NMR Spectroscopy of Alkali Metal Nuclei in Solution

1584 NMR SPECTROSCOPY OF ALKALI METAL NUCLEI IN SOLUTION

NMR Spectroscopy in Food Science See Food Science, Applications of NMR Spectroscopy.

NMR Spectroscopy of Alkali Metal Nuclei in Solution Frank G Riddell, The University of St Andrews, UK Copyright © 1999 Academic Press

The alkali metals, lithium, sodium, potassium, rubidium and caesium all possess NMR active nuclei, all of which are quadrupolar. • Lithium has two NMR active isotopes 6Li (7.4%) and 7Li (92.6%), of which 7Li is the isotope of choice due to its higher magnetogyric ratio and natural abundance. Both isotopes are available in isotopically enriched form making NMR tracer studies relatively easy. • Sodium has only one NMR active nucleus, 23Na (100%). • Potassium has two NMR active isotopes 39K (93.1%) and 41K (6.9%), of which 39K is the isotope of choice due to its much greater natural abundance and 41K is observable only with the greatest difficulty. • Rubidium has two NMR active isotopes 85Rb (72.15%) and 87Rb (27.85%), of which 87Rb is the isotope of choice due to its much higher magnetogyric ratio despite its lower natural abundance. • Caesium has only one NMR active nucleus, 133Cs (100%). Lithium is important as the treatment of choice for manic depressive psychosis and this has provoked a wide variety of NMR studies in an endeavour to probe its mode of action. Organolithium compounds are used extensively in synthetic organic chemistry and as industrial catalysts, especially in polymerization reactions. Both sodium and potassium are essential for life. Potassium is the major intracellular cation in most living cells, with sodium having the second highest concentration. These concentrations are generally reversed in the extracellular fluids. The

MAGNETIC RESONANCE Applications concentration differences across the cellular membrane are maintained by ion pumps, the most important of which is Na/K/ATPase. This enzyme pumps three sodium ions out of the cell and two potassium ions in for the consumption of one molecule of ATP. This enzyme consumes about onethird of the ATP produced in the human body, emphasizing the importance for life of maintaining the concentration gradients of these ions. In addition, large numbers of enzymes require the presence of sodium or potassium for them to function by mechanisms such as symport or antiport. The human need for sodium chloride as a part of the diet is recognized in many proverbs and sayings in common use, and in the word ‘salary’ which is a reminder that salt has in the past been used as a form of payment. Although the chemistry of rubidium is close to that of potassium it cannot be used as a substitute for potassium in biological systems in vivo, although it has been used in studies of perfused organs and cellular systems. The same applies for similar reasons to caesium. These metals can be taken into biological systems where they generally replace potassium, but the ingestion of large amounts of the salts of either metal has severe physiological consequences leading in extreme cases to death. Many reasons exist, therefore, for the development and implementation of NMR methods for the study of the alkali metals.

Nuclear properties The nuclear properties of the NMR active isotopes of the alkali metals are presented in Table 1.

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Table 1

Nuclear properties of the alkali metals

Isotope

Spin, I

6

1

Li

7

Li

3/2

23

3/2

Na

Natural abundance (%) 7.42 92.58 100

Magnetogyric ratio, Quadrupole moment J/10 7 (rad T1 s1) Q/1028(m2)

NMR frequency, Ξ (MHz)

Relative receptivity, D c

3.937

8 × 104

14.716

3.58

10.396

4.5 × 10

38.864

1.54 × 103

26.451

5.25 × 102

2

7.076

0.12

39

3/2

1.248

5.5 × 102

4.666

2.69

41

3/2

6.88

0.685

6.7 × 102

2.561

3.28 × 102

85

5/2

72.15

2.583

0.247

9.655

87

3/2

27.85

8.753

0.12

32.721

2.77 × 102

13.117

2.69 × 102

K K Rb Rb

133

Cs

7/2

93.1

100

3 × 103

3.509

43.0

;is the observing frequency in a magnetic field in which H is at 100 MHz. D c is the receptivity relative to 13C. Quadrupole moments Q are the least well determined parameters in this Table. Data taken from: NMR and the Periodic Table (1978) Harris RK and Mann BE (eds) London: Academic Press. 1

Quadrupolar relaxation and visibility The NMR spectra of the alkali metals are dominated by the fact that all the isotopes are quadrupolar. Effective use of alkali metal NMR requires an understanding of the resulting quadrupolar interactions and the best ways to make use of them and to avoid their pitfalls. Many of the problems that arise and solutions adopted are similar to those involved with the halogens. Quadrupolar nuclei have an asymmetric distribution of charge which gives rise to an electric quadrupole moment. Apart from when the nucleus is in an environment with cubic or higher symmetry, the quadrupole moment interacts with the electric field gradient (EFG) experienced by the nucleus, giving rise among other things to quadrupolar relaxation. The strength of the quadrupolar interaction between the quadrupole moment (eQ) and the electric field gradient (eq) is given by the quadrupolar coupling e2qQ/h. This can take from very small values to hundreds of MHz, depending on the magnitudes of Q and q. In solution, modulation of the EFG at the quadrupolar nucleus by isotropic and sufficiently rapid molecular motions (where ZW << 1) leads to relaxation according to the expression:

where K is the asymmetry parameter associated with the EFG. The alkali metal ions in solution are subject to relatively low quadrupolar interactions. This is particularly true for 6Li and 7Li and for 133Cs, which have inherently low quadrupole moments. Indeed 6Li

and 133Cs have the two lowest known quadrupole moments and 6Li is often referred to as an ‘honorary’ spin nucleus. In aqueous solution the ions are solvated by charge dipole interactions with the water molecules. At any one instant the pattern of water molecules around the cation does not have spherical symmetry but is always close to it. Thus the quadrupolar couplings are low but are never zero. Typically, in aqueous solution and in the absence of extraneous influences, both isotopes of lithium show Li+ line widths of < 1 Hz, Na+ and K+ show a line width of ca. 12 Hz, both rubidium isotopes show line widths of ca. 140–150 Hz and 133Cs shows a line width of ca. 1 Hz. Because of the low values of the quadrupole moments for both isotopes of lithium, dipolar relaxation becomes important. In aqueous solution at ambient temperature dipolar relaxation accounts for over 75% of the relaxation of 7Li (T1 ~ 20 s) and almost 100% of 6Li (T1 ~ 170 s). In D 2O solution with no 1H available for dipolar relaxation and virtually no quadrupolar mechanism available, the relaxation time of 6Li becomes very long (T1 ~ 830 s). In contrast, it appears that despite the low quadrupole moment of 133Cs, dipolar relaxation does not contribute significantly. In cases where molecular motion is restricted (ZW is not << 1) the situation is more complex. Such cases arise when the alkali metal is bound to the surface of a large molecule such as a protein or membrane surface and thereby has its motion restricted. The quadrupolar interaction with the nucleus shifts the energies of the Zeeman levels according to the square of the quantum number to a first approximation. Thus, the energy level splittings for a nucleus with I = (e.g. 7Li, 23Na, 39K and 87Rb) become as illustrated in Figure 1. With rapid isotropic motion, as described above, the multiple line pattern will

1586 NMR SPECTROSCOPY OF ALKALI METAL NUCLEI IN SOLUTION

Figure 1 Changes in the energy levels for a spin nucleus subject to a quadrupolar interaction. Note: The shifts in energy levels shown are exaggerated for clarity

collapse into a single line. In the absence of rapid isotropic motion the relaxation rate of the outer transitions, which combine to have 60% of the total intensity, is different from that of the inner transitions, which has 40% of the total intensity. The frequencies of the outer transitions also shift from the inner transition (dynamic frequency shift). There are three principal consequences of these changes for cases where the motion of the cation is restricted. First, line shapes become a double and not a single Lorentzian; secondly, two relaxation times are apparent; and finally, where the more rapid relaxation time becomes very short, partial or total invisibility of the signal from the outer transitions may occur. An excellent review of quadrupolar relaxation effects is given in the review by Springer given in the Further reading section.

Biological applications Contrast reagents

One of the main problems in using NMR to study the alkali metals in biological systems is that the chemical shifts of the aqueous ions are essentially independent of the ion’s surroundings in all cases except for 133Cs+, making differentiation of intra- and extracellular cations difficult. This problem can be met by

using a contrast reagent (either a shift or a relaxation agent) in one of the compartments, normally extracellular. A large number of aqueous shift reagents have been employed. They all work on the same principle, that a paramagnetic lanthanide, typically dysprosium, is enclosed in a complex by a ligand or ligands, and the resulting complex has several negative charges. With the overall charge on the complex being negative, the alkali metal cations are attracted to the negatively charged species and thereby brought into a region where the paramagnetic interaction induces a chemical shift change. Typical shift reagents for the alkali metals are given in Table 2. The resonances of the cations are also broadened by the process, but this broadening has been shown to be largely due the quadrupolar interaction of the cation with the reagent and not due to paramagnetic relaxation. For rubidium, which has a substantial line width that is comparable to the shifts capable of being induced by the best shift reagents, it is preferable to employ a relaxation agent to relax the signal from the extracellular Rb+ into the baseline noise. The first important application of alkali metal shift reagents was the use of dysprosium bistripolyphosphate (DyP3O10) (DyPPP2) to differentiate between intra- and extracellular 23Na in human erythrocytes. This was soon followed by a similar experiment revealing the intracellular signal from 39K. In both cases it seems as if the intracellular metal ions in human erytrocytes are essentially 100% visible. The spectra obtained for the 39K experiment are shown in Figure 2. The maximum shift generated by the shift reagents varies with the alkali metal according to the number of shells of electrons shielding the nucleus from the paramagnetic centre. For example the maximum shifts available from DyPPP2 are approximately: 7Li+, 40 ppm: 23Na+, 20 ppm; 39K+, 10 ppm; 87Rb+, 4 ppm; 133Cs+, 2 ppm. The shift reagent DyPPP2 is commonly used for in vitro systems such as vesicles or with isolated erythrocytes. However, it displays considerable toxicity for in vivo systems, in which cases the shift reagent TmDOTP5− (see Table 2) is preferred. For example during in vivo studies of rat kidneys using TmDOTP5−, three 23Na+ signals were resolved, corresponding to intracellular Na+, vascular Na+ and intraluminal Na+. Multiple quantum filtration

It has been shown that 23Na double-quantumfiltered NMR spectroscopy can be used to detect anisotropic motion of bound sodium ions in biological systems. The technique is based on the formation of

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Table 2

Shift reagents for alkali metal cations

Shift reagent

Acid anion

[Dy(PPP)2]7−

Tripolyphosphate (P3O10)5−

[Dy(DPA)3]3−

Dipicolinate

[Dy(NTA)3]3−

Nitrilotriacetate [N(CH2COO)3]3−

[Dy(CA)3]6−

Chelidamate

[Dy(THHA)]3−

Triethylenetetraminehexaacetate

TmDOTP5−

Thulium 1,4,7,10-tetraazacyclodecane1,4,7,10-tetrakismethylenephosphonate

Figure 2 39K NMR spectra recorded at 16.8 MHz of (A) resuspension medium containing 60 mM K+, 6 mM Dy3+ and 15 mM tripolyphosphate: (B) human erythrocytes in the same medium; and (C) difference spectrum after the subtration of 0.3 of the intensity of spectrum (A) from spectrum (B). For each spectrum 20 000 free induction decays were collected in approx. 20 min. Reproduced with permission of the Biochemical Society from Brophy PJ, Hayer MK and Riddell FG (1983) Biochemical Journal 210: 961.

1588 NMR SPECTROSCOPY OF ALKALI METAL NUCLEI IN SOLUTION

the second-rank tensor when the quadrupolar interaction is not averaged to zero. Isotropically tumbling 23Na, free in aqueous solution, is not seen by these methods. Such techniques allow, for example, the detection of 23Na+ bound to macromolecules such as proteins or membranes or the detection of intracellular 23Na+ if its motion is partially restricted. Triple-quantum-filtered spectra can also be used for similar purposes. Multiple-quantum-filtered NMR offers the possibility of monitoring the intracellular Na+ content in the absence of shift reagents provided that three criteria are met: (1) the contribution from intracellular 23Na+ to the multiple-quantum-filtered spectrum is substantial, (2) that it responds to a change in intracellular 23Na+ content and (3) that the amplitude of the extracellular multiple-quantum-filtered component remains constant during a change in intracellular 23Na+ content. Lithium NMR

The use of Li+ salts as the preferred treatment for manic depressive psychosis has spurred on the use of 7Li NMR in biological systems, particularly work on cellular systems. The use of the shift reagent DyPPP2 for 23Na and 39K to separate intra- and extracellular signals in human erythrocytes was rapidly followed by similar experiments with 7Li+. The object of these experiments was to determine lithium transport rates across the erythrocyte membrane as a model for the blood–brain barrier. Comparisons were made of the transport rates of 7Li+ into and out of the erythrocytes of manic depressive patients being treated with Li+, with those of normal controls. These experiments have demonstrated that at extracellular concentrations ranging from 50 to 2 mM the efflux rate from the erythrocytes of the patients was significantly slower than for those of the controls. Moreover, the experiments have shown that the abnormal transport rate is a consequence of Li+ treatment and is not a marker for the illness. Similar experiments have been carried out with other cellular systems including astrocytomas, neuroblastomas, rat hepatocytes and cultured Swiss Mouse 3T3 fibroblasts. The work on astrocytomas, an immortalized cell line from a human brain cancer, allowed visualization of Li+ inside the cells which were supported on microcarrier beads (Figure 3) and showed that there is an active Li+ extrusion pump present in these cells and, therefore, that there must also be a Li+ pump present in astrocytomas in the brain. It is widely believed that the enzyme interacting with Li+ when it acts to control manic depressive psychosis is inositol monophosphatase. 7Li+ NMR signals from Li+ bound to the inositol monophosphatase

Figure 3 7Li NMR spectra of astrocytomas on microcarrier beads in a buffer containing dysprosium tripolyphosphate shift reagent. Each spectrum is the sum of 48 acquisitions recorded at 194 MHz, [Li+]out = 10 mM. Reprinted from Bramham J, Carter AN and Riddell FG Journal of Inorganic Biochemistry 61: 273–284, copyright 1996, with permission from Elsevier Science.

enzyme have been observed. This suggests that 7Li+ NMR could make an important contribution to the study of this and other lithium-sensitive enzymes. NMR imaging techniques have been applied to study the concentrations and the pharmacokinetics of Li+ in various parts of the human body. Such experiments have shown that the concentration of Li+ in the brain and muscle is lower than that in the blood serum. Interestingly, they have also shown that after ingestion of Li+ there is lag in the uptake of Li+ into the brain. Maximum concentrations of Li+ in the brain occur several hours after the concentration maximum in the serum has been reached. Such

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experiments point towards better treatment regimes for patients. Sodium, potassium, rubidium and caesium NMR

A very substantial body of literature exists on the use of 23Na and 39K NMR to study cardiac function. Such experiments are typically performed on isolated perfused rat hearts, although hearts from other species including guinea pigs and dogs have also been used. Often the experiments involve shift reagents, although more recently multiple quantum filtration has been extensively used to differentiate between pools of ions. During ischaemia (low or zero blood flow which mimics a heart attack) there is an accumulation of intracellular sodium, cellular swelling and energy deficiency that participate in the transition to irreversible ischaemic injury. Such changes can be followed readily by a combination of 23Na and 31P NMR techniques. These experiments have provided valuable information on the behaviour of hearts under conditions of ischaemia and their recovery afterwards during reperfusion. They provide valuable information on methods for the resuscitation of ischaemic hearts and their protection against hypoxic injury. Although not present in normal biological systems in other than trace amounts, Rb+ and Cs+ have been used on several occasions as K+ analogues in the above experiments, thus extending the range of nuclei available for study. Other similar experiments have been performed on hearts from genetically hypertensive rats. Similar experiments have been performed on other organs such as kidney and liver from small animals. Studies of 23Na+ in cellular systems have been performed on cells such as superfused isolated rat cardiomyocytes, Methanobacterium thermoautotrophicum, porcine vascular endothelial cells, the Brevibacterium halotolerant bacterium sp., Escherichia coli, murine TM3 Leydig and TM4 Sertoli cell lines, and mouse 3T3 fibroblasts. These experiments have been employed to determine the NMR visibility of 23Na+, its intracellular concentration, membrane transport properties and dynamics and its ionic mobility inside the cells. 23Na NMR studies have contributed to the study of Na/K/ ATPase. Since the principal cytoplasmic cation is K+, NMR experiments on 39K+ in cellular systems should give valuable information on the intracellular environment. That they have been used much less frequently than experiments on 23Na+ is because of the lower receptivity of 39K. The utility of 39K+ studies is shown by work on 39K+ from E. coli after plasmolysis. The 39K+ signals are 100% visible and show biexponential relaxation, with both components relaxing very

rapidly. The result was attributed to a substantial interaction between the 39K+ and the polynegatively charged surface of the ribosomes. The uptake of Rb+ into human erythrocytes has been studied by 87Rb NMR using the relaxation agent LaPPP2 to contrast the two pools of Rb+. Uptake was linear over a 24 h period. With 113Cs+ NMR there is no need for a contrast reagent to separate the intra- and extracellular signals since the chemical shifts of the intra- and extracellular signals are well separated. Variations of the phosphate concentration in the suspension buffer are sufficient for this purpose. Uptake of 113Cs+ into human erythrocytes was observed to be linear with a rate of 0.33 mM h −1 at an extracellular Cs+ concentration of 10 mM. When the cells were removed to a Cs +-free buffer they retained the Cs+, indicating that there is no transport mechanism available for the removal of Cs+ from the cells. Cs+ was shown to replace K+ inside the cell. The favourable properties of 113Cs+ as indicated above, primarily its chemical shift range without the use of shift reagents and its low quadrupolar interactions, have led to its use as an analogue of K+ in several studies of its tissue compartmentation. Mediated membrane transport

A variety of NMR methods exists for the study of the mediated transport of alkali metal ions through model biological membranes. Substrates that mediate the transport include the ionophoric antibiotics such as monensin [1], channel forming peptides such as gramicidin and the peptaibols, other channel forming substrates such as amphotericin and the brevitoxins, or synthetic carriers, for example, those based around crown ether-like skeletons such as [2]. For such experiments large unilamellar vesicles (LUV) formed from phospholipid are prepared and a chemical shift difference between the intra- and extravesicular compartments is established by means of a shift reagent. For rapid exchange of ions across the membrane (k > 10 s −1) dynamic line broadening provides information on the transport kinetics (Figure 4). For cases where the transport rate is comparable to the relaxation rate a magnetization transfer technique can be employed. In this experiment one of the two signals, normally the extracellular signal, is inverted by a simple pulse sequence. Chemical exchange then causes a time-dependent reduction in the signal of the other resonance. Analysis of signal intensities against time gives the transport kinetics. For relatively slow exchange (k < 10 −3 s−1) isotope exchange is used, e.g. 6Li/7Li or 7Li/23Na. In such experiments the concentration gradients of the cations form the driving force for the transport. Such experiments have provided extremely valuable

1590 NMR SPECTROSCOPY OF ALKALI METAL NUCLEI IN SOLUTION

Figure 4 Changes in the 23Na NMR spectra recorded at 21.16 MHz of LUV containing 120 mM NaCl on addition of increasing microlitre amounts of a dilute solution of monensin in methanol at 303 K. The surrounding solution contains 10 mM Na5P3O10, 70 mM NaCl and 4.0 mM DyCl3. Reprinted from Riddell FG and Hayer MK Biochimica Biophysica Acta 817: 313–317, Copyright © 1985, with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.

insights into the kinetics and mechanism of mediated transport. For the ionophoric antibiotics such as monensin they have shown that one ionophore transports one alkali metal and that the rate limiting step is almost invariably release of the alkali metal ion at the membrane surface. However, several synthetic ionophores, e.g. ([2], n = 3, R1 = R2 = C10H21), which transports Na+ at a rate comparable to that of monensin, exhibit diffusion as the rate limiting step. For gramicidin for example, these experiments have confirmed that two molecules are required to come together to form a pore. For the peptaibols the transport has been shown to occur by ‘barrel stave’ assembly of peptide molecules across the membrane forming a pore inside the ‘barrel’ that is of sufficiently long duration to allow complete exchange of the intra- and extravesicular media. For the brevetoxins, selectivity for various ions was probed by changing the ions involved in the concentrations gradients. The dependence on cholesterol incorporation in the membrane was studied. So-called ‘bouquet molecules’, based on a central crown ether or cyclodextrin unit equipped with pendant arms that are also capable of complexing cations and are long enough to traverse a lipid bilayer, have been studied in vesicles with a Na+ / Li+ gradient across the membrane using both 7Li and 23Na NMR. Such systems show a one-for-one exchange of Na+ for Li+ (antiport). These molecules were found to transport Na+ at similar rates in fluidand gel-state membranes; this suggests that ion

passage occurs preferentially by a channel mechanism and not by the carrier mechanism. Monensin, known to operate as a carrier, was shown to transport at a slower rate in a gel-state membrane. Another interesting aspect of these experiments is their ability to probe the effect of changes in the membrane composition on the transport kinetics. Thus, placing positive and negative charges on the membrane surface causes changes in ionophore mediated transport rates, and the introduction of pharmaceuticals such as chlorpromazine and imipramine cause changes in the nigericin mediated Na+ transport rates.

Chemical applications Covalently bound lithium

Lithium covalently bound to carbon may be observed by 7Li NMR in lithium alkyls. In such molecules 7Li has a small chemical shift range (~12 ppm). Tables of chemical shifts and coupling constants are to be found in the review by Günther. Metal ion complexation studies

Although biological applications have been the major use of alkali metal ion NMR, it has also proved to be valuable for the study of complexation of the alkali metals in host–guest systems by suitably designed ligands, e.g. the crown ethers and cryptands. Two parameters are important in detecting complexation: chemical shift changes and decreases

NMR SPECTROSCOPY OF ALKALI METAL NUCLEI IN SOLUTION 1591

in relaxation times as a result of enhanced quadrupolar interactions. Often dynamically broadened alkali metal NMR spectra can be seen as a result of exchange between the free and complexed cation. A good example is provided by the dynamic 7Li and 23Na spectra for the interaction of Li+ and Na+ with the pendant arm macrocycle 1,4,7,10-tetrakis(2-methoxyethyl)-1,4,7,10tetraazacyclododecane [3]. The dynamic 7Li spectra are shown in Figure 5. Evidence of a slowly exchanging 1:1 complex and of a 2:1 complex in rapid equilibrium with the 1:1 complex between calixarene [4]

and Na+ is provided by studies of this system by 23Na and 1H NMR. Frequently, when the alkali metal ion is exchanging between the complex and the solution, the temperature variation of T1 and/or T2 for the metal can give information about the exchange kinetics. The complexed ion has a much shorter T1 (and T2) value due to strong quadrupolar interactions with the ligand. In the slow exchange limit the observed T1 value approaches that of the ion free in solution, whilst in the rapid exchange limit the T1 value is an average of the values for the complexed and free

1592 NMR SPECTROSCOPY OF ALKALI METAL NUCLEI IN SOLUTION

Figure 5 Typical exchange-modified 7Li NMR spectra recorded at 116.59 MHz of a dimethylsulfoxide solution of solvated Li+ and [3]. Experimental temperatures and spectra appear on the left of the figure and the best fit calculated line shape and lifetime values on the right. Complexed 7Li+ appears as the left-hand side, high frequency, peak. Reprinted with permission from Stephens AKW, Dhillon RS, Madbak SE, Whitbread SL and Lincoln SF Inorganic Chemistry 35: 2019–2024, copyright 1996, American Chemical Society.

ions. In between these extremes T1 follows a sigmoid curve when plotted against 1/T. Alkali metal anions in solution: alkalide ions

Under conditions of the most rigorous purity and using high vacuum techniques in dipolar aprotic and similar solvents, the alkali metals yield metal anions (M−) known as the alkalides. Thus sodium in hexamethylphosphoric triamide solutions gives rise to sodide (Na−) ions. Sodium and rubidium in 1,4,7,10-tetraoxacyclododecane (12-crown-4) give sodide and rubidide ions (Figure 6). These anions are most readily identified by their NMR spectra which occur substantially to low frequency of the chemical shift standards of the alkali metal salts in D2O solution. The alkalide ions are formed by the addition of one electron to the partially filled outermost S orbital. This is expected to lead to a substantial shielding increase, the observation of which is a good indication of the ion formation. Chemical shifts of the alkalide anions vary slightly with solvent and temperature but are near the following values: Na−, −62 ppm, K−, −103 ppm, Rb −, −191 ppm, Cs −, −280 to −300 ppm. The spectra of the sodide ion and the potasside ion at low temperatures show relatively sharp line widths, indicative of low ion–solvent interactions suggesting that these ions in solution resemble those

Figure 6 23Na and 87Rb NMR spectra of solutions of sodium and rubidium in 1,4,7,10-tetraoxacyclododecane (12-crown-4). Negative chemical shift values correspond to a decrease in resonance frequency and an increase in nuclear shielding. Reproduced with permission of The Royal Society of Chemistry from Holton DM, Edwards PP, Johnson DC, Page CJ, McFarlane W and Wood B (1984) Journal of the Chemical Society, Chemical Communications, 740–741.

in the gas state. On the other hand, at room temperature the line width of the rubidide ion (~1000 Hz vs. 140 Hz for Rb + in H2O) indicates that there is quadrupolar broadening and the observed shift falls short of that calculated for a gaseous-like ion, unlike the shifts of the sodide and potasside ions. This strongly suggests that there are interactions between the solvent and the rubidide ion. In the case of caesium dissolved in crown ethers the species Cs+e− has also been observed.

List of symbols eq = electric field gradient strength; e2qQ/h = quadeQ = quadrupole rupolar coupling; moment strength; I = spin quantum number; T1 = longitudinal relaxation time; T2 = transverse relaxation time; W = correlation time for molecular motion (s); ω = Larmor frequency (rad s–1). See also: Biofluids Studied By NMR; Cells Studied By NMR; Halogen NMR Spectroscopy (excluding 19F); In Vivo NMR, Applications, 31P; In Vivo NMR, Applications, Other Nuclei; Membranes Studied By NMR Spectroscopy; NMR Relaxation Rates; Perfused Organs Studied Using NMR Spectroscopy; Relaxometers.

Further reading Bramham J and Riddell FG (1994) Cesium uptake studies on human erythrocytes, Journal of Inorganic Biochemistry 53: 169–176.

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Brophy PJ, Hayer MK and Riddell FG (1983) Measurement of intracellular potassium ion concentration by NMR, Biochemical Journal 210: 961–963. Edwards PP, Ellaboudy AS and Holton DM (1985) NMR spectrum of the potassium anion K−, Nature (London) 317: 242–244. Edwards PP, Ellaboudy AS, Holton DM and Pyper NC (1988) NMR studies of alkali anions in non-aqueous solvents. Annual Reports on NMR Spectroscopy 20: 315–366. Günther H (1996) Lithium NMR, In: Grant DM and Harris RK (eds) Encyclopedia of Nuclear Magnetic Resonance, p. 2807, Chichester: Wiley. Laszlo P (1996) Sodium-23 NMR, In: Grant DM and Harris RK (eds) Encyclopedia of Nuclear Magnetic Resonance, p. 4551. Chichester: Wiley.

Lindman B and Forsén S (1978) The Alkali Metals, In: Harris RK and Mann BE (eds) NMR and the Periodic Table, London: Academic Press. Mota de Freitas D (1993) Alkali metal nuclear magnetic resonance, Methods in Enzymology 227: 78–106. Riddell FG (1998) Studying biological lithium using nuclear magnetic resonance techniques, Journal of Trace and Microprobe Techniques 16: 99–110. Sherry AD and Geraldes CFGC (1989) Shift reagents in NMR spectroscopy in lanthanide probes, In: Bünzli JCG and Chopin GR (eds) Life, Chemical and Earth Sciences, Theory and Practice, Amsterdam: Elsevier. Springer CS (1996) Biological systems, spin-3/2 nuclei. In: Grant DM and Harris RK (eds) Encyclopedia of Nuclear Magnetic Resonance, p. 940. Chichester: Wiley.

NMR Spectroscopy, Applications See Diffusion Studied Using NMR Spectroscopy; Drug Metabolism Studied Using NMR Spectroscopy; Structural Chemistry Using NMR Spectroscopy, Pharmaceuticals; Biofluids Studied By NMR; Carbo-hydrates Studied By NMR; Cells Studied By NMR; Structural Chemistry Using NMR Spectroscopy, Peptides; Proteins Studied Using NMR Spectroscopy; Nucleic Acids Studied Using NMR; Structural Chemistry Using NMR Spectroscopy, Inorganic Molecules; Structural Chemistry Using NMR Spectroscopy, Organic Molecules.

NOE See Nuclear Overhauser Effect.