Through-space nuclear overhauser effect

No such chemical exchange is observed for NP2—ImH or NP3—ImH, for which WEFT-NOESY spectra have cross peaks due only to through-space nuclear Overhauser effect interactions (91). Interestingly, these two proteins have only one proline in the A-B loop (Fig. 4). However, considerable additional NMR investigation needs to be carried out in order to determine whether the chemical exchange process shown in the NOESY spectra of NPl-ImH is indeed due to the dynamics of the A-Bloop. 

Hydrocarbon chains snch as n-tetradecane (C14) that are longer than the internal length of the capsule cavity form encapsulation complexes by assuming helical conformations as confirmed by through-space nuclear Overhauser effect (NOE) cross peaks along their chains (Figure 8.10). Apparently, the unfavorable gauche interactions are compensated by attractive CH- n interactions. 

The tlirough-space nuclear overhauser effect (NOE) can provide information on sites where metals interact even if the metals do not form stable bonds through which spin coupling can be transferred. For instance, metal-HS interactions can be studied by NOE spectroscopy (NOESY) by measuring interactions between the protons within the HS molecules before and after the addition of metals to understand the conformational changes that occur within the molecules (Kingery et al., 2001). An alternative approach is to measure the heteronuclear overhauser effect (HOE) directly between the metal ion and the HS proton in close proximity by HOE spectroscopy (HOES), as has been demonstrated for organo-Li complexes (Bauer, 1995). 

NMR also can show how one atom in a molecular may be nearby (in space) to another atom in the same or even a different molecule through the nuclear Overhauser effect. 

Another technique often used to examine the stmcture of double-heUcal oligonucleotides is two-dimensional nmr spectroscopy (see AfAGNETiC SPIN resonance). This method rehes on measurement of the nuclear Overhauser effects (NOEs) through space to determine the distances between protons (6). The stmcture of an oligonucleotide may be determined theoretically from a set of iaterproton distances. As a result of the complexities of the experiment and data analysis, the quality of the stmctural information obtained is debated. However, nmr spectroscopy does provide information pertaining to the stmcture of DNA ia solution and can serve as a complement to the stmctural information provided by crystallographic analysis. 

Nuclear Overhauser effects (NOEs) cause changes in the intensity of NMR signals by through-space dipole iipole interactions [36]. The magnitude of an observed NOE between two magnetic nuclei gives useful information on the distances between them. The observed NOE is also related to Brownian... 

Whereas spin decoupling, COSY and TOCSY techniques are used to establish connectivities between protons through bonds, techniques that make use of the nuclear Overhauser effect (NOE), such as 1-D NOE and NOESY, 1- and 2-D GOESY, 1- and 2-D ROESY, can establish connectivities through space. Before looking at these techniques in detail, it s worth spending a little time considering the NOE phenomenon itself - in a nonmathematical manner, of course ... 

As with the COSY experiment, the sequence starts with a pulse followed by an evolution period, but now the mechanism that couples the two spins (which must be in close proximity, typically <6 A) is the Nuclear Overhauser Effect (NOE). The second pulse converts magnetization into population disturbances, and cross-relaxation is allowed during the mixing time. Finally, the third pulse transfers the spins back to the x-y-plane, where detection takes place. The spectrum will resemble a COSY spectrum, but the off-diagonal peaks now indicate through-space rather than through-bond interactions. 

Nuclear Overhauser effect Occurs as a result of cross-relaxation between dipolar-coupled spins resulting from spin spin interactions through space. Phase diagram Summarizes the pressure and temperature conditions at which each phase of a homogeneous material is most stable. 

Chemical shift correlated NMR experiments are the most valuable amongst the variety of high resolution NMR techniques designed to date. In the family of homonuclear techniques, four basic experiments are applied routinely to the structure elucidation of molecules of all sizes. The first two, COSY [1, 2] and TOCSY [3, 4], provide through bond connectivity information based on the coherent (J-couplings) transfer of polarization between spins. The other two, NOESY [5] and ROESY [6] reveal proximity of spins in space by making use of the incoherent polarization transfer (nuclear Overhauser effect, NOE). These two different polarization transfer mechanisms can be looked at as two complementary vehicles which allow us to move from one proton atom of a molecule to another proton atom this is the essence of a structure determination by the H NMR spectroscopy. 

H is particularly important in NMR experiments because of its high sensitivity and natural abundance. For macromolecules, 1H NMR spectra can become quite complicated. Even a small protein has hundreds of 1H atoms, typically resulting in a one-dimensional NMR spectrum too complex for analysis. Structural analysis of proteins became possible with the advent of two-dimensional NMR techniques (Fig. 3). These methods allow measurement of distance-dependent coupling of nuclear spins in nearby atoms through space (the nuclear Overhauser effect (NOE), in a method dubbed NOESY) or the coupling of nuclear spins in atoms connected by covalent bonds (total correlation spectroscopy, or TOCSY). 

This phenomenon should not be confused with the interaction through distance between two nuclei that exchange magnetisation because the structure of the molecule is such that they are close in space, although a great number of bonds separates them. This is the Nuclear Overhauser Effect that causes modifications in signal intensities. 

Nuclear Overhauser effect spectroscopy NOESY Interactions through space/chemical exchange... 

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... 

The nuclear Overhauser effect (NOE) is manifest as an intensity change in a one-dimensional (ID) spectrum or a cross peak in two-dimensional (2D) NOESY spectrum that reflects a through space dipolar coupling interaction between nuclei. The size of the NOE is proportional to the reciprocal of the distance of the two nuclei to the power of six and is a function of the correlation time xc of the molecule, as indicated in Eq. 1 ... 

The nuclear Overhauser effect (NOE) is a consequence of the modulation of the dipole-dipole interactions (through space) between different nuclei and is correlated with the inverse sixth power of the internuclear distance. Experimentally, the NOE is the fractional change in intensity of one resonance when another resonance is irradiated in a double-irradiation experiment. The NOE phenomenon is intimately related to spin relaxation. The NOE varies as a function of the product of the Larmor frequency, co0, and the rotational correlation time, tc. In small molecules, tc is short relative to uo"1. In this extreme motional narrowing situation, the frequency... 

NUCLEAR OVERHAUSER EFFECT, DIFFERENCE SPECTROMETRY, H H PROXIMITY THROUGH SPACE 1 73... 

In Chapter 3 (Section 3.16), there is a description of the nuclear Overhauser effect difference experiment, an experiment that provides information about H— H through-space proximity. Review of this section is helpful before proceeding here. The ROESY experiment, rotating-frame Overhauser effect spectroscopy, is a useful 2-D analogue of the nuclear Overhauser effect difference experiment. This experiment is useful for molecules of all sizes whereas the related experiment, NOESY (nuclear Overhauser effect spectroscopy), is not very useful with small molecules. NOESY is used primarily with biological macromolecules. Both NOESY and ROESY experiments correlate protons that are close to each other in space, typically 4.5 A or less. 

What we need is a method that allows us to tell which groups are close to one another in space (though not necessarily through bonds) even when there are no coupling constants to help out. Very fortunately, an effect in NMR known as the nuclear Overhauser effect allows us to do this. 

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