ABSTRACT
Electron spin resonance (ESR), also known as electron paramagnetic resonance (EPR), is a unique research tool that can directly identify free radicals and measure their concentration. It is a technique based on fundamental quantum physics, which has found widespread application in the analysis of skin. In most studies, skin biopsies or homogenates are used for analysis. However, in vivo data may be collected by making special adjustments during the operation of the ESR instrument. Regardless, far less work has been completed in vivo—probably due to limited sensitivity. ESR can directly detect endogenous long-lived (persistent) free radicals in skin, such as the ascorbate and melanin radicals. By monitoring their concentration, we can subject the sample, directly in the instrument, to treatments or stresses such as UV irradiation and monitor the formation or reduction of radical species. Moreover, ESR can provide information about the overall redox state of skin, for example, how many radicals are present. ESR can also furnish data regarding the degree of oxygenation of tissues (oximetry) and the biophysical properties of lipid membrane structures. In most cases, this type of data is generated using nitroxide probes (nitroxyl spin labels)—stable radicals that are not easily metabolized. Thus, they react with and incorporate themselves into the structure of radical biological species and serve as an indicator of the redox state of the tissue.
