Relaxation of proton

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... 

The T2 relaxation of protons and deuterium in muscle water is much faster than T2 relaxation of pure water. 

The T2 relaxation of protons in muscle tissue is non-mono-exponential. 

The nature of the chemical shift tensor is a potential source of complications in relaxation studies. For sugar carbons, the CSAs are around 40 ppm and their contribution to relaxation of protonated carbons is nearly negligible. On the other hand, CSA values of the protonated carbons of the bases are between 120 and 180 ppm, the tensors deviate quite significantly from axial symmetry and none of their principal components is colli-near with the C-H bond. This makes interpretation of the relaxation rates in terms of molecular dynamics prohibitively complicated or, if neglected, introduces an error whose magnitude has not yet been evaluated. 

Both, longitudinal and transverse relaxation of protons in tissue depend on the microstructure and on the chemical composition of several microscopic compartments. Relaxation properties are not necessarily constant for the diiferent compartments inside the cells (cytosol and cavities in cell organella) and in the extracellular space (interstitium and vessels). However, water exchange processes between the compartments are often fast enough to generate one effective relaxation time, which can be assessed by monoexponential fitting of the relaxation dependent data. 

NOE factors smaller than 1.988 indicate participation of other mechanisms in the spin-lattice relaxation of protonated 13C nuclei. The percentange contribution of the DD mechanism can be ascertained from the measured NOE factor according to eq. (3.19) ... 

The influence of major variables indicated above on the electrokinetic potential and degree of hydration of AS, BAS and BAC hydrolysis product particles has been studied in detail. The electrophoretic mobility of particles was measured by microelectrophoresis. As a measure of hydration of HPP the time of spin-spin relaxation of protons of adsorbed/adherent water (7j) was used. The 7j values were measured by NMR with a pulse relaxometer by the null method.5 The results obtained can be summarised as follows. 

Reuben (291) has latterly suggested that the way round this problem is to investigate both proton and deuteron relaxation rates. In the Solomon expression for Tj [equation (17)] the deuterons are less susceptible than protons to the dipolar relaxation by a factor ( h/ d) which equals 424. Thus if Tm Tim the ratio of the induced relaxation of protons to that of deuterons is unity, but this ratio increases with increasing TimAm values until it reaches a value of 424 for the case when Tm T m- When applied to the Gd " system it is found that 0 03Tim = 3-3 X 10 s. 

The principle of the NMR approach to semi-local properties of polymeric melts is considered in Section 2 it is shown how the existence of a temporary network structure is detected from the relaxation of the transverse magnetisation of protons attached to chains. The observation of segmental motions from the longitudinal relaxation of proton magnetisation is described in Section 3 it is also shown how local motions in concentrated polymeric solutions can be probed from the diffusion process of small molecules. Section 4 is devoted to the analysis of the effect of entanglement relaxation on NMR properties. 

In this Section, the attention is focused on properties of the tramsverse relaxation of protons attached to polymer molecules it is sensitive to the presence of temporary network structures in molten polymers. Any high polymer melt is picttjred as an ensemble of chain segments with temporarily fixed ends. 

NMR relaxation of protons in proteins is dominated by the dipole-dipole relaxation mechanism. For aromatic, geminal or methyl protons this involves protons separated by a fixed intemuclear distance, and for which the internal motion of the groups is simple and well understood (9-11). The NMR relaxation of protons relaxing via the intramolecular dipole-dipole mechanism [where the contributions of internal motion are not yet considered] is given by... 

The dilatometric method is time-consuming and subject to the bias introduced by the convention described. More recently pulsed Nuclear Magnetic Resonance (pNMR) has been used to measure the relative amounts of liquid and solid fat in a sample, based upon the difference in rates of relaxation of protons in the two phases after the sample has been pulsed (AOCS Method Cd 16-81). With proper calibration this gives a direct determination of the percentage of solid fat, and the results are termed sohd fat content (SFC). The analysis takes less time than dilato-metry, but the equipment is more expensive. 

B. D. Boss and E. O. Stejskal, "Restricted, anisotropic diffusion and anisotropic nuclear spin relaxation of protons in hydrated vermiculite crystals," J. Colloid Interface Sci. 26, 271-278 (1968). 

Little Is known about the exact nature of these local motions or the relative amplitudes and frequencies of motions at specific sites. Resolution of these questions requires data that Is sufficiently accurate to differentiate the behavior at different sites, and to choose a model that appropriately describes the nature of these potentially complex motions. Despite low sensitivity at the natural abundance level, NMR methods are probably most useful In this regard. The chemical shift range for resonances Is large and shift dispersion due to local variations In environment Is modest. In practice this means that many DNA sites can be monitored, even though Intrinsic llnewldths are large compared to small molecule standards. In addition, the dominance of dipolar C-H coupling contributions to the relaxation of protonated carbons simplifies Interpretation of T. and NOE parameters. 

G. D. Boss and E. O. Stejskal, Restricted, anisotropic diffusion and anisotropic nuclear spin relaxation of protons in hydrated vermiculite crystals, J. Colloid Interface Sci. 26 271 (1968). S. Olejnik and J. W. White, Thin layers of water in vermiculites and montmorillonites Modification of water diffusion. Nature Phys. Sci. 236 15 (1972). J. Hougardy, J. M. Serratosa, W. Stone, and H. van Olphen, Interlayer water in vermiculite Thermodynamic properties, packing density, nuclear pulse resonance, and infrared absorption, Spec. Disc. Faraday Soc. 1 187 (1970). 

Hansen et alP describe a proton NMR method for detecting transient interactions between protein molecules in solution. The method relies on the intermolecular paramagnetic contribution to the NMR relaxation of protons at or close to the interaction surface. The method was apphed to the soluble electron transfer protein, plastocyanin, and used to locate surface patches that may be involved in electron transfer. 

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