23Na NMR

Sodium octanoate (NaO) forms reversed micelles not only in hydrocarbons but also in 1-hexanol/water. The hydration of the ionogenic NaO headgroups plays an important role in this case too. For this reason Fujii et al. 64) studied the dynamic behaviour of these headgroups and the influence of hydration-water with l3C and 23Na NMR measurements. Below w0 = [H20]/[NaO] 6 the 23Na line-width... 

The alkalides. The first crystalline alkalide to be prepared in this manner was [Na+(2.2.2)].Na. This salt is obtained as shiny, gold-coloured crystals (Dye etal., 1974). The 23Na nmr spectrum yields a narrow upfield signal for the Na- ion (Dye, Andrews Ceraso, 1975) the X-ray structure indicates close-packed sodium cryptate cations with Na" anions occupying octahedral holes between the cryptate layers (Tehan, Barnett Dye, 1974). 

The dimer 12 can only be dissolved in ether solvents, when it undergoes dissociation, as corroborated by cryoscopic measurements and 23Na NMR spectra (24). In contrast, the Narduster salt 14 cannot be dissolved in any common organic solvents without decomposition... 

The proportion of the /rans-O-alkylated product [101] increases in the order no ligand < 18-crown-6 < [2.2.2]-cryptand. This difference was attributed to the fact that the enolate anion in a crown-ether complex is still capable of interacting with the cation, which stabilizes conformation [96]. For the cryptate, however, cation-anion interactions are less likely and electrostatic repulsion will force the anion to adopt conformation [99], which is the same as that of the free anion in DMSO. This explanation was substantiated by the fact that the anion was found to have structure [96] in the solid state of the potassium acetoacetate complex of 18-crown-6 (Cambillau et al., 1978). Using 23Na NMR, Cornelis et al. (1978) have recently concluded that the active nucleophilic species is the ion pair formed between 18-crown-6 and sodium ethyl acetoacetate, in which Na+ is co-ordinated to both the anion and the ligand. 

Fig. 20. 23Na NMR results chemical shifts vs. donicity of the donor solvent D...

More recent studies show some variations to the above generalities. 13C and 23Na NMR studies indicate that for Na+ the complexation sequence is 18-crown-6 > 15-crown-5 > benzo-lS-crown-5,482 hinting at the need for ligand flexibility conduction data reveal that for 18-crown-6 the stability sequence is K+>Rb+>Cs+>Na+ 483 extraction studies using metal picrates show that for benzo-15-crown-5, Na+ > K+ > Rb+ > Li+ > Cs+ for 1 1 complexes and that for the corresponding 1 2 complexes K+ > Rb+ > Cs+ 484 further extraction work indicated... 

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.

For an oriented sample with a uniform director orientation the 23Na NMR spectrum at a weak radiofrequency field thus consists of three equally spaced peaks. The relative magnitude of the three resonance lines is governed by the transition probabilities between the different energy levels. It turns out (9) that the central line contributes 40% of the total intensity while each of the satellites contributes 30%. The deuteron NMR spectrum consists of two equally intense peaks. 

Figure 5. 23Na NMR spectra of a lamellar mesophase sample of the system sodium octanoate-decanol-water. The rf field amplitude was increased in the order a,b,c,dt Peaks arising from double quantum transitions are clearly shown (cf. Figure 1).

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. 

Fig. 2.17. Plots of 23Na+ NMR chemical shifts versus the inverse concentration of sodium octanoate ( ), sodium octylsulfate (o), sodium octylsufonate ( ) and sodium dodecylsulfate (X). A positive shift is upfield. (From Ref.57))...

Table 1. The tortuosity is defined by the ratio Ta = Do/Daa where D0 is the self-coefficient of the bulk water (1.92x 1CT9 m2s-1). Avqax is the maximum quadrupolar splitting detected by 23Na NMR. The Laponite samples noted by an asterisk were prepared by oedometric compression.

Cryptands were found to react with metal solutions in basic solvents to generate the alkali metal cryptate and an alkali anion (alkalide), for example (Na[2.2.2])+Na (62, 63). 23Na-NMR measurements of this salt in methylamine, tetrahydrofuran, and ethylamine solutions showed that the Na resonance is shifted strongly upheld from the Na resonance (free or complexed) as shown in Fig. 7. The anion resonates at approximately the same frequency as that calculated for the free... 

Fig. 7. 23Na-NMR spectra of (Na[2.2.2])+Na solutions in three solvents (chemical shifts are referenced to Na at infinite dilution 5 = 0 ppm) (reproduced with permission).

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... 

Fig. 15. Comparison between the static, MAS and MQMAS 23Na NMR spectra of multisite sodium salts. Contour levels in the 2D plots were taken at 70%, 35%, 16%, 8%, and 4% of the maximum spectral intensities asterisks indicate the spinning sidebands. (Reproduced, with permission, from Medek et a/.209)...

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