Stability and reactivity of free radicals: a physicochemical perspective with biological implications

Several factors control the reactivity of radicals and can provide the strategies to convert highly reactive species into more persistent species that are easier to detect in an experiment. A reaction can only proceed if sufficient mobility and thermodynamic driving force are provided and the reaction is allowed by steric considerations and by electronic states of the reagents and products. A violation of at least one of these conditions can make the radical relatively stable. In certain cases, these factors occur naturally, in other situations, they can be purposefully manipulated to reduce the reactivity of highly reactive radicals, prolonging their lifetime and increasing their concentration. The discussed examples cover a vast range of lifetimes, from 10(-9) seconds to 10(9) years, at concentration levels down to 10(3) radicals per sample (10(-18) M), and stress that stability and reactivity are not independent notions and are the two sides of the same coin.