Nuclear Overhauser enhancement dipole interaction

The main contribution to the spin-lattice relaxation of C nuclei which are connected to hydrogen is provided by the dipole-dipole interaction (DD mechanism, dipolar relaxation). For such C nuclei a nuclear Overhauser enhancement of almost 2 will be observed during H broadband decoupling according to ... 

The relative stereochemistry of hyperaspine 93 was determined by 2-D NMR spectroscopic and mass spectrometry (MS) methods. It has a m-fused bicyclic conformation 93a <2001TL4621>. The trans-fused one is disfavored by an axial pentyl group at C-8 and by a destabilizing dipole-dipole interaction between the N- and O-atoms, which does not exist in the alternative //.(-conformation. The geminal coupling constant of C( 1 )H2 in 93 (11.0 Hz), and that of its 6-hydroxy derivative (11.2 Hz), indicates that they exist preferentially in / //-conformations, whereas their 6-epimers adopt trans-conformations (9.3 and 8.4 Hz, respectively) <2005EJ01378>. Nuclear Overhauser enhancement spectroscopy (NOESY) studies also confirmed the stereochemistry of 93 by the marked nuclear Overhauser effect (NOE) correlation between H-3 and H-4a <20030L5063>. 

The 13C NMR sensitivity can sometimes be a problem, but for the kind of samples studied here the effective concentration of monomer units is several molar which does not place excessive demands on present Fourier transform NMR spectrometers. In addition to the sensitivity of the chemical shift to structure (9), the relaxation of protonated carbons is dominated by dipole-dipole interaction with the attached proton (9). The dependence of the relaxation parameters T, or spin-lattice, and Tor spin-spin, on isotropic motional correlation time for a C-H unit is shown schematically in Figure 1. The T1 can be determined by standard pulse techniques (9), while the linewidth at half-height is often related to the T2. Another parameter which is related to the correlation time is the nuclear Overhauser enhancement factor, q. The value of this factor for 13C coupled to protons, varies from about 2 at short correlation times to 0.1 at long correlation... 

As mentioned above, the determination of atomic level structure, i.e., the backbone torsion angles for an oriented protein fiber, is possible by using both solid-state NMR method described here and specifically isotope labeling. This is basically to obtain the angle information. Another structural parameter is distance between the nuclei for atomic coordinate determination. The observation of Nuclear Overhauser Enhancements (NOEs) between hydrogen atoms is a well known technique to determine the atomic coordinates of proteins in solution [14]. In the field of solid-state NMR, REDOR (rotational echo double resonance) for detection of weak heteronuclear dipole interactions, such as those due to C and N nuclei [15, 16] or R (rotational resonance) for detection of the distance between homonuclei, are typical methods for internuclear distance determination [17,18]. The REDOR technique has been applied to structure determination of a silk fibroin model compound [19]. In general, this does not require orientation of the samples in the analysis, but selective isotope labeling between specified nuclear pairs in the samples is required which frequently becomes a problem. A review of these approaches has appeared elsewhere [16]. 

Homonuclear correlation via the nuclear Overhauser effect (NOESY). The NOESY experiment correlates peaks by means of the nuclear Overhauser enhancement and so identifies pairs of nuclei which are sufficiently close together in space to relax by their dipole-dipole interaction. This technique is not so applicable in determining stereochemical assignments as those described previously, but may be extremely useful in determining the chain conformation as demonstrated by Mirau et al. in a study of the alternating copolymer of styrene and methyl methacrylate [45] (see chapter 4). 

Rates of nuclear relaxations can be altered by the deliberate addition to the solutions used for NMR spectroscopy of species containing paramagnetic nuclei. The usual relaxation agent employed with substrates dissolved in organic solvents is the chelate chromium tris(acetylacetonate) (Cr(acac)3), [Me C6 CH C(6)Me]3Cr, i.e. chromium(III)-2,4-pentane-dionate. It acts to reduce values of via dipole-dipole interactions, leaving chemical shift values unaltered. Cr(acac)3, when used at concentrations of about 0.05 M, substantially reduces values of T, particularly those for quaternary nuclei simultaneously it removes the nuclear Overhauser enhancement from proton-decoupled and other resonances. Use of this reagent could therefore considerably shorten the times required for quantitative NMR measurements. 

Nuclear Overhauser effect—The nuclear Overhauser effect (NOE) occurs only between nuclei that share a dipole coupling, i.e., their nuclei are so close that their magnetic dipoles interact. Techniques that use NOE enhance spectra and allow spacial relationships of protons to be determined. 

Energy levels with Overhauser effect (a) Relaxation due to a time-dependent isotropic contact electron-spin-nuclear-spin hyperfine interaction a(t)l S which has a zero time-average, but allows processes X and Y and enhances nuclear spin transitions when the electron populations are made equal by saturation, (b) Relaxation is due to all dipole-dipole interactions, which allow processes X, V, and PNi nuclear spin transitions are forced into emission by the Overhauser effect. In (a) the relative Boltzmann populations before saturation are shown. 

Nuclear Overhauser effects, NOEs, involve dipole-dipole relaxation phenomena which result in signal enhancements. For two interacting protons, the maximum NOE, rjmax is ... 

Consider a very simple AX system in which the two spins interact by a magnetic dipole-dipole interaction. We expect two lines in the spectrum, one from A and the other from X. However, when we irradiate the system with radiofrequency radiation at the resonance frequency of X using such a high intensity that we saturate the transition (that is, we equalize the populations of the X levels), we find that the A resonance is modified. It may be enhanced, diminished, or even converted into an emission rather than an absorption. That modification of one resonance by saturation of another is called the nuclear Overhauser effect (NOE). 

Judeinstein et al have conducted direct measurement of through-space NMR interactions that provide definitive evidence for spatial proximity of different species. Dipole-dipole interactions can be measured in principle between any NMR active nuclei with heteronuclear correlation experiments in the liquid or solid state." The dipole-dipole interactions decay quickly with the internuclear distances (r ), and are difficult to evaluate for long-range distances and even more difficult when exchange, conformation, or motion phenomena are present. However, the measurement of the nuclear Overhauser method" based on the dipole-dipole-induced crossrelaxation, was proposed to successfully measure intermolecular interactions" and the formation of ion pairs." " In agreement with recent studies, the pulsed field gradient enhanced inverse HOESY (heteronuclear Overhauser enhancement spectroscopy) sequence is usually preferred because it is more sensitive for isotope pairs H- Li and also improves the digital resolution in the H crowded spectrum." ... 

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