Nuclear Overhauser effect system

TTie TOCSY 2D NMR experiment correlates all protons of a spin system, not just those directly connected via three chemical bonds. For the protein example, the alpha proton, Ft , and all the other protons are able to transfer magnetization to the beta, gamma, delta, and epsilon protons if they are connected by a continuous chain—that is, the continuous chain of protons in the side chains of the individual amino acids making up the protein. The COSY and TOCSY experiments are used to build so-called spin systems—that is, a list of resonances of the chemical shift of the peptide main chain proton, the alpha proton(s), and all other protons from each aa side chain. Which chemical shifts correspond to which nuclei in the spin system is determined by the conventional correlation spectroscopy connectivities and the fact that different types of protons have characteristic chemical shifts. To connect the different spin systems in a sequential order, the nuclear Overhauser effect spectroscopy... 

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. 

NOESY spectra 146 nuclear Overhauser effect 140 phosphorus-31 140,295 of adenylate system 303 of muscle 304... 

The distinction between regioisomeric cycloadducts of nonsymmetrical, bridged bicyclic systems and organic azides has been achieved by observing the NMR chemical shifts, the coupling constants, and the nuclear Overhauser effect. For example, the exo regioisomers 73 and 74 show the following characteristics ... 

A modification of the design of the flow cell can also lead to a further increase in sensitivity [7,8]. The addition of a proton RF coil for selective excitation of the protons before entering the 13C flow cell enables the full build-up of the nuclear Overhauser effect in a continuous-flow system (Figure 8.1.5). The combination of this NMR flow cell design, together with the use of immobilized paramagnetic species, could lead to the registration of continuous-flow 13C NMR spectra of separated species. 

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 structure determination of biopolymers using NMR spectroscopy usually involves interactions of protons[216,33. Typically, interactions of protons (nuclear Overhauser effect, NOE) that are close in space but separated by several subunits of the biopolymer are used to establish the folding of the backbone. Distance restraints are then used to compute a structure which is checked by back-calculation of the NOE spectra and comparison with experimental results 361. For large and highly flexible systems molecular dynamics is invaluable for scanning the conformational space. 

A slightly more complicated case of nuclear Overhauser effect was recently observed in our laboratory in the photoreduction of furil. Furil is considered as a weakly coupled AMX three-proton spin system and the dipolar cross relaxation involves the interplay of all three spins. The experiments were done in a light-modulation mode with a FT spectrometer and a detailed theoretical analysis of the experimental data will allow the evaluation of the various different cross-relaxation rates (138). ... 

---