Saturation transfer

The technique of saturation transfer has been used to measure in vivo the unidirectional rates of the ATPase and creatine kinase reactions, which will be discussed in later sections. In this technique, developed and first applied by Forsen and Hoffman (1963), the nuclear spin magnetization of one chemical species is perturbed from its equilibrium value and the appearance of nonequilibrium magnetization in a second, product, species is monitored to determine the reaction rate. The technique measures reaction rates in equilibrium or steady state conditions and has a time resolution of 1 s or less. Consider a second-order reaction  

As already mentioned (Sections 1.7.4 and 3.13), when a nuclear spin system experiences fluctuating magnetic fields, relaxation occurs. If these fields are 

We have already defined the equilibrium magnetization of a spin / in a given magnetic field Bq as Mz(oo), where the (oo) refers to the fact that the sample must have been exposed to the field for time sufficiently long for equilibrium magnetization to be virtually achieved. After any perturbation from equilibrium of the nuclear spin system such that, at time zero after the perturbation, Mz(0) Mz(oo), the system will tend to return to equilibrium with a simple rate law of the type 

R being the rate constant for the longitudinal relaxation process. In the case of an inversion recovery experiment, Afz(0) = —Mz oo), and Eq. (4.23) reduces to 

The rate equations for a spin system in chemical exchange between two sites 

2 The transfer of spin population between sites A and B can be written as magnetization transfer through Eqs. (1.26), (1.27) and (1.33). 

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This technique is the most widely used and the most useful for the characterization of molecular species in solution. Nowadays, it is also one of the most powerful techniques for solids characterizations. Solid state NMR techniques have been used for the characterization of platinum particles and CO coordination to palladium. Bradley extended it to solution C NMR studies on nanoparticles covered with C-enriched carbon monoxide [47]. In the case of ruthenium (a metal giving rise to a very small Knight shift) and for very small particles, the presence of terminal and bridging CO could be ascertained [47]. In the case of platinum and palladium colloids, indirect evidence for CO coordination was obtained by spin saturation transfer experiments [47]. 

Mayer, M., Meyer, B. Characterization of ligand binding by saturation transfer difference NMR spectroscopy. Angew. Chem. Int. Ed. 1999, 38, 1784-1788. 

FIGURE 10.5 Estimating xc from saturation transfer. In the dispersion spectrum of a spin label (TEMPO) the ratio of I /I runs from approximately unity in the rigid limit, when the rotation correlation time xc > ICE3, to approximately zero for xc 10 7. 

Hyde, J.S. and Dalton, L.R. 1979. Saturation-transfer spectroscopy. In Spin labeling II Theory and Applications, ed. LJ. Berliner. New York Academic Press. 

Thomas, D.D., Dalton, L.R., and Hyde, J.S. 1976. Rotational diffusion studied by passage saturation transfer electron paramagnetic resonance. The Journal of Chemical Physics 65 3006-3024. 

Beside classical MRI, based on the measurement of water abundance in tissues and variation of its relaxation rates, other techniques such as chemical exchange saturation transfer (CEST) and its improved form using paramagnetic compounds (PARACEST) are currently investigated to improve the potential of MRI diagnosis. 

Figure 10.13. (a) SEM image of ZnO nanorods coated with octylamine. Scale bar, 200 nm. (b) Uniform nanorod film fabricated by spin coating of ZnO nanorods. Scale bar, 500 nm. The nanorods assemble into domains with nematic ordering, (c) Saturated transfer characteristics for a thin-film transistor fabricated by spin coating of ZnO nanorods with different ligands octylamine (solid line), butylamine (dashed line). Vi = 60V. (d) Output characteristics of a spin-coated device made from octylamine-stabilized ZnO nanorods.The device structure is shown in the inset in (c). Reproduced from Ref. 83, Copyright 2006, with permission from the American Chemical Society. 

Shirzadi A, Simpson MJ, Xu Y, Simpson AJ (2008) Application of saturation transfer double difference NMR to elucidate the mechanistic interactions of pesticides with humic acids. Environ Sci Technol 42 1084—1090... 

In principle, there are different ways to characterize the complexes On one hand, in favourable cases, it is possible to measure the NMR parameters of the receptor in the free and bound states, i.e., differences in chemical shift and NOE-contacts, before and after the formation of the complex. On the other hand, it is usually more feasible to observe and detect changes in the ligand NMR signals before and after the binding process. In this latter case, the use of STD (saturation transfer difference) and TR-NOE (transferred NOE) experiments is of paramount importance. 

Firstly, the (negative) values of the NOE for residues of the unstructured N-terminus that do not interact with the DPC micelle surface are larger. This result is most probably due to increased saturation transfer from the water and results from increased exchange of amide protons at the used pH of 6.0 compared to that used in the absence of DPC (pH 3.1). Secondly, the values for residues from the C-terminal pentapeptide are negative in the case of NPY free in solution whereas they are positive in the micelle-bound form. This clearly indicates that the C-terminal pentapeptide is significantly rigidified upon binding to the micelle. The result is supported by the structure calculation that displays rather low RMSD values for that part... 

The 15N,1H shift correlation maps are most conveniently recorded with a sensitivity-enhanced HSQC sequence with incorporated water flip-back pulses for reduced saturation transfer and pulsed-field gradients for coherence selection. The pulse sequence of the experiment is shown in Fig. 14.4 A. 

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