REDOR technique

An example of a spin-1/2 and spin-1 pair, i.e. C and in enriched DL-[2- H,3- C] alanine was later studied by the REDOR technique , proving the applicability of the method in these cases as well. Another spin-1/2 and spin-1 pair of interest in the present context is C- Li. In Figure 4, the dipolar coupling between Li and C is given as a function of the carbon-lithium distance. It is clear from this graph that distances up to ca 4 A can be measured by this method and successful applications of the REDOR technique for the measurement of C-Li distances have been reported (see Section II.D). 

As mentioned in Sections I.B.2.b and II.A, the dipolar coupling between Li- C may complicate solid state NMR spectra of organolithium compounds and its elimination is often desirable. On the other hand, dipolar coupling constants are related to atomic distances and their determination can yield important structural information. It is therefore of general interest that the REDOR technique, briefly described in Section I.B.2.b, provides a means to determine these parameters. 

In conclusion, the investigations presented show that the Li- C REDOR technique can be a valuable tool for the structural characterization of organolithium complexes. Contrary to X-ray, there is no need for single crystals. Although the carbon-lithium distances are to some extent overestimated by the REDOR technique, the precision is sufficient to discriminate between various possible lithium positions in the complexes. 

The rotational echo double resonance (REDOR) technique is essentially the MAS version of SEDOR, affording a site-resolved measurement of heteronu-clear dipolar coupling information on rotating samples. Figure 8 shows two commonly used pulse sequences [21-24]. 

REDOR technique has been applied to natural abundance nuclei coupled to a singly labeled nucleus to determine the interatomic... 

Solid-state NMR has been used to study the new aluminophosphate material A1P04-Z0N. The location of fluorine in the AIPO4-ZON framework has been verified by measuring the intemuclear distances between F and all Al sites using the MQ-REDOR technique. MQ-REDOR has been also used to measure the distances between and Al nuclei in this material. 

As mentioned above, the determination of atomic level structure, i.e., the backbone torsion angles for an oriented protein fiber, is possible by using both solid-state NMR method described here and specifically isotope labeling. This is basically to obtain the angle information. Another structural parameter is distance between the nuclei for atomic coordinate determination. The observation of Nuclear Overhauser Enhancements (NOEs) between hydrogen atoms is a well known technique to determine the atomic coordinates of proteins in solution [14]. In the field of solid-state NMR, REDOR (rotational echo double resonance) for detection of weak heteronuclear dipole interactions, such as those due to C and N nuclei [15, 16] or R (rotational resonance) for detection of the distance between homonuclei, are typical methods for internuclear distance determination [17,18]. The REDOR technique has been applied to structure determination of a silk fibroin model compound [19]. In general, this does not require orientation of the samples in the analysis, but selective isotope labeling between specified nuclear pairs in the samples is required which frequently becomes a problem. A review of these approaches has appeared elsewhere [16]. 

In 1989 Gullion and Schaefa described the rotational-echo, double-resonance (REDOR) technique. Distances measured by this method may be used in the detemination of bound-ligand conformations without the limitations imposed by crystallization, molecular weight or solubility. They have performed REDOR NMR... 

Fig. 9.14 The determination of the distance between the specific spin labels in [1- C, N]acetyl-L-carnitine by the REDOR technique. The difference, AS, and reference, Sq, REDOR spectra are shown for the 34-fR experiment. (Reproduced from [62].)...

Solid-echo double resonance (SEDOR) and rotational-echo double resonance (REDOR) techniques can be used to determine the proximity of the two types of nuclei, e.g., to determine which Na ions are closest to A1 atoms in the framework of a zeolite, or to determine internuclear distances, or to help locate the position of a guest molecule relative to the host framework. ... 

REDOR technique on the quadmply labeled analog 15.3.2 (489), and these results coupled with fluorescence data led to the proposal of stmcture 15.3.3 as the most probable conformation of taxol on tubulin. 

In another work based on the use of the REDOR technique the interchain i3C- F dipolar coupling was measured for mixtures of differently labelled (KIAGKIAjs peptides in phospholipid bilayers. ... 

An example of how the REDOR technique was used was for determining the 0-H distance of bridging hydroxyl groups in zeolites H-Y and H-ZSM-5 in high magnetic fields [27] and the P-B distances in SAPO-based mesoporous materials [28]. 

A.L. Blumenfeld, J.J. Ftipiat etal.. Characterization of Bronsted and Lewis acidity in zeolites by solid-state NMR and the recent progress in the REDOR technique. Magn. Reson. Chem. 37,5118-5125(1999)... 

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