Solidstate Nmr Techniques For Alkali Metal Ions

All NMR-active alkali metal isotopes have an atomic nucleus, the nuclear spin quantum number of which is greater than 1/2 (S > 1/2). These nuclei are known as quadrupolar nuclei. In this chapter, we focus on 23Na (natural abundance = 100%) and 39K (natural abundance = 93%), which are both S = 3/2 nuclei. The quadrupolar nature of these alkali metal nuclei makes it necessary to use solid-state NMR techniques that are quite different from those developed for more familiar spin-1/2 nuclei such as 13C and 15N. Very often, the resolution in NMR spectra for quadrupolar nuclei is poor, as the quadrupole interactions are usually large (e.g., on the order of several megahertz for 23Na and 39K).

For half-integer (or noninteger) quadrupolar nuclei such as 23Na and 39K, one often detects only the central transition (m = +1/2 o m = -1/2), which is independent of the quadrupole interaction to the first order if the language of perturbation theory is used. Under some favorable conditions, such as for quadrupolar nuclei with small quadrupole moments or at very high magnetic fields, reasonable resolution can be obtained in central-transition spectra simply by using a conventional technique known as magic-angle spinning (MAS).15 However, the biggest problem in MAS spectra for half-integer quadrupolar nuclei is the incomplete averaging of second-order quadrupole interactions, which often results in residual line broadening much larger than the chemical shift dispersion. Under such circumstances, it is very difficult to obtain high-resolution NMR spectra from which chemical information can be extracted. In 1995, Frydman and coworkers16,17 introduced a new solid-state NMR technique known as multiple-quantum MAS (MQMAS). Using this technique, it is possible to achieve complete removal of the second-order quadrupole interactions. Because MQMAS is a two-dimensional (2D) NMR experiment, it is more time consuming than a one-dimensional (1D) MAS experiment. However, the tremendous benefit in resolution enhancement by using MQMAS has made this technique a remarkably powerful tool in many applications.

In the discussion that follows, we use a hydrated Na salt of guanosine 5'-monophosphate, Na2(5'-GMP)7H2O (in its orthorhombic form), as an example to illustrate how to extract NMR parameters from a combined analysis of 1D MAS and 2D MQMAS spectra. As seen in Figure 13.1, the 23Na MAS spectrum for Na2(5-GMP)7H2O does not show any recognizable feature that might be useful for extracting information about the number of Na+ sites and the corresponding NMR parameters. In the 23Na MQMAS spectrum, however, four spectral regions are clearly observed, indicating that there are four distinct Na+ sites. From individual Fj slice spectra, we can obtain for each Na site three 23Na NMR parameters: isotropic chemical shift (8iso), quadrupole coupling constant (Cq), and asymmetry parameter (Hq). Finally, we can compare the simulated total MAS spectrum with the experimental spectrum. For orthorhombic Na2(5-GMP)7H2O, we obtain the following data: Na1, CQ = 1.30 MHz, nQ = 0.7, 8iso = -4.5 ppm; Na2, CQ = 1.85 MHz, nQ = 0.5, 8iso = -2.0 ppm; Na3, CQ = 1.85 MHz, nQ = 0.6, 8iso = -2.0 ppm; Na4, CQ = 2.30 MHz, nQ = 0.7, 8iso = -5.5 ppm. Because the crystal structure for orthorhombic Na2(5-GMP)7H2O is known,18 an assignment of these NMR parameters to individual

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FIGURE 13.1 Experimental and simulated 23Na MAS spectra for orthorhombic Na2(5'-GMP) 7H2O at 11.75 T (left) and partial crystal structure and two-dimensional 23Na MQMAS spectrum of Na2(5'-GMP)-7H2O (right).

FIGURE 13.1 Experimental and simulated 23Na MAS spectra for orthorhombic Na2(5'-GMP) 7H2O at 11.75 T (left) and partial crystal structure and two-dimensional 23Na MQMAS spectrum of Na2(5'-GMP)-7H2O (right).

Na+ sites can be made on the basis of a simple correlation between Cq and ion-binding geometry. Na1 is assigned to the Na site with four water molecules and two hydroxyl groups from the ribose groups. Na2 and Na3 correspond to the two fully hydrated Na sites. Na4 is the site coordinated to four water molecules and two N7 nitrogen atoms from the pyrimidine moieties. A more reliable method for 23Na spectral assignment is to use ab initio electric-field-gradient calculations.19 20 It is important to point out that in the central-transition NMR spectra for half-integer quadrupolar nuclei, the center of mass of a peak (or a line shape) does not correspond to the true isotropic chemical shift. In this chapter, we use the symbol 5 to indicate the center of mass for a peak (in parts per million) and 5iso for the isotropic chemical shift (also in parts per million); 5iso is alwayslargerthan 5. Thedifferencebetween these two quantities is the isotropic second-orderquadrupoleshift,whichapproaches zero at the very high magnetic field limit.

Figure 13.2 shows 23Na MAS spectra for a series of Na-nucleotides. For systems containing only a single Na+ ion, spectral analysis is straightforward. For systems containing multiple Na+ sites, we must use both 1D and 2D MQMAS spectra to extract 23Na NMR parameters. A general procedure is similar to what has been described in the example of orthorhombic Na2(5-GMP)7H2O. Several 23Na MQMAS spectra for Na-nucleotides are shown in Figure 13.3. Asummaryofsolid-state 23Na NMR parameters observed for these Na-nucleotides is given in Table 13.1. The Na+ ions in Na-nucleotides exhibit a variety of coordination environments. The oxygen

FIGURE 13.2 1D 23Na MAS spectra for Na-nucleotides at 11.75 T.

FIGURE 13.2 1D 23Na MAS spectra for Na-nucleotides at 11.75 T.

ligand comes from several sources: water (W), phosphate (P), hydroxyl (S), and carbonyl groups (B). There are also several cases in which nitrogen atoms from the base are involved in the first-sphere Na+ coordination. The coordination number for Na+ ions varies from 5 to 6 to 7. To our knowledge, Table 13.1 represents the only collection of experimental solid-state 23Na NMR data for Na-nucleotides. We anticipate that continuing accumulation of solid-state 23Na NMR data will lead to a better understanding of the relationship between 23Na NMR parameters and Na+ binding structure. In addition to the work from our laboratory, there are also a few scattered reports in which solid-state 23Na NMR was used to study DNA-related systems. Klinowski and coworkers studied the solid-state 23Na NMR spectra of Na-DNA with and without competing species.21 Ding and McDowell reported 23Na MQMAS spectra for hydrated adenosine 5'-triphosphate (5'-ATP).22 Madeddu demonstrated the importance of water content in analysis of solid-state 23Na NMR spectra.23 Frydman and coworkers recently introduced a new NMR experiment for Na+ site assignment by using 1H-23Na dipolar interactions.24

As one of the low-y quadrupolar nuclei, 39K is notoriously difficult to study by NMR. The difficulty of solid-state 39K NMR experiments is primarily twofold. First, the low 39K NMR frequency not only makes the overall NMR sensitivity very low but also causes severe second-order quadrupole broadening. This is because the second-order quadrupole broadening is inversely proportional to the NMR frequency of the nucleus under observation. Second, the 39K chemical shift range (ca. 200 ppm) usually is much smaller than the second-order quadrupole broadening, making the 39K NMR spectra lack of site resolution. For these reasons, solid-state 39K NMR experiments are often time consuming and produce very broad spectra at low and moderate magnetic fields (e.g., 11.75 T or lower). To date, solid-state 39K NMR studies have been largely restricted to simple inorganic salts.25

One simple (but not necessarily easy) solution to improving NMR sensitivity is to perform solid-state 39K NMR measurement at a high magnetic field. Figure 13.4

FIGURE 13.3 2D 23Na MQMAS spectra for Na-nucleotides at 11.75 T.

shows the high-field (19.6 T) 39K MAS NMR spectra for hydrated K+ salts of adenosine 2'-monophosphate, K(2'-AMP)1.5H2O, and adenosine 5'-diphosphate, K(5'-ADP)2H2O. From an analysis of these spectra, we obtain the following 39K NMR parameters: 2'-AMP, 5iso = -55 ppm, CQ = 1.85 MHz, nQ = 0.80; 5-ADP, 5iso = -105 ppm, CQ = 2.05 MHz, nQ = 0.25. The K+ ion in 2'-AMP is coordinated to two phosphate oxygen atoms, two water molecules, and two hydroxyl groups from the ribose,26 representing a typical environment for phosphate-bound K+ ions. In comparison, the K+ ion K(5'-ADP) is coordinated to seven ligands: four phosphate groups, one water, one hydroxyl, and a nitrogen atom (N3) from the adenine base.27 The participation of N3(A) in K+ coordination is quite unusual for K nucleotides. Perhaps this unusual coordination is responsible for the very shielded environment observed in K(5'-ADP), 5iso = -105 ppm. Although the 39K chemical-shift range is relatively small, the resolution observed in the 39K MAS spectra at 19.6 T is

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