Double-quantum coherence, sensitivity

Although the natural abundance of nitrogen-15 [14390-96-6] leads to lower sensitivity than for carbon-13, this nucleus has attracted considerable interest in the area of polypeptide and protein stmcture deterrnination. Uniform enrichment of is achieved by growing protein synthesi2ing cells in media where is the only nitrogen source. reverse shift correlation via double quantum coherence permits the... 

SELINQUATE (Berger, 1988) is the selective ID counterpart of the 2D INADEQUATE experiment (Bax et al., 1980). The pulse sequence is shown in Fig. 7.4. Double-quantum coherences (DQC) are first excited in the usual manner, and then a selective pulse is applied to only one nucleus. This converts the DQC related to this nucleus into antiphase magnetization, which is refocused during the detection period. The experiment has not been used widely because of its low sensitivity, but it can be employed to solve a specific problem from the connectivity information. 

In the following, we will discuss heteronuclear polarization-transfer techniques in four different contexts. They can be used as a polarization-transfer method to increase the sensitivity of a nucleus and to shorten the recycle delay of an experiment as it is widely used in 1H-13C or 1H-15N cross polarization. Heteronuclear polarization-transfer methods can also be used as the correlation mechanism in a multi-dimensional NMR experiment where, for example, the chemical shifts of two different spins are correlated. The third application is in measuring dipolar coupling constants in order to obtain distance information between selected nuclei as is often done in the REDOR experiment. Finally, heteronuclear polarization transfer also plays a role in measuring dihedral angles by generating heteronuclear double-quantum coherences. 

By allowing multiple-quantum coherence to process during the evolution period of a two-dimensional experiment, Drobny et al. were able to detect its effects indirectly. This idea subsequently blossomed into the new technique of filtration through double-quantum coherence. Multiple-quantum coherence of order n possesses an n-fold sensitivity to radiofrequency phase shifts, which permits separation from the normal single-quantum coherence. This concept inspired the popular new techniques of double-quantum filtered correlation spectroscopy (DQ-COSY) and the carbon-carbon backbone experiment (INADEQUATE), both designed to extract useful connectivity information from undesirable interfering signals. 

Pulsed methods [82] increase the range of distance sensitivity. They can be used to separate the dipole-dipole interaction from other contributions of the spin Hamiltonian. At very large available microwave power, distances can be measured well by double quantum coherence (DQC) that uses a single frequency. With the power available on commercial spectrometers, double electron electron resonance [DEER, an acronym which is synonymously used with PELDOR (Pulsed Electron Double Resonance)] is the more sensitive technique and is thus most widely applied in the... 

The peaks in the 2D DQ spectrum correspond to double-quantum coherences between two spins which must be relatively close neighbors in space in order to contribute significantly to the peak intensity as follows from the strong distance sensitivity of the dipolar coupling. From the existence of the corresponding peaks therefore through space dipolar connectivities can be easily established. 

Until now the double-quantum coherence spectroscopy has not often been used for silicon frameworks. In double-quanmm coherence spectroscopy the silicon nucleus is almost ten times as sensitive as the C nucleus because of the higher natural abundance of Si (4.7% compared with 1.1% C). Disadvantages of the Si nucleus are the long relaxation times T i) and the negative nuclear Overhauser effect (NOE). In practice, this means that a very long recording time is necessary unless a relaxation reagent such as Cr(acac)3 is used. 

In the past decades, continuous-wave (CW) and pulse [double quantum coherence (DQC) and PELDOR] ESR spectra of double-spin-labeled systems have been reported [90, 91]. The high sensitivity provided by DQC and PELDOR spectra [92] allows reliable determination of distances (1.6-6.0 nm) between labels in frozen solution but cannot be used for distances shorter than 1.6 nm because of the large electron dipolar interaction and the presence of relevant scalar electron exchange interactions prevents the irradiation of a single electron spin, which is a prerequisite for their application [92]. 

The HMQC experiment gives exactly the same result as the HSQC, and the data is processed in the same way. There are some differences in sensitivity and peak shape that depend on the size and complexity of the molecule, and the pros and cons of the two experiments are the subject of some debate in the literature. Because it relies on double-quantum and zero-quantum coherences (DQC and ZQC) during the evolution (t ) period, the HMQC is more difficult to explain and understand than HSQC, which uses only the familiar singlequantum transitions that can be diagramed and analyzed using vectors. We discuss it here because it forms the basis of the HMBC (multiple-bond) experiment. 

Finally, the very recently proposed double-quantum (DQ) and double-quantum filtered (DFQ) STMAS experiments [42] allow filtering out diagonal (and outer satellite transitions) peaks of STMAS spectra with no loss on sensitivity. The experiments efficiently convert inner ST coherences from single to doublequantum with a central-selective transition 71 pulse. The conversion allows the selection of double-quantum transfer pathways with phase cycling, filtering out the unwanted peaks. 

The fact that multiple quantum coherences can only be measured indirectly by their influence on the amplitude and phase of a subsequently acquired single quantum signal makes this technique a phase-encoding method. The necessity to acquire the spatial information in time-consuming extra dimensions is a penalty in all phase-encoding techniques. However, in the case of double quantum imaging of quadrupolar nuclei like, e.g., H, the wideline information of the single quantum spectrum can be utilized for contrast, because the quadrupolar interaction usually dominates all other spectral features and is a sensitive probe for molecular dynamics and orientations [80-83]. 

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