Recently, it has been shown that 23 Na double-quantum-filtered result from the formation of the third-rank tensor T 3,1 which NMR spectroscopy can be used to detect anisotropic motion of evolves into both double-and triple-quantum coherence tenbound sodium ions in biological systems. The technique is based sors T 3,2 and T 3,3 , respectively. The even-rank tensor T 2,1 , on the formation of the second-rank tensor when the quadrupolar on the other hand, can be formed only in ordered systems, interaction is not averaged to zero. Using this method, anisotropic where the quadrupolar interaction is not averaged to motion of bound sodium in human and dog red blood cells was zero (4).
detected, and the effect was shown to depend on the integrity Using the method described above, anisotropic motion of of the membrane cytoskeleton. In the present study, multiplesodium ions was detected in a variety of biological systems, quantum-filtered techniques were applied in combination with a such as connective tissues-cartilage, tendon and skin, quadrupolar echo to measure the transverse-relaxation times, T 2f brain, and muscle (5-7) as well as in human and dog red and T 2s . Line fitting was performed to obtain the values of the residual quadrupolar interaction, which was measured for sodium blood cells (RBC) (8). In connective tissues, the anisotropic in a variety of mammalian erythrocytes of different size, shape, motion was related to the interaction of the sodium ions rheological properties, and sodium concentrations. Human unwith the fibrous collagen molecules (2, 9). The effect of sealed white ghosts were used to study sodium bound at the anisoanisotropy was present in the RBC ghosts and was not found tropic sites on the inner side of the RBC membrane. Modulations in the cytosol, and, thus, it was concluded that it stemmed of the conformation of the cytoskeleton by the variation of either from the interaction of the sodium ions with the membrane. the ionic strength or pH of the suspending medium caused drastic The plasma membrane of mammalian RBC is underlined changes in both the residual quadrupolar interaction and T 2f due by an ordered network, the cytoskeleton, comprising seven to changes in the fraction of bound sodium ions as well as changes different proteins and anchored to the inner side of the memin the structure of the binding sites. By combining the two spectrobrane. The major component of the cytoskeleton is spectrin, scopic parameters, structural change can be followed. The changes a fibrous protein, which like collagen (10) has tertiary strucin the structure of the sodium anisotropic binding sites deduced by this method were found to correlate with known conformational ture based on the triple helix motifs (11). The cytoskeleton changes of the membrane cytoskeleton. Variations of the medium is crucial for maintaining the biconcave shape of the RBC pH affected both the fraction of bound sodium ions and the strucand controls the deformation of the cells while passing ture of the anisotropic binding sites. Sodium and potassium were through capillaries (10). Evidence was provided that the shown to bind to the anisotropic binding sites with the same affineffect was dependent on the integrity of the cytoskeleton ity.