ENDOR blind spots

Muns ENDOR mvolves observation of the stimulated echo intensity as a fimction of the frequency of an RE Ti-pulse applied between tlie second and third MW pulse. In contrast to the Davies ENDOR experiment, the Mims-ENDOR sequence does not require selective MW pulses. For a detailed description of the polarization transfer in a Mims-type experiment the reader is referred to the literature [43]. Just as with three-pulse ESEEM, blind spots can occur in ENDOR spectra measured using Muns method. To avoid the possibility of missing lines it is therefore essential to repeat the experiment with different values of the pulse spacing Detection of the echo intensity as a fimction of the RE frequency and x yields a real two-dimensional experiment. An FT of the x-domain will yield cross-peaks in the 2D-FT-ENDOR spectrum which correlate different ENDOR transitions belonging to the same nucleus. One advantage of Mims ENDOR over Davies ENDOR is its larger echo intensity because more spins due to the nonselective excitation are involved in the fomiation of the echo. 

As a result of equation (9), the Mims ENDOR signal is minimal ( blind ) at t = (n/Aiso), n = 0 (which is experimentally meaningless), 1,2,..., and is maximum at T = [(In -F l)/2Aiso]. Mims ENDOR spectra of a broad H pattern can clearly show the oscUlating blind spot pattern... 

Another advantage of ReMims (Doan) ENDOR is the ability to observe this implicit-TRIPLE effect for a wider range of hyperfine couplings. A given hyperfine coupling could be deleferiously affecfed by Mims blind spots, which would prevent observation of implicit TRIPLE. However, use of ReMims (Doan) ENDOR would allow selection of T value(s) that do not cause troublesome blind spots while simultaneously suppressing ESEEM, so that the presence of the implicit TRIPLE effect could be explored. 

T = 2n + l)/2Aiso]. Mims ENDOR spectra of a broad H pattern can clearly show the oscillating blind spot pattern... 

Although it has the ability for hyperfine selection, the Davies ENDOR sequence is not so sensitive to blind spots as in a Mims ENDOR, and thus it is more useful for detecting powder-pattern ENDOR line shapes. The Davies sequence, however, generally does not give so large a percent ENDOR effect typical Davies ENDOR effects in proteins are approximately 1-15% of the spin echo amplitude. 

Fig. 10. Diagram representing general choice of techniques based on their sensitivity as a function of hyperhne coupling magnitude. Consideration of the pulsed EPR techniques is based on operation-at X-band (9 GHz) microwave frequencies. The dotted line for Mims ENDOR at large A values indicates that the technique is sensitive but suffers from blind spots as described in the text.

More recently, pulsed ENDOR methods have been introduced. There are two commonly used ENDOR pulse sequences, both of which are based on the impact of RF pulses on the intensity of a spin echo that is formed from a series of three pulses at the microwave frequency. These techniques are sometimes called ESE-ENDOR. In Mims ENDOR the pulse at the RF frequency is applied between the second and third microwave pulses whereas in Davies ENDOR the RF pulse is applied between the first and second microwave pulses. The Mims ENDOR experiment is particularly effective for weakly coupled nuclei, but has some blind spots (frequencies that cannot be observed). It is often advantageous to combine data from both ENDOR methods. 

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