Spin Coupling Networks

The most important difference in the spectrum as compared with RGs released by RG-hydrolase action (Colquhoun et al., 1990) was a doublet at 5 5.81 (J = 3.4 Hz). From the COSY experiments this doublet was found to belong to a four-proton spin-coupling network... 

The experiments described above include the most commonly used and most sensitive ones. As mentioned earlier, many more pulse sequences exist that can provide important information. Some of the experiments are also able to distinguish between certain types of amino acids based on their unique spin coupling network or chemical shifts [74—79]. Whether these experiments are used during the assignment process depends on the individual project. 

In this chapter, the discussion will be focused on the ID TOCSY (TO-tal Correlation SpectroscopY) [2] experiment, which, together with ID NOESY, is probably the most frequently and routinely used selective ID experiment for elucidating the spin-spin coupling network, and obtaining homonuclear coupling constants. We will first review the development of this technique and the essential features of the pulse sequence. In the second section, we will discuss the practical aspects of this experiment, including the choice of the selective (shaped) pulse, the phase difference of the hard and soft pulses, and the use of the z-filter. The application of the ID TOCSY pulse sequence will be illustrated by examples in oligosaccharides, peptides and mixtures in Section 3. Finally, modifications and extensions of the basic ID TOCSY experiment and their applications will be reviewed briefly in Section 4. 

These experiments provide identification of the through-bond spin coupling network as well as through-space proximity between spins. Examples of the application of these techniques have demonstrated the efficiency of these techniques in establishing the assignment, sequence and linkage site information for oligo- and poly-saccharides [59-60, 62-65], and for saponins [29]. 

The construction of proton spin-coupling networks is achieved by a variety of COSY experiments (Section 7-7). Standard COSY and DQF-COSY spectra of the 15-mg sample of the unknown compound are shown in Figure 7-1 la, and the data from these contour plots are summarized in Table 8-3. Coupling constants, which have been measured in the NMR spectrum, correspond to cross peaks, and are given in parentheses in the table. 

Selective isotropic mixing transfer. Simplification of spin coupled network... 

The most efficient way to speed up spin diffusion is the so-called r.f.-driven spin-diffusion experiment [15, 19] where the chemical-shift differences are removed by r.f. irradiation. For small chemical-shift differences, r.f.-driven spin diffusion can be implemented by applying a continuous-wave r.f. field to the S-spins which can theoretically be described by a transformation into a tilted rotating frame (see Appendix B). To zeroth-order average Hamiltonian theory the chemical-shift differences are removed (fl, — fty = 0 for all spins i and j) and the dipolar-coupling frequencies are scaled by a factor s = -1/2. The scaled-down (or ideally vanishing) chemical-shift difference allows one to keep the zero-quantum line narrow by decoupling the protons. This results in fast spin-diffusion rates. Furthermore, the rate constants are now determined by the S-spin coupling network, and the proton spins need not be considered for the data analysis. 

Fig. 9.6. Two-dimensional (2D) COSY spectrum ofa-CyD in D2O. Off-diagonal cross peaks allow us to follow the spin-spin coupling network and assign subsequent signals starting at the anomeric H-1 proton characteristic of its large chemical shift.

Homonuclear NMR Structural elucidation becomes much more difficult as spectral complexity increases. Under these circumstances, proton NMR spectroscopy has benefited considerably from the use of 2D NMR techniques. For example, homonuclear correlation spectroscopy (COSY) identifies spin-coupled pairs of nuclei as well as spin-coupled networks of nuclei in a molecule, even without prior structural information. The 2D /-resolved spectroscopy method permits even highly overlapping resonances to be resolved into readily interpretable multiplets. This enables chemical shift assignments to be made in a very straightforward manner. Both of... 

Bax A and Freeman R 1981 Investigation of complex networks of spin-spin coupling by two-dimensional NMR J. Magn. Reson. 44 542-61... 

The DQFCOSY spectrum of RpII in D O is shown in Figure 2. Each cross peak in this spectrum identifies a pair of coupled spins of the amino acid side chains. Since couplings are not propagated efficiently across amide bonds, all groups of coupled spins occur within individual amino acids. The chemical structure of an amino acid side chain is reflected in the characteristic coupling network and chemical shifts (13). Valine spin system (CH-CH-(CH3)2) explicitly shown in Figure 2 as an example. 

Morello, A., Stamp, P.C.E. and Tupitsyn, I. (2006) Pairwise decoherence in coupled spin qubit networks. Phys. Rev. 

Cross-correlated dipolar relaxation can be measured between a variety of nuclei. The measurement requires two central nuclear spins, each of which is directly attached to a remote nuclear spin (Fig. 16.4). The central spin and its attached remote spin must be connected via a large scalar coupling, and the remote spin must be the primary source of dipolar relaxation for the central spin. The two central spins do not need to be scalar coupled, although the necessity to create multiple quantum coherence between them requires them to be close together in a scalar or dipolar coupled network. In practice, the central spins will be heteroatoms (e.g. 13C or 15N in isotopically enriched biomolecules), and the remote spins will be their directly attached protons. 

Heteronuclear NOE experiments yield additional, but due to their inherent low sensitivity only sparsely exploited information on molecular structures. They are most useful to unravel structural features in the vicinity of quaternary carbons often behaving as barriers when exposed to standard routine experiments dedicated to evaluate coupling networks. Heteronuclear NOE data - in some sense complementary to the data from heteronuclear long-range couplings - are based on dipolar spin-spin interactions and strongly depend on intemuclear distances. In contrast to the sometimes similar Jch... 

C This pulse sequence, the popular homonuclear 2D COSY (COrelation Speciro-scopY) experiment, was designed to determine the entire H/ H-coupling network of a molecule within a single experiment. The sequence consist of all four elements, i.e. a preparation, a evolution, a mixing and a detection period. The evolution period serves to introduce the second time (tl) domain of a 2D experiment and in the mixing period, which is actually a pulse, polarizations are exchanged among the coupled spins. 

Since a single experiment with a fixed spin-lock period is usually performed, this experiment does not, in contrast to the H/ H decoupling experiment, establish the complete coupling network. For this purpose a series of ID TOCSY experiments, with the length of the mixing period increasing step-wise from experiment to experiment, has to be performed. 

In the solid state a metallic element such as sodium consists of an ordered network of positively charged sodium ions and spin-coupled electrons which move freely through the field owing to the positive ions. In this state the conductance is high. 

---