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Relaxation resulting from the modulation of dipolar interaction is commonly used for estimating distances in molecules in solutions. However, for most nuclei with spin I > ½ the single-quantum-transition relaxation by dipolar interaction is masked by quadrupolar relaxation. In the present study, it is shown that even in systems where single-quantum relaxation times are dominated by quadrupolar interaction, dipolar relaxation can be measured by following the -m $\leftrightarrow$ m transitions. This is demonstrated for 7Li in the complex [Li-Kryptofix 211]+X- (X = Cl, Br) dissolved in glycerol at temperatures for which slow motion prevails and no 1H-7Li NOE can be observed. The relaxation times that are most important for the assessment of the dipolar interaction of 7Li are -½ $\leftrightarrow$ ½ and -${{3}\over{2}}$ $\leftrightarrow$ ${{3}\over{2}}$ and they are measured by multiple-quantum-filtration techniques. For estimating the quadrupolar interaction, the relaxation times of the populations and those of the transitions ±½ $\leftrightarrow$ ±${{3}\over{2}}$ were measured. The longitudinal and transverse relaxation times of 6Li as well as the 1H-6Li NOE were also measured and, together with the 7Li measurements, were used to obtain the strengths of dipolar (D) and quadrupolar (chi) interactions. The experimental data were analyzed using several models to describe the motion. The model that gave the best fit and resulted in parameters that were physically meaningful encompassed a whole-body isotropic motion as well as internal anisotropic motion. For this particular model, the following values for the quadrupolar and the dipolar interactions strength were obtained: D(7Li)/2pi = 6.8 kHz, chi(7Li)/2pi = 85 kHz and D(6Li)/2pi = 1.4 kHz, chi(6Li)/2pi = 2.6 kHz. From the value of D, an estimate of the average lithium-proton distance was calculated to be 3.3 Å, which is in fair agreement with crystallographic studies. The sizes of the quadrupolar and dipolar interactions were independently confirmed by the 7Li NMR powder spectra of the complexes that were used for the solution studies.

Recently, it has been shown that 23Na double-quantum-filtered NMR spectroscopy can be used to detect anisotropic motion of bound sodium ions in biological systems. The technique is based on the formation of the second-rank tensor when the quadrupolar interaction is not averaged to zero. Using this method, anisotropic motion of bound sodium in human and dog red blood cells was detected, and the effect was shown to depend on the integrity of the membrane cytoskeleton. In the present study, multiple-quantum-filtered techniques were applied in combination with a quadrupolar echo to measure the transverse-relaxation times, T2f and T2s. Line fitting was performed to obtain the values of the residual quadrupolar interaction, which was measured for sodium in a variety of mammalian erythrocytes of different size, shape, rheological properties, and sodium concentrations. Human unsealed white ghosts were used to study sodium bound at the anisotropic sites on the inner side of the RBC membrane. Modulations of the conformation of the cytoskeleton by the variation of either the ionic strength or pH of the suspending medium caused drastic changes in both the residual quadrupolar interaction and T2f due to changes in the fraction of bound sodium ions as well as changes in the structure of the binding sites. By combining the two spectroscopic parameters, structural change can be followed. The changes in the structure of the sodium anisotropic binding sites deduced by this method were found to correlate with known conformational changes of the membrane cytoskeleton. Variations of the medium pH affected both the fraction of bound sodium ions and the structure of the anisotropic binding sites. Sodium and potassium were shown to bind to the anisotropic binding sites with the same affinity.

Double-quantum-filtered NMR spectra of 23Na in bovine nasal cartilage tissue and its constituents were measured. The presence of even-rank tensors was detected in the cartilage tissue and in a suspension of collagen fibers, indicating anisotropic motion of the sodium ions. Quantitative analysis of the spectra was performed by calculating the time evolution of the second- and third-rank tensors by solving a modified Redfield equation. Analytical expressions for the spectra were obtained. It is shown that the anisotropy stems from local rather than macroscopic order. A good fit to the observed spectra was obtained for several models that assume isotropic distribution of the directors of the locally ordered sites. The local residual quadrupolar interaction was found to be 550 Hz, demonstrating the sensitivity of double-quantum filtration in the detection of anisotropic motion.