NITROXIDE RADICAL BIOSTABILITY IN SKIN

Nitroxide radicals are important chemical tools in dermatologic resear ch (e.g., for studying biophysical properties of skin lipids and epide rmal membranes with the method of electron paramagnetic resonance, EPR , spectroscopy). However, nitroxides may loose their paramagnetic prop erties in biological tissues, which could limit their usefulness in bi omedical applications. We analyzed the biostability of various chemica l types of nitroxide radicals in keratinocytes, epidermis homogenate, and intact skin. EPR signal loss of imidazoline, pyrrolidine, piperidi ne, and oxazolidine nitroxides is attributed to their reduction to the corresponding hydroxylamine. The rate of nitroxide reduction in skin varies considerably with nitroxide ring structure and substitution. Th e order of nitroxide stability in isolated human keratinocytes, mouse epidermis homogenate, and intact mouse and human skin is imidazoline > pyrrolidine > di-t-butylnitroxide (DTBN) > piperidine > oxazolidine. Cationic nitroxides are reduced much faster than neutral or anionic pr obes, presumably due to transmembrane electron shuttle or internalizat ion. The results indicate that imidazoline- and pyrrolidine-type nitro xides should be used when high biostability of nitroxides is needed. P iperidine-type nitroxides are versatile probes for studying one-electr on transfer reactions in skin.