Coupled network

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. 

How can we correlate protons that are coupled to each other (proton coupling network) ... 

Homonuclear shift-correlation spectroscopy (COSY) is a standard method for establishing proton coupling networks. Diagonal and off-diagonal peaks appear with respect to the two frequency dimensions. 

In the case of a COSY-NOESY spectrum having an unequal mixing, let us consider two nuclei (say, A and B) that are spatially close but belong to different coupling networks, and nuclei B and C, which have scalar coupling with each other but are spatially distant. The only transfers allowed in this situation are ... 

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 information available on aqueous polymer blends is qualitative in nature because of the lack of a suitable theory to interpret the experimental observations. Mixed gels can be comprised of an interpenetrating network, a coupled network (as discussed above), or a phase-separated network [2]. The latter is the most common as the blends have a tendency to form two phases during gelation. In such cases the miscibility and thermodynamic stability have to be empirically investigated and proper conditions for miscible blends identified. This involves a phase diagram study as is described in [3]. 

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. 

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. 

TOCSY Two-dimensional total correlation spectroscopy To elucidate structure of organic molecules To establish proton coupling network and molecular connectivity... 

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... 

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]. 

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. 

TOCSY spectra usually give no information about direct coupling interactions and hence details of J-coupling networks. This missing information is commonly obtained either from ID homodecoupling or more efficiently from 2D COSY spectra. 

The experiment is mainly used to establish the H/ H J-coupling network and to help assign the proton resonances of a molecule. Additional infonnation, i.e, the evaluation of coupling constants can be obtained if the phase sensitive DQ-filtered COSY is used. 

Assign the signals using the COSY information, and try to establish the dipolar coupling network, i.e. to find spatial proximity between protons. Compare the results with the results extracted from the spectra of the corresponding 1D ROE experiments. 

Coupling network evaluated starting with either the anomeric protons (some ambiguities in the range 5.2 - 4.9 ppm) or with H-5 or H-b, protons (showing characteristic multiplets) as entry points... 

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