Structural information using overhauser

In solution, dipole-dipole interactions constitute a relaxation mechanism, and the dipolar relaxation which is the basis for the well-known nuclear Overhauser effect (NOE), mostly used in the homonuclear H, H case. The 2D HOESY method between H and Li has been used to obtain structural information of many organolithium systems in solution and this field was reviewed in 1995. Li is commonly used as the relaxation is dominated by the dipole-dipole mechanism and the relaxation time is relatively long. Knowledge of the proximity of the lithium cation relative to protons in the substrate is used to derive information about the structure and aggregation of organolithium systems in solution. In a few cases quantitative investigations have been made °. An average error of the lithium position of ca 0.2 A was reported. 

The use of spin labels to induce general broadening of solvent-exposed resonances in the NMR spectra of proteins has been a focus of attention of several workers (reviewed in ref. 210). Studies have so far focused on well-characterized, small proteins, e.g. bovine pancreatic trypsin inhibitor,211 212 whose X-ray crystal structures are available, and the correlations between solvent accessibilities determined by both methods have been good. In the latter study, spin label-induced attenuation of the nuclear Overhauser effects between the protein and its bound water molecules was determined. Conceivably, these techniques could be applied to proteins of larger size and those for which there is little detailed structural information available. 

The identification of both short chain and long chain branches in polyethylene at concentrations of 1 per 10,000 carbon atoms has become feasible with the availability of improved probes and improved computer hardware/ software capabilities. Reviewed in this chapter are the methods and computations as well as the basic requirements for sound quantitative analyses namely, correct choice of solvent, a consideration of concentration effect on line widths and satisfying nuclear Overhauser effects and spin lattice relaxation time requirements. Finally, the NMR generated structural information is put to use in correlations with polyethylene physical properties and measurements of number average molecular weight. 

Quantitative information may also be gained from the data. If the amino acid composition is accurately known, it may prove possible to integrate portions of the 13C-n.m.r. spectra in order to gain quantitative information about the oligosaccharide structure. This also assumes that no differential nuclear Overhauser effects (n.O.e. s) exist between the various resonances and that none of the resonances in question are attenuated by the use of short recycle-times. 

When one resonance in an NMR spectrum is perturbed by saturation or inversion, the net intensities of other resonances in the spectrum may change. This phenomenon is called the nuclear Overhauser effect (NOE). The change in resonance intensities is caused by spins close in space to those directly affected by the perturbation. In an ideal NOE experiment, the target resonance is completely saturated by selected irradiation, while all other signals are completely unaffected. An NOE study of a rigid molecule or molecular residue often gives both structural and conformational information, whereas for highly flexible molecules or residues NOE studies are less useful. 

The relationship between the aromatic donor molecule binding site and the heme group has been explored further using ID and 2D H NMR, and enzyme inactivation studies (see Section H1,D). Nuclear Overhauser enhancement data obtained in ID NOE and 2D NOESY experiments (these provide information on proton-proton distances in a structure, with an upper limit of 5 A) indicate that binding occurs relatively close to heme methyl CI8H3 (229), in agreement with the conclusion from inactivation work that substrates interact with the... 

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. 

The nuclear Overhauser effect was predicted in 1953 [37], experimentally demonstrated in 1955 [38], and widely used since then to obtain structural and conformational information in diamagnetic small molecules. As it appears from... 

High resolution multidimensional NMR experiments can provide the dendrimer chemist with a wealth of additional information extending far beyond the determination of the molecular structure. In the interpretation of (2D)-NOESY (NOESY=nuclear Overhauser enhancement spectroscopy) spectra, a knowledge of the spatial interrelationships between protons in different parts of the dendrimer scaffold can be acquired from proton-proton NOE interactions. At the same time, the prevailing conformation of the dendritic branches in the solvent used can be deduced from this information. Furthermore, studies of dendrimer/sol-vent interactions and the influence of solvent on the spatial structure of the dendrimer are also possible [22]. Thus the information content of such NMR experiments resembles that of small-angle scattering experiments on dissolved dendrimers (see Section 7.6). 

---