NOESY experiments exchange spectroscopy

Figure 1.45 Coherence transfer pathways in 2D NMR experiments. (A) Pathways in homonuclear 2D correlation spectroscopy. The first 90° pulse excites singlequantum coherence of order p= . The second mixing pulse of angle /3 converts the coherence into detectable magnetization (p= —1). (Bra) Coherence transfer pathways in NOESY/2D exchange spectroscopy (B b) relayed COSY (B c) doublequantum spectroscopy (B d) 2D COSY with double-quantum filter (t = 0). The pathways shown in (B a,b, and d) involve a fixed mixing interval (t ). (Reprinted from G. Bodenhausen et al, J. Magn. Resonance, 58, 370, copyright 1984, Rights and Permission Department, Academic Press Inc., 6277 Sea Harbor Drive, Orlando, Florida 32887.)...

Oil and 0)2, and (b) 2D shift-correlation spectra, involving either coherent transfer of magnetization [e.g., COSY (Aue et al, 1976), hetero-COSY (Maudsley and Ernst, 1977), relayed COSY (Eich et al, 1982), TOCSY (Braunschweiler and Ernst, 1983), 2D multiple-quantum spectra (Braun-schweiler et al, 1983), etc.] or incoherent transfer of magnedzation (Kumar et al, 1980 Machura and Ernst, 1980 Bothner-By et al, 1984) [e.g., 2D crossrelaxation experiments, such as NOESY, ROESY, 2D chemical-exchange spectroscopy (EXSY) (Jeener et al, 1979 Meier and Ernst, 1979), and 2D spin-diffusion spectroscopy (Caravatti et al, 1985) ]. 

In addition to X-ray crystallographic studies, two-dimensional NMR solution experiments (i.e., COSY, 1D-NOE, and NOESY, discussed in Sections 3.5.9 and 3.5.10) have been carried out on many lanthanide(III), Ln(ffl), chelate complexes to confirm that the structure of the MRI imaging agent, used in aqueous solution, will correspond to the solid-state X-ray crystallographic structure. Two-dimensional exchange spectroscopy (2D-EXSY) has been applied to lanthanide chelates to study the dynamics of conformational equilibria (how acetate arms chelate and how... 

In this chapter, we will introduce a new level of theoretical tools—the density matrix— and show by a bit of matrix algebra what the product operators actually represent. The qualitative picture of population changes in the NOE will be made more exact, the precise basis of cross-relaxation will be revealed, and a new phenomenon of cross-relaxation— chemical exchange—will be introduced. With these expanded tools, it will be possible to understand the 2D NOESY (nuclear Overhauser and exchange spectroscopy) and DQF-COSY experiments in detail. 

Similar to the STD experiment, exchange-transferred NOE spectroscopy (et-NOESY) provides information about a binding event between a small molecule and a high molecular weight biomolecule, even when the biomolecule itself is not amenable to NMR studies because of broad line widths, low solubility, or fast T2 relaxation. 

A unique aspect of this form of exchange spectroscopy concerns the ability to detect species whose concentration is so low that they escape detection in a conventional one-dimensional experiment. Figure 7 shows a section of the H NOESY spectrum for a mixture of isomeric palladium phosphino-oxazoline, 1,3-diphenylallyl complexes. One observes a major component in exchange with a visible minor component (ca. 10% of the more abundant isomer). However, there are additional, very broad, exchange cross-peaks from the main isomer to an invisible species, which would easily have gone undetected. 

Other basic 2-D experiments include J spectroscopy, which displays scalar-coupled multiplets along the second dimension, and nuclear Overhauser effect spectroscopy (NOESY, pronounced nosy ), which exploits through-space dipolar couplings to provide conformational information. In the NOESY experiment, protons are selectively irradiated as data are acquired, and the effect of neighboring nuclei is monitored by their mutual enhancement. NOESY is one example of an exchange experiment, in which magnetization is transferred from spin to spin. 

The effect of exchange processes can be observed in two-dimensional spectrum and be analyzed in a very similar way. The 2D exchange spectroscopy (EXSY) is in principle, identical to the NOESY experiment. Cross peaks in 2D EXSY experiments arise from noncoherent magnetization transfer between sites with different resonances. Noncoherent magnetization transfer takes place either by exchange of nuclei between different sites or by cross-relaxation (NOE). However, the mixing time in EXSY is usually chosen to be shorter,... 

NOE Measurement of the NOE between the a protons of residue i and the amide protons of residue i + 1 provides NMR information about the protein sequence. The correct experimental path can be selected by comparing the NMR results with known amino acid sequences. However, NOEs arise from residues that are close together only in space, not in the chain sequence. For correct information about the geometry, there must be some way to distinguish between what is in the sequence and what is in the space. Several special programs are available to solve the problem, such as multidimensional NMR of NOESY (nuclear Overhauser and exchange spectroscopy) and ROESY (rotatory-frame Overhauser spectroscopy). Both offer some means to obtain the same Overhauser effect information for aU nuclei in a molecule by a single experiment. 

Figure 3.1 The various time periods in a two-dimensional NMR experiment. Nuclei are allowed to approach a state of thermal equilibrium during the preparation period before the first pulse is applied. This pulse disturbs the equilibrium ptolariza-tion state established during the preparation period, and during the subsequent evolution period the nuclei may be subjected to the influence of other, neighboring spins. If the amplitudes of the nuclei are modulated by the chemical shifts of the nuclei to which they are coupled, 2D-shift-correlated spectra are obtained. On the other hand, if their amplitudes are modulated by the coupling frequencies, then 2D /-resolved spectra result. The evolution period may be followed by a mixing period A, as in Nuclear Overhauser Enhancement Spectroscopy (NOESY) or 2D exchange spectra. The mixing period is followed by the second evolution (detection) period) ij.

Nuclear Overhauser enhancement spectroscopy ( H- H NOESY and NOE) experiments show only the TTC isomer in equilibrium with the TTT isomer for 6,8-dinitro-BIPS [36,55] and 6-nitro,8-bromo-BIPS. The TTC form dominates the equilibrium. Spectral broadening for several proton resonances in the spectra of 6,8-dinitro-BIPS and 6-nitro,8-bromo-BIPS indicate a rapid exchange between these two isomeric forms. The activation energy for this isomerisation is reported to be 43.6 kJ mol and the energy difference between the the TTC and TTT forms is 4.6 kJ mol [55]. 

A further field of application for the 2D NOESY and the 2D ROESY experiment.s are dynamic systems, where exchange processes may be recognized and may be analysed quantitatively (EXCSY-spectroscopy). 

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