Screened reactive center

Free radicals are neutral or charged particles with one or more uncoupled electrorrs. Unlike usual (short-living) radicals, stable ones (long-living) are characteristic of paramagnetic substances whose chemical particles possess strong delocalized uncoupled electrons and sterically screened reactivity centers. This is the very catrse of the high stability of many classes of nitroxyl radicals of aromatic, fatty-aromatic and heterocyclic series, and ion radicals and their complexes. 

If enzymatic tracers are used, the actual detection reaction is frequently disturbed by influences exerted on the reactive center of the enzyme, especially those due to interactions of the antibody with polyfunctional groups, e.g., with the humic substances in natural water samples [15]. This should be taken into aceount, especially in the use of immunoassays for soil screening. In competitive immunoassays, analytes that are sparingly soluble in water can be solubilized by the addition of surfactants [16]. On the other hand, the surfactant can alter the tertiary structure of the antibody, and possibly of the enzyme involved. The use of organic solvents is being intensively studied [17]. In some cases, immunoassays are known to tolerate more than 10 vol % of solvent. 

Subsequently, Overberger and coworkers proposed that insensitivity of reactivity ratios of these experimental parameters resulted from preferential solvation of the propagating carbenium ion by the most polar component present in the system (either nitrobenzene from the mixed solvent system they employed or chlorostyrene monomer). The active center in m-DIPB/m-DMB copolymerizations could be preferentially solvated by free m-DMB thus screening any effect of changing bulk solvent. However, Overberger, Ekrig, and Tanner 1 noted no dependence of reactivity ratios on initiation system whereas m-DMB/ m-DIPB copolymerizations are very dependent on the nature of the initiation system. Obviously, a more detailed study is required. 

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