Electron paramagnetic resonance (EPR) measurements using various fatty acid spin labels were performed on membranes of intact human erythrocytes at physiological, and at low ionic strength. In the case of spin probes bearing the nitroxide near the polar head group, a less restricted motion at low ionic strength was seen than with those labels with a nitroxide deeper within the hydrophobic tail of the membrane. Although these data clearly show an influence of ionic strength on membrane structure, and possibly a modified protein-lipid interaction, they cannot be simply discussed in terms of an altered membrane fluidity.

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