23Na NMR spectroscopy

Fig. 25. Schematic representation of the transport experiment in which aqueous NaCl solution is added to a preparation of liposomes containing bouquet molecules in the membrane in aqueous LiCl solution, creating opposing gradients in Na+ and Li+ ion concentrations. The entry of Na+ ions, initially found only in the external volume, into liposomes is followed by 23Na NMR spectroscopy the exit of Li+ may also be followed by 7Li NMR spectroscopy.

Another way to assess ion channel conductance is to use artificial phospholipid vesicles (liposomes) as cell models. These structures (described in more detail in the next chapter) are commonly used to transport vaccines, drugs, enzymes, or other substances to target cells or organs. The vesicles, which are several hundred nanometres in diameter, do not suffer from interference from residual natural ion channel peptides or ionophores, unlike purified natural cells. A liposome model was used to test the ion transport behaviour of the redox-active hydraphile 12.36. The compound transports Na+ and the process can also be monitored using 23Na NMR spectroscopy.26 The presence of the ferrocene-derived group in the central relay allows the ion transport to be redox-controlled - oxidation to ferrocinium completely prevents Na+ transport for electrostatic reasons. Some representative data from a planar bilayer measurement is shown for hydraphile 12.36 in Figure 12.16. 

In a recent publication (184), alkali metal (7Li and 23Na) NMR spectroscopy confirmed that in solution as well as in the crystalline state, [LiTi(0-/-Pr)5]2 exists as a dimer with a rapid isopropoxide exchange, whereas a tight ion pair exists between Na+ and the Ti(0-/-Pr)5- anion. 

P and 19F NMR spectroscopy), and attempts to record a CP/MAS solid-state NMR spectrum (19F, 31P, 23Na) failed because of the large line-broadening by the 23Na ions (I = 3/2) and strong spin-spin coupling. 

MAS has been applied to a highly viscous cubic phase of a lyotropic LC formed by 1-monooleolyl-rac-glycerol and water in order to obtain liquid-like and 13C spectra.330 Deuterium, sodium, and fluorine NMR spectroscopy have been applied to study the phase behaviour of several dilute lamellar systems formed by low concentrations of an ra-hexadecylpyridinium salt, a sodium salt (e.g., NaBr, NaCl, or sodium trifluoroacetate), 1-hexanol, and D20.331 The 2H, 19F, and 23Na splittings were used to monitor the phase equilibria. The last two studies are motivated by the search of new lyotropic LC for the alignment of biomolecules. 

Nuclear Magnetic Resonance (NMR) Spectroscopy. Longitudinal and transverse relaxation times (Ti and T2) of 1H and 23Na in the water-polyelectrolytes systems were measured using a Nicolet FT-NMR, model NT-200WB. T2 was measured by the Meiboom-Gill variant of the Carr-Purcell method (5). However, in the case of very rapid relaxation, the free induction decay (FID) method was applied. The sample temperature was changed from 30 to —70°C with the assistance of the 1180 system. The accuracy of the temperature control was 0.5°C. 

The characteristics of the water pool of reverse micelles has been explored by H, 23Na, 13C, 3IP-NMR spectroscopy. Since the initial association process in RMs is not totally understood, and because of the low CMC, aggregation studies from NMR are rather scarce. Direct determination of a CMC in the diethyl hexyl phosphate /water/benzene system (at Wo = 3.5) was possible because the chemical shift of 31P in phosphate groups is very sensitive to hydration effects. The structure and state of water in RMs and particularly at low water content has received considerable attention. The proton chemical shifts have been explored in AOT/water/heptane, methanol, chloroform, isooctane and cyclohexanone. The water behavior in small reverse micelles is close to that of the corresponding bulk ionic solution. Until now, the effect of a solute on micellar structure was not well... 

Na NMR can be used to follow transmembrane Na+ transport. Na+ is added to a suspension of liposomes incorporating Na+-transporting compounds, or Na+-specific channels, which are subjected to NMR spectroscopy. It has been shown... 

Analysis of molecular motion in solids using NMR spectroscopy has been a developing field during the past decades. In this context, 2H NMR is the most used approach but recently a number of other quadrupolar nuclei have been studied. This includes spin-1 as well as half-integer quadrupolar nuclei such as 7Li and 23Na. 

Characterization of Y Zeolites by 23Na Magic-Angle-Spinning NMR Spectroscopy... 

Nuclei that are typically analyzed with this technique include those of 13C, 31P, 1SN, 2SMg, and 23Na. Different crystal structures of a compound can result in perturbation of the chemical environment of each nucleus, resulting in a unique spectrum for each form. Once resonances have been assigned to specific atoms of the molecule, information on the nature of the polymorphic variations can be obtained. This can be useful early in drug development, when the single-crystal structure may not be available. Long data acquisition times are common with solid-state NMR, so it is often not considered for routine analysis of samples. However, it is usually a very sensitive technique, and sample preparation is minimal. NMR spectroscopy can be used either qualitatively or quantitatively, and can provide structural data, such as the identity of solvents bound in a crystal. 

Solid-state NMR spectroscopy has been used to study polymer electrolytes [27, 259-266]. Among the various nuclei that have been used as probes 7Li and 23Na have received maximum attention. However, the quadrupolar nature of these nuclei compounded with the solid state of the sample result in considerable line broadening of the NMR signal. Thus, a lot of valuable information pertaining to the structure of polymer electrolytes is not accessible. However, an analysis of the linewidths and the spin lattice relaxation times of the nuclei affords considerable information on the nature of the ions present in a polymer electrolyte. 

H, 2h, and chemical shifts and Ti measurements have been made on Field-swept NMR spectra of in Pyrex glass and in NbN perturbed by quadrupole interaction have been reported. Sc and Nb NMR spectroscopy has been used to investigate PMN-PSN relaxors. 23Na- V double resonance TRAPDOR experiments have been presented for two different sodium vanadates. chemical shielding and quadrupole... 

We begin, in Section 2, with a detailed theoretical examination of the physics of quadrupolar, spin-5 nuclei, using the matrix operator formalism to discuss the various NMR experiments used in biological spectroscopy of quadrupolar ions and the biexponential relaxation of these ions. The effect of interaction of the Rb+ ion with its chemical environment on relaxation characteristics of the 87 Rb nucleus is considered in Section 3. Section 4 deals with methods of differentiating between intra- and extracellular species of ions. In Section 5 we discuss the kinetics of Rb+ in a variety of tissues, as studied by 87Rb NMR. Where appropriate, studies of 23Na and 39K are also cited. 

Last but not least nuclear magnetic resonance spectroscopy has to be mentioned in this connection. New advances which have made available more nuclei for a quantitative resonance study increased the applicability of this technique for a study of complex bio-organic structures. NMR-studies114,244,26S based on the resonances of H, 23Na and 13C in order to detect metal ion-ionophore interactions (nonactin, dibenzo-18-crown-6 and valinomycin) have been described recently. 

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