Sodium interaction with ordered structures in mammalian red blood cells detected by Na-23 double quantum NMR

Na-23 double and triple quantum filtered NMR spectra of intact dog and human red blood cells were measured with the pulse sequence 90 degrees-tau/2-180 degrees-tau/2-theta degrees-t1-theta degrees-t2(Acq). For theta = 90 degrees the triple quantum filtered spectra exhibited the typical multiple quantum filtered lineshape, characteristic of isotropic media, while the double quantum filtered ones presented a superposition of two signals, whose proportion depended on the creation time tau. This effect is due to the formation of both second and third rank tensors. The formation of the second rank tensor, T21 results from non-zero residual quadrupolar interaction and is related to the anisotropic motion of sodium ions. Measurements of the double quantum filtered spectra with theta = 54.7 degrees enabled the detection of the contribution of T21 exclusively. No residual quadrupolar interaction was detected for sodium in the cytoplasm, while unsealed ghosts displayed the double quantum filtered spectral pattern, similar to that of intact cells. The anisotropy of motion of the sodium at the plasma membrane of mammalian erythrocytes depended on the integrity of the cytoskeleton network. Theoretical analysis of the double quantum filtered spectra gave a value of residual quadrupolar splitting of approximately 20 Hz for intact unsealed ghosts. The data presented prove that double quantum filtering is a sensitive technique for detection of motional anisotropies in biological systems.