NOE and Chemical Exchange

A modification of GOESY experiment was proposed by Dixon et al In the new version of the experiment called M-GOESY the target proton is selectively excited using excitation sculpting method but not decoded before NOE 

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The most important NMR parameters obtained for the hydroxyl protons are chemical shifts (6), vicinal proton-proton coupling constants (3J7hc,Oh), temperature coefficients (AS/7), deuterium-induced differential isotope shifts, and exchange rates ( ex)-119-123 These parameters may provide information on hydrogen bond interactions and hydration as well. Moreover NOEs and chemical exchanges involving hydroxyl groups observed by NOESY and ROESY experiments also add to the number of distance restraints used in conformational analysis. 

NOE and chemical exchange Selective water inversion in 2D NOE-NOESY WaterLOGSY... 

Macura S, Wurthrich K, Ernst R R 1982 Separation and suppression of coherent transfer effects in two-dimensional NOE and chemical exchange spectroscopy. J Magn Res 46 269-282... 

The number of NMR parameters available for measurement is rather small, consisting of the chemical shift, relaxation rates (/1 and lo), scalar (J) couplings, dipolar (D) couplings, cross-relaxation rates (the NOE), and hydrogen exchange rates. All of these have been quantified for many of the amide protons of A131 A, and most of the data suggest the presence of little persistent structure. 

For small molecules, NOE is positive for large molecules and chemical exchange, peaks appear negative. 

Nuclear Overhauser Effects. Nuclear Overhauser effects (NOEs) can be used to measure both interactions through space, and chemical exchange (Neuhaus and Williamson, 2000). In a system where a contaminant interacts strongly with NOM, NOEs should be measurable between the NOM and the contaminant. In theory, using such an approach should provide information as to which components in DOM the contaminant is associated, as well as possible information on exchange rates, molecular dynamics, and strength of the interactions. Relatively few studies have used NOEs extensively to study NOM-contaminant interactions directly. The... 

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. 

J. Jeener, B.H. Meier, P. Bachmann and R.R. Ernst, J. Chem. Phys., 71 (1979) 4546. Please note this is the first appearance of the NOSY sequence however, this publication deals with chemical exchange. For a quite comprehensive text on NOE please see the above reference by Neuhaus and Williamson.. 

On the basis of Eq. (1), NOEs are usually treated as upper bounds on interatomic distances rather than as precise distance constraints, because the presence of internal motions and, possibly, chemical exchange may diminish the strength of an NOE [23]. In fact, much of the robustness of the NMR structure determination method is due to the use of upper distance bounds instead of exact distance constraints in conjunction with the observation that internal motions and exchange effects usually reduce rather than increase the NOEs [5]. For the same reason, the absence of an NOE is in general not interpreted as a lower bound on the distance between the two interacting spins. 

Eq. (VII. 12) is the starting point to derive not only the equations relevant for the NOE phenomenon (Chapter 7) but also Eq. (3.15) and the following ones (Section 3.4). A somewhat different form of Eq. (VII. 12) has already been encountered when dealing with transfer of magnetization between two sites in chemical exchange (Section 4.3.4). 

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. 

Figure 12.22. Aromatic region of NOES Y spectrum of a 1 1 mixture of Hoechst vs. d(CTTTTCGAAAAG)2 recorded with a 200-ms mixing time. Chemical exchange cross peaks between protons of the free DNA and the 2 1 HoechstrDNA complex are labeled with their identifying base pair. Below the diagonal the H6 and H8 cross peaks are shown, whereas those of the adenine H2 resonances are highlighted in the upper portion of the figure and labeled with a subscript 2. (Reprinted with permission from Ref 92. Copyright 1990, Oxford University Press.)...

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