Dipolar correlation spectroscopy

Figure 22. A representation of the aggregation shifts, i.e., the difference between the chemical shift in the aggregate (i.e., the solid state) and in the monomer (i.e., in solution), as determined for the H and 13C nuclei in a uniformly 13C enriched bacteriochlorophyll (BChl) c in intact chlorosomes of Chlorobium tepidum, using 2D H-13C and 3D H-13C-13C dipolar correlation spectroscopy at a magnetic field of 14.1 T. Circles correspond to upfield changes upon aggregation with the size of the circle indicating the magnitude of the change. The different representations in I and II correspond to the experimental observation of two separate components. (Reproduced with permission from ref 162. Copyright 2001 American Chemical Society.)...

The backbone and side-chain and N signals of a solid 62-residue (U- C, N)-labelled protein containing the a-spectrin SH3 domain were assigned by 2D MAS N- C and dipolar correlation spectroscopy at... 

Water-protein interactions in microcrystalline Crh were measured by H- C solid state NMR. Using H- C dipolar correlation spectroscopy, proton exchange on the millisecond time scale was observed between water molecules and protein protons in a solid sample. These interactions were related to important structural features of the protein such as hydrogen-bonding or salt-bridge networks. 

B.J. Van Rossum, H. Foerster, H.J.M. de Groot, High-field and high-speed CP-MAS 13C NMR heteronuclear dipolar-correlation spectroscopy of sohds with frequency-switched Lee-Goldburg homonuclear decoupHng, J. Magn. Reson. 124 (1997) 516-519. 

Nuclear Overhauser effect (nOe) The change in intensity in the signal of one nucleus when another nucleus lying spatially close to it is irradiated, with the two nuclei relaxing each other via the dipolar mechanism. Nuclear Overhauser effect correlation spectroscopy (NOESY) A 2D... 

Each of these experiments has pros and cons. The SEDOR experiment is easiest to set up but lacks the resolution since it is made on a static sample, and is often limited in the range of accessible dipolar couplings by T2. The REDOR experiment is more elaborate to set up and requires stable spinning, but its increased resolution enables multisite systems to be explored, while the longer T2 enables longer distances to be determined. TEDOR is less sensitive than REDOR as it involves a transfer step with a theoretical maximum efficiency of 50 percent. However, it is not a difference experiment, and is therefore less prone to experimental errors. An important application of TEDOR is its use in 2D-correlation spectroscopy. 

In this review, we shall introduce some basic concepts about sohd-state NMR of half-integer quadrupolar nuclei and discuss the most useful and promising methods presently available to study them. These include older methods, such as Double Rotation (DOR) and Dynamic Angle Spinning (DAS), and novel techniques including MQMAS, Quadrupolar Phase Adjusted Spinning Sidebands QPASS, SateUite Transition (ST) MAS and Inverse-STMAS NMR, and Fast Amplitude Modulation (FAM). We also discuss several techniques based on dipolar interactions between quadrupolar and spin-1/2 nuclei, such as Cross-Polarization (CP) MQMAS, MQ Heteronuclear Correlation Spectroscopy... 

At the qualitative level, NMR studies devoted to this issue exploit heteronuclear dipolar recoupling to correlate resolved MAS NMR signals of the nuclear species associated with different network formers. This information comes from the dephasing of the observe nuclei in REDOR, TRAPDOR, or REAPDOR experiments, or from one- or two-dimensional correlation spectroscopy involving coherence transfer via TEDOR (transferred echo double resonance) or... 

General symmetry principles for rotor-synchronized pulse sequences in MAS solid-state NMR have been presented. The synunetry theory has been extended to the case of generalized Hartmann-Hahn sequences, in which rotor-synchronized r.f. irradiation is applied simultaneously to two isotopic spin species. The symmetry theory has been used to design pulse sequences which implement heteronuclear dipolar recoupling at the same time as decoupling homonuclear spin-spin interactions, and which also suppress CSAs. Experimental demonstrations of heteronuclear 2D correlation spectroscopy, heteronuclear MQ spectroscopy, and the estimation of intemuclear dipolar couplings have been given. 

Today, spin diffusion is often used in a quite general way to describe multispin polarization-transfer processes, whether or not the process can actually be described by a diffusion equation. In this overview, we will interpret spin diffusion in this broad sense. If applied in the context of two-dimensional homonuclear experiments, it becomes synonymous with total through-space correlation spectroscopy (TOSSY) [2], the dipolar equivalent of the liquid-state TOCSY [3] experiment. 

Two-dimensional homonuclear correlation spectroscopy (COSY) is well established for the study of liquids [24,25] and has recently been shown to be effective with solid samples [8,26-38], many of which are of catalytic interest [30,37]. The COSY spectrum contains diagona] peaks and off-diagonal cross-peaks. The cross-peaks arise because of coherence transfer between spins, and they indicate that the resonances at the relevant shift positions on the two axes are coupled. In the solid state, the necessary coherence transfer may occur through dipolar or scalar interactions. Experiments based on scalar couplings have been more popular, because in favorable circumstances they allow the spectroscopist to establish unambiguously the atomic connectivities within molecules in solid samples [8,27-38] including complex zeolitic frameworks [8,30-37]. 

---