|M.Sc Student||Lipson Osnat-Shemer|
|Subject||DANTE-Based Frequency-Selective Reintroduction of Dipolar|
Interactions: An Experimental and Theoretical
Solid State NMR Study
|Department||Department of Chemistry||Supervisor||Professor Asher Schmidt|
In High-resolution solid state NMR, rapid rotation of the sample about an axis slanted away from B0 by the magic-angle is used (MAS - magic-angle spinning). However, the high resolution is achieved at the expense of structural information, such as the dipolar coupling constant (nD = hgIgSrIS-3). Nevertheless, the dipolar coupling can be reintroduced by rf irradiation, and various pulse sequences were developed for measuring dipolar coupling. Rotational-echo, double-resonance - REDOR, is one of these sequences which reintroduce dipolar coupling between nuclei of two different types, thus enabling measurements of dipolar interaction strength and therefore internuclear distance determinations.
There is a wide interest in reintroduction of only a selected dipolar coupling within the studied molecule. It is especially important in the study of large uniformly labeled molecules such as proteins, where reintroduction of selected interactions is not possible by the REDOR experiment. DANTE-based, Frequency-Selective REDOR, dbFSR, developed earlier in our lab, is one approach that achieves this goal.
In the present work, we have successfully applied this scheme on a model compound [4-13C,15N]asparagine, showing good agreement of the frequency dependence of dbFSR between experimental and calculated data. In the theoretical part of this work, we present a graphical explanation of the recoupling performance at low rf-power DANTE (n1 < Dn). In particular, we have focused on the DANTE part alone. We show by calculation of the magnetization trajectory, that while its path depends on the rf irradiation intensity, an almost perfect inversion is obtained also at low rf-power. Also, the performance of the non-rotor synchronized DANTE was studied theoretically. We found that it will yield the desired nutation and frequency response curve for a ratio wCSA/ wR < 1.6. Last, an additional dbFSR pulse scheme designed to overcome inefficient S0 refocusing, was proposed. Its theoretical characterization shows that it is achieving the desired theoretical frequency response curve.