Paramagnetic quenching

The main feature of such a paramagnetic quenching is that not only an active 02 molecule is formed during this process, but that the second partner, i.e., the aromatic molecule, has also acquired chemical reactivity, having been transformed into the biradical A (triplet). We expect, therefore, that paramagnetic quenching will be accompanied by association or dimerization processes, induced... 

Jelicks, L.A. Gupta, R.K. (1989). Double-quantum NMR of sodium ions in cells and tissues. Paramagnetic quenching of extracellular coherence. J. Magn. Reson. 81,586-592. 

Inherent low sensitivity of NMR spectroscopy has been a major disadvantage, especially to study biomolecules like membrane proteins. Ultralow temperatures were used to enhance the NMR sensitivity for solid-state NMR spectroscopy. However, this has led to excessively long times. This was successfully overcome by using paramagnetic quenching agents. 

Is the paramagnetic adduct between CO and Cluster A a kinetically intermediate in acetyl-CoA synthesis Questions have been raised about whether this adduct is a catalytic intermediate in the pathway of acetyl-CoA synthesis 187, 188) (as shown in Fig. 13), or is formed in a side reaction that is not on the main catalytic pathway for acetyl-CoA synthesis 189). A variety of biochemical studies have provided strong support for the intermediacy of the [Ni-X-Fe4S4l-CO species as the precursor of the carbonyl group of acetyl-CoA during acetyl-CoA synthesis 133, 183, 185, 190). These studies have included rapid ffeeze-quench EPR, stopped flow, rapid chemical quench, and isotope exchange. 

Obviously, only parts of the defects created on the surface are paramagnetic, and thus EPR active. Nonetheless subsequent XiCl4 deposition on samples covered with a non-epitaxial MgCl2 film quenches the EPR signal by 40% [21], This can be taken as a clear indication that some of the defects are localized on the surface of the MgCl2 film, while most of the defects are bulk defects not influenced by adsorbed TiCl4. 

Bimolecular reactions with paramagnetic species, heavy atoms, some molecules, compounds, or quantum dots refer to the first group (1). The second group (2) includes electron transfer reactions, exciplex and excimer formations, and proton transfer. To the last group (3), we ascribe the reactions, in which quenching of fluorescence occurs due to radiative and nonradiative transfer of excitation energy from the fluorescent donor to another particle - energy acceptor. 

In this group, there are collisional interactions, which are responsible for quenching of excited states by molecular oxygen, paramagnetic species, heavy atoms, etc. [1, 2, 13-15]. Probability of such quenching can be calculated as ... 

Calibration curves are linear over several orders of magnitude but eventually show curvature and even reversal due to quenching effects. These are caused by partial or total absorption of the emitted radiation by unexcited analyte molecules, dissolved oxygen and other species, particularly if they are paramagnetic. Unlike absorptiometry, sensitivity can be improved by increasing the intensity of the exciting radiation, I0. 

While there are only a few examples that can be used for direct detection of desired analytes, many simple molecules and ions do not have optical activity under regular conditions, a chemical reaction is needed to generate an optically active species. The reactions can be acid-base, ion pairing, complexation reactions, or quenching of fluorescence by 02, paramagnetic molecules, etc. Optical sensors for a few analyte or group analytes are summarized below. 

In the case of second-site screening with a spin-labeled first ligand, the dependence of the paramagnetic relaxation enhancement on the inverse sixth power of the distance (Eq. (1)) leads to differential quenching effects on the second ligand, depending on its... 

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