Coherence Transfer Problems

As outlined in the introduction the maximum sensitivity gain shown in Table 5.13 for direct or detection experiments and Table 5.21 for indirect detection experiments cannot be guaranteed for a real sample. 

Comparative sensitivity gain by coherence transfer in heteronuclear correlation spectroscopy [5.3]. 

In the following Check its the possible deficiencies of the heteronuclear coherence transfer step in combination with the preparative evolution step are shown. In an attempt to represent a real sample a number of different values for J(C, H) are used to simulate the simultaneous chemical shift and homonuclear coupling evolution. 

200 Hz in 20 Hz increments. The value of d2 1/(2 140) = 0.00357s. Inspect the processed spectrum and note how the Intensity and antiphase coherence depends upon the coupling constant. To study the same effects for nj(i3c, 1H) replace the current spin system with the spin system file ch5511b.ham which has values from 2.5 Hz to 12.5 Hz in 2.5 Hz increments. Change the value of d2 1/(2 8) = 0.0625s and repeat the simulation. 

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I also have a question for Prof. Fleming When you treated the problem of coherence transfer phenomenologically using Redfield s equation, did you examine details of the potential that leads to this robustness ... 

It should be noted that the concept of label coherence transfer is also being addressed in the earlier as well as the following chapters in Section 9.18.8. This chapter, however, was written with the specific purpose of demonstrating how labeling coherence tracking in combination with retrobiosynthetic comparison can provide elegant solutions for problems that are not easily resolved by other techniques. 

To overcome the problem of low dispersion of sugar resonances in nucleotides Hu et al. proposed to incorporate HCN transfer into HCCH-type sequences resulting in 3D MQ-HCN-CCH-TOCSY and 3D MQ-HCN-CCH-COSY experiments. The coherence transfer pathway starts with HT-CT transfer followed by an out-and-back transfer to N1/N9 for indirect detection and subsequent transfer through network. As a result the CCH spectra are additionally resolved with chemical shift. During the C -N1/N9 transfer MQ HT/CT coherence is active to reduce the relaxation losses due to relaxation. The experiments were successfully applied to a 23-mer RNA aptamer. 

The relayed-COSY experiment shall be considered only very briefly because it has essentially been superseded by the far superior TOCSY experiment described below, although one may still encounter references to this experiment in older literature however. The relayed-COSY experiment (Fig. 5.57) attempts to overcome problems caused by coincidental overlap of crosspeaks in COSY that can lead to a breakdown in the stepwise tracing of coupling networks within a molecule. It incorporates an additional coherence transfer step in which that transferred from a spin. A, to its partner, M, as in the standard COSY, is subsequently relayed onto the next coupled spin X in the sequence. This produces a crosspeak in the spectrum between spins A and X even though there exists no direct coupling between them, by virtue of them sharing... 

The first of these arises when the long spin-lock pulse acts in an analogous fashion to the last 90" pulse of the COSY experiment so causing coherence transfer between J-coupled spins. The resulting peaks display the usual antiphase COSY peak stmcture and tend to be weak so are of least concern. A far greater problem arises from TOCSY transfers which arise because the spin-lock period in ROESY is similar to that used in the TOCSY experiment (Section 5.7). This may, therefore, also induce coherent transfers between J-coupled spins when these experience similar rf fields, that is, when the Hartmann-Hahn matching condition is satisfied. Since the ROESY spin-lock is not modulated (i.e. not a composite pulse sequence), this match is restricted to mutually coupled spins with similar chemical shift offsets or to those with equal but opposite... 

In this chapter NMR-SIM is used to illustrate the theoretical principles of NMR spectroscopy instead of the more usual pure mathematical description. There are many textbooks, reviews and original papers in the literature dealing with the fundamentals of NMR spectroscopy and the reader is referred to them [2.1 - 2.7]. Alternatively the reader can use the list of references at the end of chapter 5 but these relate primarily to the pulse sequences discussed in that chapter. The design of any new experiment always starts with a formal analysis of the problem and an examination of the coherence transfer processes necessary to obtained the required information. The present chapter focuses on three items ... 

A perennial problem with coherence transfer experiments is the choice of coupling constant to use for calculating the free precession period prior to the coherence transfer step. The same problem arises when calculating the refocusing delay after a coherence transfer step prior to data acquisition under decoupling. 

One important source of distance constraints for paramagnetic proteins are cross correlations between Curie spin relaxation and H-X dipolar relaxation (X = H, C, or N). These cross correlations can in principle be detected by the same pulse sequences as are commonly used in diamagnetic systems to detect cross correlation between chemical shift anisotropy and H- N dipolar relaxation. However, in paramagnetic systems, rapid transverse self-relaxation tends to quench the build-up of relaxation allowed coherence transfer. To overcome this problem, Kateb and Piccioli have proposed a modified HSQC experiment to observe and quantify relaxation-allowed coherence transfer before it is quenched by a short T2. 

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