Relayed transfer

Fig. 7 (a) 2D PAR spectrum of uniformly 13C, 15N-labeled N-/-MLF-OH (structure inserted) diluted to 10% in natural abundance providing cross peaks (circled) corresponding to intemuclear 13C-13C distances of more than 4 A. (b) Simulations supporting that the transfer is dominated by the MC- H -LC third-spin assisted transfer, i.e., relayed transfer is negligible (reprinted from [45] with permission)... 

Fig. 6.6 Schematics of hydrogen bonds between purine and pyrimidine bases with indicated trans-hydrogen bond scalar interactions and related coupling constants, which can be measured using NMR. In addition to correlations between exchangeable protons and nitrogens, also a relayed transfer to nonexchangeable aromatic protons, shown by a dashed arrow, can be employed. The...

In a ID COSY-RELAY experiment [38] (fig. 13(a)) a multistep relay transfer is applied after the filtration. If the filtration is performed on the H-2 proton, the CSSF is incorporated into the first spin-echo. If there is not sufficient chemical shift separation between H-2 protons, the filter is shifted to the second spin-echo. The method is illustrated for the separation of the spin systems of two terminal /3-glucopyranose residues of a modified LPS (5) containing a total of nine saccharide units [76]. The anomeric proton resonances of the two /3-glycopyranoses overlapped almost completely, with a chemical shift difference of only 1.9 Hz, while the corresponding H-2 resonances were separated by 55.0 Hz. The length of the filtration interval, Ti, was adjusted to yield a maximum antiphase magnetization of H-2 pro-... 

A type I thioesterase domain is present at the NHj-terminal of the animal FAS and is responsible for catalyzing hydrolysis of the completed fatty acyl chain from the enzyme. The active site contains both conserved serine and histidine residues [87] and is thought to function via a mechanism similar to that of the serine proteases [50] however, no conserved acidic residue is present to complete the charge relay/transfer. A second variety of thioesterase (type II) is encoded as a separate protein and interacts with the multifunctional FAS to release medium chain fatty acids [88, 89]. This enzyme has a weak sequence similarity to the type I thioesterase, which includes the conserved active site serine and histidine residues. These enzymes are also homologous to proteins encoded by genes involved in the synthesis of peptide antibiotics [90,91] (see below). 

Heteronuclear Hartmann-Hahn sequences also effect homonuclear Hartmann-Hahn transfer, resulting in (heteronuclear and homonuclear) total correlation spectroscopy (TOCSY Bearden and Brown, 1989 Zuiderweg, 1990 Brown and Sanctuary, 1991 Ernst et al., 1991). Simultaneous heteronuclear and homonuclear magnetization transfer can be beneficial in relayed transfer experiments (Gibbs and Morris, 1992 Tokles et al., 1992 Majumdar et al., 1993). However, as pointed out by Ernst et al. 

TOCSY Provides multistep (relayed) transfers to overcome ambiguities arising from crosspeak overlap. High sensitivity. In-phase lineshapes can provide correlations even in the presence of broad resonances. Number of transfer steps associated with each crosspeak not known, a priori. In-phase lineshapes tend to mask crosspeak fine-structure and may preclude J-measurement. 

Crotonaldehyde has been used in the literature to test the homonuclear and heteronuclear relayed COSY experiment [5.142]. In the following Check its a spin system based on crotonaldehyde will also be used but because the maximum number of coupled spins in a cluster is restricted to nine, see section 1.2, and to speed up the calculation the CH3 group has been replaced by a CHX2. Two spin systems have also been created for this crotonaldehyde type molecule one for a pure isotopomer and a second with the natural ratio of and DC isotopomers the latter being used for pulse sequence with relay transfer through the DC nucleus. 

Load the configuration file ch54113.cfg an6 run the simulation of the 2D 130-1H relay COSY experiment of the crotonaldehyde type spin system. In this modified spin system the coupling between H-1 and H-2 is excluded to show the relay transfer from H-1 to H-2 trough C-1. Compare the result with the basic 1H COSY spectrum. Repeat both simulations using the spin system relcspsy.ham. Note however that in this spin system the 13C nucleus is 100% abundant. 

In contrast to the NOE experiment the cross-peaks of the ROE experiment are of opposite sign with respect to the chemical-exchange peaks. This is a particularly valuable property in studies of complex formation where peaks arise both from cross-relaxation and chemical exchange. Furthermore, the assignment of stereo-specific restraints to the methylene protons that are often subject to spin-diffusion can be facilitated by the ROESY measurement. The ROE cross-peaks that stem from the relayed transfer have opposite sign with respect to the direct transfer and thus the methylene ROE signals do not tend to equal values. 

Bolton PH (1985) Heteronuclear relay transfer spectroscopy with proton detection. J Magn Reson 62 143-146... 

An immediate suggestion from the above study is that the rotation along the axis of C — N in polypeptide becomes far more difficult by the presence of water molecules for two reasons They may stabilize the zwitter ionic structure as a solvent effect. Besides, water molecules may cause proton relay transfer in the reverse manner of the present state, that is, from keto form (peptide) to enol form, which results in creation of a strong double bond in C — N. Thus the presence of water molecules around polypeptide can make the dramatic difference from gas phase reaction. Although the extent of possibility of free rotation around the C - N bond in polypeptide is well-known as one of the classic issues in protein science [244], one should... 

The rate constant for this radical relay transfer reaction has the same activation energy as that of the j8-relaxation in the corresponding polymer. This result again suggests that the kinetics of macroradical reactions in solid polymers is sensitive to small-scale molecular dynamics characterized by the secondary... 

Relayed transfer of two-dimensional double-quantum coherence (2QT) has recently been described in homonuclear and heteronuclear correlated spectral system. The pulse sequence used in the relayed 2QT experiment is shown in Figure 5.88. The first three mixing pulses (90°, 180°, 90°) serve to mix double-... 

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