Paramagnetic effects, enhancement

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... 

Maliakal AJ, Turro NJ, Bosman AW, Cornel J, Meijer EW. Relaxivity studies on dinitroxide and polynitroxyl functionalized dendrimers effect of electron exchange and structure on paramagnetic relaxation enhancement. J Phys Chem A 2003 107 8467-8475. 

This enhancement of intersystem crossing by combining heavy atom and paramagnetic effects explains the relative insensitivity of the Gd phosphorescence lifetime (Table IV) to any additional heavy atom effect (as in the chelate with iodo-BTFA), or to deuteration of solvent or ligand which, by inhibiting nonradiative deactivation, usually increases the lifetime of organic phosphorescence. This insensitivity of the lifetime of the Gd chelate permits us to assign the value of ca. 3 X sec." as the intrinsic radiative rate for the triplet state for Gd BTFA chelates, and a similar value should apply for the Eu compounds. 

The use of the ATP resonances as the reporter has been described recently. Reduction of the ATP STD NMR signals by a competitive inhibitor, permitted a direct measurement of the inhibitor K, with respect to the natural substrate ATP. After this initial measurement, the assay was combined with paramagnetic relaxation enhancement effect. In a second step, Maganese ions were added to the samples, which turned the Mg2+/ATP complex into a paramagnetic probe. The proximity of a potential non-ATP competitive compound can be inferred [81]. 

Figure Ic shows the corresponding Cl NMR linewidth measurements as a function of slow ageing. Clearly, the linewidth enhancements generated with ageing time correlate well with the Mn(II) formation, both in the presence and absence of added chloride ions. Upon addition of 20 fiK EDTA to mask the paramagnetic effects of the Mn(II) ions, the residual linewidth enhancement, which would reflect chloride binding and/or viscosity effects, is the same in completely aged samples as in fresh samples. This result indicates that the ageing response does not lead to the exposure of new NMR-detectable chloride interaction sites.

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