NMR relaxation rates

See also Chemical Exchange Effects in NMR Fourier Transformation and Sampling Theory NMR Relaxation Rates NMR Spectrometers Nuclear Overhauser Effect Parameters in NMR Spectroscopy, Theory of. 

Effects in NMR Chiroptical Spectroscopy, Orientated Molecules and Anisotropic Systems Heteronuclear NMR Applications (Ge, Sn, Pb) Heteronuclear NMR Applications (O, S, Se, Te) Liquid Crystals and Liquid Crystal Solutions Studied By NMR Magnetic Resonance, Historical Perspective NMR Data Processing NMR in Anisotropic Systems, Theory NMR Relaxation Rates NMR Spectroscopy of Alkali Metal Nuclei in Solution P NMR Parameters in NMR Spectroscopy, Theory of Product Operator Formalism in NMR Relaxometers Xenon NMR Spectroscopy. 

See also Biofluids Studied By NMR Chromatogra-phy-NMR, Applications Diffusion Studied Using NMR Spectroscopy Magnetic Fieid Gradients in High Resolution NMR NMR Data Processing NMR Principles NMR Pulse Sequences NMR Relaxation Rates NMR Spectrometers Product Operator Formalism in NMR Proteins Studied Using NMR Spectroscopy Two-Dimensional NMR, Methods. 

Applications, Clinical MRI Applications, Clinical Flow Studies MRI Theory NMR Relaxation Rates NMR Spectroscopy of Alkali Metal Nuclei in Solution Nuclear Overhauser Effect. 

See also Contrast Mechanisms in MRI Diffusion Studied Using NMR Spectroscopy Food and Dairy Products, Applications of Atomic Spectroscopy Food Science, Applications of Mass Spectrometry High Resolution Solid State NMR, Industrial Applications of IR and Raman Spectroscopy Labelling Studies in Biochemistry Using NMR MRI Applications, Biological MRI Instrumentation MRI Theory MRI Using Stray Fields NMR Data Processing NMR Relaxation Rates NMR of Solids. 

See also Fourier Transformation and Sampling Theory Heteronuclear NMR Applications (Sc-Zn) Heteronuclear NMR Applications (Y-Cd) NMR Relaxation Rates NMR Spectrometers. 

Kowalewski J and Maler L 1997 Measurements of relaxation rates for low natural abundance /=1/2 nuclei Methods for Structure Elucidation by High-Resolution NMR ed Gy Batta, K E Kdver and Cs... 

N-protonation the absolute magnitude of the Ad values is larger than for Af-methylation <770MR(9)53>. Nuclear relaxation rates of and have been measured as a function of temperature for neat liquid pyridazine, and nuclear Overhauser enhancement has been used to separate the dipolar and spin rotational contributions to relaxation. Dipolar relaxation rates have been combined with quadrupole relaxation rates to determine rotational correlation times for motion about each principal molecular axis (78MI21200). NMR analysis has been used to determine the structure of phenyllithium-pyridazine adducts and of the corresponding dihydropyridazines obtained by hydrolysis of the adducts <78RTC116>. 

Table I reports the observed NMR linewidths for the H/3 protons of the coordinating cysteines in a series of iron-sulfur proteins with increasing nuclearity of the cluster, and in different oxidation states. We have attempted to rationalize the linewidths on the basis of the equations describing the Solomon and Curie contributions to the nuclear transverse relaxation rate [Eqs. (1) and (2)]. When dealing with polymetallic systems, the S value of the ground state has been used in the equations. When the ground state had S = 0, reference was made to the S of the first excited state and the results were scaled for the partial population of the state. In addition, in polymetallic systems it is also important to account for the fact that the orbitals of each iron atom contribute differently to the populated levels. For each level, the enhancement of nuclear relaxation induced by each iron is proportional to the square of the contribution of its orbitals (54). In practice, one has to calculate the following coefficient for each iron atom ...

In addition to the standard constraints introduced previously, structural constraints obtainable from the effects of the paramagnetic center(s) on the NMR properties of the nuclei of the protein can be used (24, 103). In iron-sulfur proteins, both nuclear relaxation rates and hyperfine shifts can be employed for this purpose. The paramagnetic enhancement of nuclear relaxation rates [Eqs. (1) and (2)] depends on the sixth power of the nucleus-metal distance (note that this is analogous to the case of NOEs, where there is a dependence on the sixth power of the nucleus-nucleus distance). It is thus possible to estimate such distances from nuclear relaxation rate measurements, which can be converted into upper (and lower) distance limits. When there is more than one metal ion, the individual contributions of all metal ions must be summed up (101, 104-108). If all the metal ions are equivalent (as in reduced HiPIPs), the global paramagnetic contribution to the 7th nuclear relaxation rate is given by... 

The relaxation rates of the individual nuclei can be either measured or estimated by comparison with other related molecules. If a molecule has a very slow-relaxing proton, then it may be convenient not to adjust the delay time with reference to that proton and to tolerate the resulting inaccuracy in its intensity but adjust it according to the average relaxation rates of the other protons. In 2D spectra, where 90 pulses are often used, the delay between pulses is typically adjusted to 3T] or 4Ti (where T] is the spin-lattice relaxation time) to ensure no residual transverse magnetization from the previous pulse that could yield artifact signals. In ID proton NMR spectra, on the other hand, the tip angle 0 is usually kept at 30°-40°. 

Slowly tumbling large molecules, such as proteins, undergo rapid transverse relaxation, which causes line broadening in the NMR spectrum. This imposes an upper limit on the size of molecules whose structures can be usefully interpreted by NMR. Small molecules tumble at high rates and have much slower relaxation rates, and therefore a sharper well-resolved NMR spectrum. 

NMR relaxation of liquids such as water in porous solids has been studied extensively. In the fast exchange regime, the spin-lattice relaxation rate of water in pores is known to increase due to interactions with the solid matrix (so-called surface relaxation ). In this case, T) can be described by Eq. (3.5.6) ... 

The key to obtaining pore size information from the NMR response is to have the response dominated by the surface relaxation rate [19-26]. Two steps are involved in surface relaxation. The first is the relaxation of the spin while in the proximity of the pore wall and the other is the diffusional exchange of molecules between the pore wall and the interior of the pore. These two processes are in series and when the latter dominates, the kinetics of the relaxation process is analogous to that of a stirred-tank reactor with first-order surface and bulk reactions. This condition is called the fast-diffusion limit [19] and the kinetics of relaxation are described by Eq. (3.6.3) ... 

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