Lipid-Protein Interactions: From Crystalline Lipoproteins to Intact Membranes
The molecular structure of phospholipid molecules suggests at least three types of interactions: (1) ionic interactions of the polar lipid headgroups with metal ions and other charged molecules dissolved in the aqueous phase, (2) lipid-water interactions (hydration) at the level of the polar groups and the glycerol backbone region, and (3) hydrophobic interaction of the fatty acyl chains with non-polar molecules. 2H-NMR is a particularly sensitive method to detect such interactions as is illustrated in figure 1 for the binding of metal ions to bilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). In these experiments the choline head group was selectively deuterated at the α-CH2 segment and the 2H-NMR spectra were recorded as a function of metal ion concentration at 59 °C . In the absence of ions the quadrupole splitting of the α-segment was 6 kHz, but decreased considerably upon addition of ions. Monovalent ions like Na+ and K+ had only small effects (of about a few hundred Hz) on the quadrupole splitting, whereas multivalent ions induced changes up to the order of 10 kHz. 2H-NMR may thus be used as a convenient means to monitor ion binding to bilayer membranes . If the ion-binding studies are repeated with the deuterium label in the fatty acyl chains, no change in the quadrupole splitting is observed. The effect of ion-binding is thus limited to a conformational change in the head group region of the phosphatidylcholine bilayer.
KeywordsHead Group Quadrupole Splitting Glycerol Backbone Fatty Acyl Chain Deuterium Label
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