 58 +/- 3 degrees C with a breadth delta T"m = 15 +/- 5 degrees C. The NMR spectra indicate that exchange among these various UO2(2+)/DPPC complexes is slow. In addition, computer simulations of the 31P-, 13C-, and 2H-NMR powder spectra show that axial diffusion of the DPPC molecules about their long axes is quenched by addition of UO2(2+) and acyl chain isomerization is the dominant motional mode. The isomerization is best described as two-site hopping of the greater than C-D bond at a rate of approximately 10(6) s-1, a motional mode which is expected for a kink diffusion.

Microscopic Methods to Observe the Distribution of Lipids in the Cellular Membrane

“…The membrane disturbance in thawing cryosections may be alleviated with uranyl acetate, which can stabilize phospholipids by binding to the phosphate group. , The simplest method is to add uranyl acetate to the solution that is used to retrieve cryosections (Figure a). By this method, labeling of cholesterol is confined to the membrane …”

“…The effect of uranyl acetate is based on the binding of uranyl ions to the phosphorus groups of phospholipids , (with the exception of phosphatidic acid). One uranyl ion can bind up to four lipid phosphorus groups, thereby effectively cross-linking membrane phospholipids and inhibiting lateral lipid diffusion. Additionally, binding of the positively charged uranyl ions to the membrane affects the polarity and extractability of the lipids.…”

Analysis of Lipid Droplets in Hepatocytes

“…Use of uranyl acetate at a certain step helps preserve membrane structure by binding to the phosphate group of phospholipids (Ginsburg & Wolosin, 1979;Huang, Blume, Das Gupta, & Griffin, 1988). Three different methods have been reported.…”

Microscopy of membrane lipids: how precisely can we define their distribution?

“…The cryosections prepared by this method (the Tokuyasu method [22]) may be used to label lipids, but as discussed above, quick-frozen specimens are more appropriate for observing lipid distribution in situ. In this method, uranyl acetate stabilizes phospholipids by binding to their phosphate groups [24,25]. In one method called RHM (rehydration method) [23], frozen samples are kept in acetone (or other organic solvents) containing uranyl acetate at a sub-zero temperature (e.g.…”