Nuclear Overhauser enhancement mechanism

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 ... 

Conformational analyses of JOM-13 and [L-Ala3]DPDPE have proven to be critical for the determination of the bioactive conformation of enkephalin-like peptides at the delta receptor. H-NMR studies of JOM-13 in aqueous solution revealed that this tetrapeptide exists in two distinct conformations on the NMR time scale as evidenced by two sets of resonances [63]. Large differences in the observed chemical shifts and coupling constants for the D-Cys2 residue in the two conformers suggested that the major differences between the two NMR conformers reside in the disulfide portion of the molecule however, a paucity of conformationally informative nuclear Overhauser enhancement (NOE) interactions precluded the development of a detailed structural model from the NMR studies. In order to develop such a model a thorough conformational analysis of JOM-13 was undertaken, in which the NMR data were complemented by x-ray diffraction results and by molecular mechanics calculations [64]. The results indicate that the 11-... 

NMR observations basically contain spin relaxation processes which are associated with molecular motions with different specific frequencies in a given system. For quantitative measurements to determine the compositions of the system or selective measurements of particular components with different relaxation parameters, it is essential, therefore, to understand the principle of the relaxation mechanism. When our interest is focused on molecular motions, spin relaxation parameters such as spin-lattice relaxation time T, spin-spin relaxation time T2, and the nuclear Overhauser enhancement (NOE), are directly measured as a function of temperature or field frequency by using appropriate pulse sequences. Such temperature or frequency dependencies of the spin relaxation parameters are analyzed in terms of appropriate models to obtain detailed information of molecular motions with frequencies of Hz in the system. In this chapter, the basic theories and analyses... 

For Rexco branched/cyclics, gating of the H-noise decoupling (so that it is applied only during signal acquisition) in order that nuclear Overhauser enhancement (n.O.e.) is suppressed causes the intensities of the methylene (29.85 p.p.m.) to decrease relative to that of the methyl (1. 15 p.p.m.). Presumably different relaxation mechanisms predominate in the two cases (j U) for the methylene group, relaxation is mainly dipolar, with positive Overhauser enhancement, whereas the methyl group may relax by spin-rotation with negative Overhauser enhancement. 

E. Molecular Dynamics of Gel Network and Gelation Mechanism To characterize the molecular dynamics of a polymer-chain in the gel state, relaxation parameters such as the spin-lattice relaxation times, nuclear Overhauser enhancements and linewidths were compared among the gel state, solution state of the random-coil form for higher alkaline conditions and lower molecular weight fractions... 

Such pulse programs are also used to enable other special ID experiments such as saturation or nonexcitation of a large solvent resonance (these are different in that the former method will also saturate NH or OH protons in the molecules under study through the mechanism of chemical exchange), or the measurement of nuclear Overhauser enhancement effects... 

Nuclear Overhauser enhancements (NOEs) are a consequence of the dipole-dipole relaxation mechanism that operates through space (as opposed to the... 

Such pulse programs are also used to enable other special one-dimensional experiments such as saturation or nonexcitation of a large solvent resonance (these are different in that the former method will also saturate NH or OH protons in the molecules under study through the mechanism of chemical exchange), or the measurement of nuclear Overhauser enhancement (NOE) effects which are often used to provide distinction between isomeric structures or to provide estimates of internuclear distances. Pulse programs are also used for measuring NMR spectra of nuclei other than and sometimes in order to probe connectivity between protons and the heteronucleus. In this case, pulses or irradiation can be applied on both the heteronucleus and channels in the same experiment. The commonest use is in NMR where all spin-spin couplings between the nuclei and nuclei are removed by... 

Sometimes, nuclear Overhauser enhancement fartors (NOEFs or // values) are reported. If relaxation has contributions from mechanisms other than DD interactions with the decoupled nuclei, then the observed NOE will be attenuated from its theoretical maximum value. 

The conformation of the mixed p agonist/5 antagonist H-Tyr-c[-D-Orn-2-Nal-D-Pro-Gly-] in comparison to that of H-Tyr-c[-D-Orn-Phe-D-Pro-Gly-] was studied in DMSO-d6 by NMR spectroscopy and by molecular mechanics calculations [62,64]. Neither peptide showed nuclear Overhauser effects between C H protons or chemical exchange cross peaks in spectra obtained by total correlation and rotating frame Overhauser enhance-... 

The NMR signals of insensitive nuclear spins can be enhanced by transferring polarization from a more sensitive species to which they are coupled. The well-known pulse sequences as the polarization transfer techniques are insensitive nuclei enhanced by polarization transfer (INEPT), distortionless enhancement by polarization transfer (DEPT), and reverse insensitive nuclei enhanced by polarization transfer (RINEPT) The INEPT sequence is an alternative to the nuclear Overhauser effect. The INEPT experiment does not require any particular relaxation mechanism and therefore a better enhancement factor can be obtained. Furthermore it is demonstrated that INEPT sequence can be used to determine the multiplicity of each signal in a NMR spectrum. More recently, the INEPT and DEPT experiments were used for the coherence transfer via heteronuclear J-coupling between spin-1/2 and quadrupolar nuclei in the solids. " Fyfe et showed that coherence transfer via the scalar coupling between spin-1/2 and quadrupolar nuclei can be obtained in the solid state by using INEPT experiment. 

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