The benefits of using magnetic field gradients in high resolution spectroscopy have been well documented. In high resolution MAS similar benefits are seen from the use of gradients, in particular a reduction in a variety of artifacts, such as t 1 -ridges and repetition artifacts, while also saving time by eliminating the need to complete a full phase cycle. A number of gradient high resolution MAS experi-ments are shown below on a variety of samples.
Figure 7.12 shows the gradient COSY of a swollen Wang resin, used in combinatorial chemistry. The use of gradients eliminates the repetition artifacts and t 1 -ridges. Note that the spinning sidebands in the f 1 -direction are aliased since no bandwidth filters are used in the indirect domain and the spectrum is not symmetrized.
Figure
7.12. Gradient COSY of a swollen Wang resin
500
MHz gradient 1 H-1
H COSY spectrum of an N-FMOC-N-Boc-L-Lysine deriva-tized
Wang resin swollen with CDCl 3 .
MAS 3.5 kHz, 1 ms gradients, 10 G/cm.
Gradients are frequently used in inverse detected heteronuclear correlation experiments to select the magnetization from protons coupled to a 13 C spin and suppress the uncoupled protons magnetization. The spectra below show 1 H-13 C hr-MAS HMQC ( figure 7.13) and HMBC ( figure 7.14) spectra of a Wang resin, used in combinatorial chemistry. Note the excellent suppression of t1-noise in the gradient HMQC spectrum (figure 7.13, left) versus the phase cycled spectrum (figure 7.13, right) and the clean noise floor in the HMBC spectrum (figure 7.14). Note that no attempts were made to improve the spectral quality (e.g. by t 1 -ridge sub-traction).
Figure 7.15 shows another example of a gradient HR-MAS HMBC spectrum. The spectrum is obtained from a human lipoma tissue and demonstrates the usefulness of this type of high resolution MAS spectroscopy to the study of biological tissues.
Figure
7.13. 400 MHz 1 H-13
C HMQC spectra of an N-FMOC-N-Boc-L-Lysine derivatized
Wang resin swollen with CDCl 3 .
The left spectrum is acquired using gradients to suppress the magnetization
from uncoupled proton spins, while the spectrum on the right is obtained
with phase cycling. No attempt was made the improve the appearance of the
spectra. MAS 5 kHz, 1 ms gradients, 10,10 and 5 G/cm.
Figure 7.14. 400 MHz 1 H-13 C HMBC spectra of an N-FMOC-N-Boc-L-Lysine derivatized Wang resin swollen with CDCl 3 . A J-evolution time of 50 ms is used to reveal long-range proton- carbon correlations. No 13 C decoupling is applied during acquisition. The right spectrum shows the noise floor and demonstrates excellent artifact suppression. MAS 5 kHz, 1 ms gradients, 10,10 and 5 G/cm.
Figure 7.15. 500 MHz 1 H-13 C Gradient hr-MAS HMBC spectrum of a human Lipoma tissue (sample courtesy of Dr. Singer, Brigham and Women's Hospital). MAS 5 kHz; 1 ms sine-shaped gradient pulses.