Applications of EPR spectroscopy in hair science
This short review describes applications of the technique of electron paramagnetic resonance (EPR) spectroscopy that are relevant to understanding processes that occur in hair; these also include studies on melanins, which are responsible for the colour of hair as well as skin and eyes. EPR spectroscopy detects specifically paramagnetic chemical species, and does not provide any information on components of materials that do not possess unpaired electrons. However, it is able to detect and characterize free radicals and molecular species that contain paramagnetic transition ions. Thus it provides information that is of relevance to understanding the origins of colour, reactions of transition metals in hair, and the performance of hair treatment products. In addition, a description of potential future applications of the technique in hair science is presented.
INTRODUCTION TO EPR SPECTROSCOPY
Electron paramagnetic resonance is a spectroscopic technique that is specifically based on the detection of materials containing unpaired electrons. It makes use of the principle that the degeneracy of the electron spin energy levels is removed in the presence of a magnetic field, and that transitions between them are induced by absorption of electromagnetic radiation in the microwave region (Figure 1a). The g-value is affected by coupling between the energy level(s) containing the unpaired electron(s) with other (filled or empty) electron energy levels, and additional interactions between the unpaired electron(s) and nuclei with non-zero spin produces hyperfine structure (Figure 1b). The number of hyperfine peaks is determined by the nuclear spin, and their separations by a combination of the nuclear magnetic moment and the electron density on that nucleus.
STRUCTURE AND COMPOSITION OF HAIR
Hair is a protein fibre that has three distinct components, the cuticle, the cortex, and the medulla. The cortex, which constitutes the bulk of the hair, contains tightly packed elongated cells, which ...