Probes, in NMR

FIG. 24 The range of distance scales, from molecular to macroscopic, probed in NMR. See text for details. 

Fluorinated analogues of the vitamin D3 metabolites have also been used as probes in NMR, to perform studies on the conformation of the complex between vitamin D and its receptor [160],... 

By introducing fluorine atoms to the polyenic system of retinal, the geometry, electronic properties, hydrophobicity, and absorption properties of the molecule will be modified. Thus, fluoro derivatives of retinal are useful tools to understand the interactions between retinal and opsin, especially on the level of charge and hydrophobic effects at the protein site. Moreover, fluorine atoms are probes in NMR and allow studies on model molecules of visual pigments Consequently, syntheses of mono-, di-, and trifluoro derivatives of retinal have been the subject of many investigations. 

Aromatic amino acids are biogenetic precursors of neuroamines (dopamine, serotonin, histamine, etc.). On the other hand, phenylalanine (Phe) is frequently present in peptide sequences, while tyrosine is an important site of phosphorylation of proteins. Aromatic amino acids and neuroamines fluorinated on the aromatic ring have been the focus of many investigations. Indeed, after incorporation in polypeptides and proteins, they can be used as probes in NMR and in PET. 

The following table lists the gyromagnetic ratio, y, of some important nuclei that are probed in NMR spectroscopy.1-12 The gyromagnetic ratio is the proportionality constant that correlates the magnetic moment (p) and the angular momentum, p p = yp. 

This splitting is proportional to B but also to the shielding tensor a. By measuring transitions between different nuclear spin states, information about the chemical shielding and, thus, the underlying electronic structure of the molecule can be obtained. These are the transitions that are probed in NMR spectroscopy [73]. 

Fraissard using 129xe as a probe in NMR technique (64). The chemical shift is a function of coverage by a gas as hydrogen and of the nature of the zeolite. Moreover acidic feature may also be determined (65). 

Several properties probed in NMR spectroscopy can be used to study the rate of chemical processes. This section is restricted to the most easily accessible properties the NMR chemical shifts and the line width of resonance signals. ... 

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