HRMAS

Fig. 50 Phosphorus-31 CP/MAS spectrum of the nickel complex shown, which is immobilized on silica gel, recorded at a spinning rate of 4 kHz (lower spectrum) and HRMAS spectrum of an acetone suspension, recorded at a spinning rate of 2 kHz (upper spectrum)...

HRMAS 13c-NMR is a promising method to investigate the nature of carbon-containing residues trapped in zeolitic structures when running hydrocarbons and oxygenates conversion reactions (10). 

This hypothesis does not agree with the HRMAS Si-NMR results of Bodart et al. [3], who found that the Al atoms in chabazite are distributed statistically. However, the low resolution of the Si-NMR spectra and the superimposition of different Si(nAl) lines for zeolites of the chabasite group with two independent T sites make these results uncertain. 

HRMAS Al and Si-NMR spectra frequently give useful information for the determination of Si-Al distribution in zeolites, but this technique should be considered as complementary to x-ray or neutron diffraction. Their combined use often provides a most complete and reliable description of the Si-Al distribution in zeolites. The limits of NMR spectra in zeolites with disordered Si-Al distributions and three or more symmetrically independent T sites must also be considered. 

Brenna, S., Posset, T, Furrer, J. et al, N NMR and two-dimensional suspension iR and i C HRMAS NMR spectroscopy of ionic liquids immobilized on silica,... 

Fig. 5. 500 MHz HRMAS H NMR spectra of FMOC-isoleucine on Wang resin swollen in DMF- 7 and spun at 4 kHz. Spectrum A shows the presence of dissolved phenylalanine peaks, particularly around 3.0 ppm from its methylene protons, and residual protons of the solvent. Application of a diffusion filter using gradients selectively removes these signals in spectrum B. Reproduced with permission from Ref. 50. Copyright 1999 Elsevier.

Fig. 6. 500 MHz H HRMAS NMR spectra of FMOC-Ala-Ile-Asp-Wang swollen with DMF- 7 and spun at 5 kHz. Spectra were obtained without (a) and with (b) using a ID TOCSY variant54 to reduce polymer signals. Note the substantially reduced signals arising from polystyrene in the aromatic (6.5-7.5 ppm) and aliphatic (1.5-2.5 ppm) regions.

The Sharpless asymmetric dehydroxylation of resin-bound olefins was monitored using 3H, 13C and HMQC HRMAS NMR.63 The authors found 13C HRMAS NMR to be particularly suited to evaluating the progress of this reaction and permitted the enantiomeric excesses of the products to be determined before they were cleaved from the support. Most importantly, they were able to evaluate the types of substrates amenable to this reaction on solid supports, showing the ability of HRMAS NMR to contribute to synthetic questions. Transformation of the unnatural amino acid Lys(NH2) on a poly (ethylene glycol)-dimethylacrylamide (PEGA) resin to 6-hydroxynorleucine was confirmed by application of TOCSY HRMAS experiments.64... 

Fig. 7. 300 MHz H HRMAS NMR spectra of a resin suspension swollen with DMF-dq from a reaction mixture and spun at 4 kHz. The spectrum (A) was obtained with a single-pulse sequence. The spectra in (B), liberated from the reaction vessel at the times indicated, were obtained using a diffusion filter to reduce the signals from non-bound species. Note the excellent suppression of the solvent DMF peak at 8 ppm. Reproduced with permission from Ref. 75. Copyright 2000 American Chemical Society.

While much of the application of HRMAS to solid-phase organic chemistry has been via H NMR, 13C HRMAS NMR has been used to determine resin loading.78 Two methods were applied, using both internal standards added to the resin in known amounts, or using features of the resin structure itself as a standard for calibration of the spectral intensities. The first method... 

The first applications of solid phase synthesis were to peptides, hence it is no surprise that there have been several reports of HRMAS studies of peptide systems attached to a support. One of the earliest reports of HRMAS in a supported sample was that of Wang-bound lysine, whose structure was determined by TOCSY and HMQC HRMAS NMR.38 More recently, HRMAS NMR has been used to identify several peptidomimetic inhibitors of hepatitis C virus NS3 protease while on the resin.79 However, it is perhaps a bit surprising that more has not been made of HRMAS in attacking problems of relevance to peptide synthesis, although most recent interest is moving that way. Combinatorial chemistry and solid phase organic chemistry has been a much more active area using HRMAS techniques. 

Traditional protein NMR spectroscopy of smaller proteins relies of 15N-filtered experiments, due to the relatively low expense of introducing 15N labels into proteins (compared to 13C) and the concomitant ability to use heteronuclear filtering to improve resolution in the H NMR dimension. Jelinek et al. were the first to demonstrate the ability to transfer this approach to peptides on TantaGel.80 They also showed the ability to detect pronounced peptide structure through the appearance of strong NOE correlations in 3H NOESY HRMAS spectra as shown in Fig. 8. This had important implications for the search of biological activity in peptides attached to supports, as the structure on the support may be different or more pronounced than in solution, if present at all in solution in peptides of this small size. 

Fig. 8. 750 MHz H-I5N HMQC (A) and H NOESY (B) HRMAS NMR spectra of Gly-Pro-Gly-Arg-Ala-Phe on TantaGel resin swollen with DMSO-

While both solution and solid-state NMR has been routinely applied to polymers for many years, there have been a few recent applications of HRMAS to polymer systems, analyzing polymerization mechanisms and characterizing the resulting polymers in the swollen state. The vulcanization of butadiene rubber by cyclic disulfides was shown to follow two different mechanisms with two different classes of sulfur compounds - cross-linking progressed... 

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