Heteronuclear recoupling

The most common technique used for heteronuclear distance measurement is rotational-echo double resonance - REDOR [239,240,241]. It is now relatively commonplace to label proteins with the spin-1/2 isotopes and N, and C-N distance constraints can be derived using REDOR [242, 243] and used to determine elements of secondary structure. More importantly for molecules of interest to the pharmaceutical industry is the measurement of distances between and the quadmpolar deuterium ( H) nucleus [244]. Introduction of the H isotope is relatively straightforward and the measurement of distances at natural abundance over several bonds can be used to provide torsional constraints [245]. The measurement of distances (up to 6.8 A) 

RFDR methodology has been used to measure inter-fluorine distances of up to 12 A (-I-/-2 A) [255], this application is expected to be of great utility in the solid state NMR analysis of fluorine containing pharmaceuticals. RFDR can be used to provide distance measurements between a labelled nucleus and others of the same isotope at natural abundance. For instance, such measurements were made between a singly labelled guest molecule and the natural abundance carbons of the host in an organic clathrate [256]. 

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Wi, S. Frydman, L. Heteronuclear Recoupling in Solid-State Magic-Angle-Spinning NMR via Overtone Irradiation. J. Am. Chem. Soc. 2001,123, 10354-10361. 

SHRIMP (scalar heteronuclear recoupled interaction by multiple pulse Weitekamp et al., 1982) and WIM-24 (windowless isotropic mixing sequence Caravatti et al., 1983) were the first heteronuclear isotropic-mixing sequences that were proposed in the literature (see Table 6). The SHRIMP sequence consists of hard 90° pulses separated by delays, whereas WIM-24... 

ROESY-TOCSY experiment Selective inverse detection of C-H correlation Scalar heteronuclear recoupled interaction by multiple pulse Simulation program one Selectively inverted soft PICSY Singular value decomposition Tailored correlation spectroscopy Triple-resonance J cross-polarization Total correlation spectroscopy TOCSY-ROESY experiment TOCSY without NOESY... 

Thin-film amorphous hydrogenated boron carbide (BxC Hy) from an orthocarborane precursor was studied by solid state NMR. Using Lee-Goldburg cross-polarization and heteronuclear recoupling experiments are used to confirm atomic connectivities which allowed a model to be proposed for this. 

The REDOR scheme for heteronuclear recoupling has been extensively used for distance measurements [11]. It is constituted of 180° pulses apphed every half rotor period under supercycled conditions (XY-4 [30], XY-8 or XY-16 [31]) that increase the robustness to oflset and RP inhomogeneity. Although used only once in the D-HMQC context [18], the REDOR scheme may be an interesting option as it is a weU-estabhshed technique in the measurement of distances and because its setup is easy, as only the power and length of the 180° pulses must be determined. 

HARDSHIP heteronuclear recoupling with dephasing by strong homonnclear interactions of protons Hb hemoglobin... 

In the last decades a series of homonuclear and heteronuclear recoupling experiments have been developed that combine a 2D approach with fast MAS and dipolar multiple-quantum NMR spectroscopy. In this way, the chemical shift resolution of different chemical environments allows sites of interest to be distinguished and fully identified, and then it is possible to obtain structural information since dipole-dipole couplings are sensitive to the distances between the coupled nuclei as well as to the orientation of the internuclear vector. 

Takegoshi and Terao " proposed a heteronuclear recoupling scheme based on FS-LG. The pulse sequence consists of pairs of back-to-back FS-LG pulses with given phases and frequency offsets. When the sample spinning period is equal to one or two cycle times of the pulse sequence, a zero-quantum term of the dipolar Hamiltonian is recovered, allowing this sequence to work in the spinning speed ranging from a few kilohertz to 30 kHz. 

Heteronuclear and homonuclear dipolar recoupling under magic-angle spinning... 

Recently, numerous studies reported the application of homonuclear and heteronuclear selective recoupling schemes on uniformly labelled ligand interacting with membrane receptors. The polarization exchange curves were fitted with the two-spin model and showed that it is possible to determine intemuclear distances up to 4.5 A.118... 

The REDOR experiment has formed the basis for a large number of ideal pulse type recoupling experiments, and later finite pulse variants, for heteronuclear dipolar recoupling. These include experiments such as frequency selective REDOR (FS-REDOR) [80], TEDOR (Transferred Echo DOuble Resonance) [25], and 3D variants of TEDOR [81, 82], which have found important applications, e.g., for measurement of intemuclear 13C-15N distances in biological solids. We should also mention that rotor-encoded variants of TEDOR, such as REPT, HDOR [83], and REREDOR [84], have been proposed for 1H13C dipolar recoupling under high-speed MAS conditions. 

Dipolar recoupling may also be accomplished using continuous rf irradiation as demonstrated in the heteronuclear and homonuclear case by the CP at MAS conditions (or for low-y heteronuclear spins called double-cross-polarization, DCP [103]) and HORROR (homonuclear rotary resonance) [26] experiments, respectively. These experiments may easily be described by transforming the description into the interaction frame of the rf irradiation using (14a) exploiting... 

Recently a new type of proton assisted recoupling experiments has been developed for coherence transfer where rf irradiation is taking place on all involved rf channels. This embraces the homonuclear proton assisted recoupling (PAR) [45, 140, 141] and the later resonant second-order transfer (RESORT) [142] experiments, as well as the heteronuclear proton assisted insensitive nuclei (PAIN) cross polarization [44] experiments. In PAR and PAIN, spin-lock CW irradiation is applied on both passive ( H) and active spins (13C, 15N) without matching rotary resonance conditions. In RESORT a phase alternation irradiation scheme for the passive spins is used. 

In the simplest setup, the two strong field components may be set identical to Ci = Cs = C. The relatively large CIX or CSX term averages isotropic and anisotropic chemical shift effects as well as the heteronuclear dipolar coupling interaction between 15N or 13C and H. The difference of - or the sum of - the B coefficients selects the form of the recoupled heteronuclear dipole-dipole coupling interaction, as expressed in terms of the effective Hamiltonian in the interaction frame of the rf irradiation... 

Any heteronuclear dipolar recoupling techniques developed for systems containing multiple 3H spins must have the property to suppress actively the dipolar... 

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