Nitroxide radicals biological properties

Although nitroxide radicals do not seem to exist in naturally biological systems, they are relatively non-toxic, and survive as such for considerable periods after administration. This property makes them important as spin-labels in biological systems, ESR spectroscopy being an ideal technique for studying their behaviour. However, in these studies their radical nature is not significant chemically it is the presence of an unpaired electron, and the remarkable stability of these species that is utilized. However, nitroxides can act as electron donors and acceptors, and we compare them here with ascorbate radicals (Section 1.4). 

The electron-exchange properties of melanins have been studied with a number of special reagents in order to elucidate the electron exchange mechanism itself and the role of the melanin redox properties in biological systems. It was thus found that nitroxide radicals were reversibly reduced by melanins in the dark (5/9) and that the redox equilibria were altered on irradiation (see Section VI). Moreover, the reduction of nitroxides (R2NO ) was inhibited by oxygen. The equilibrium... 

The basic idea underlying the physical labelling approach is the modification of the chosen sites of the object in question by specific compounds, which are boimd covalently (labels) and/or non-covalently (probes), whose properties make it possible to trace the state of the surrounding biological matrix by appropriate physical methods. The following main types of compounds are used as labels and probes to monitor the dynamic parameters of proteins (1) centers with unpaired electrons (stable nitroxide radicals, radical pairs and paramagnetic complexes) exhibit electron spin resonance (ESR), (2) luminescent fluorescence and phosphorescence chromophores, and (3) Mossbauer atoms (e.g. Pe) which gives the nuclear y-resonance (NGR) spectra. 

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