Analysis of wavelength dependence of the photophysics of dry samples of synthetic eumelanin and pheomelanin

PURPOSE: Pheomelanin constitutes one quarter of epidermal melanin. Detection of pheomelanin directly by electron paramagnetic resonance (EPR) spectroscopy is difficult if eumelanin component predominates. The aim of this research was to examine UV-induced and visible light-induced increase in melanin radicals concentration and melanin temperature using synthetic eumelanin and pheomelanin to assess the usefulness of the observed effects for developing a new method of melanin characterization. MATERIALS AND METHODS: Dry samples of synthetic eumelanin and pheomelanin were used. 50 W narrow-band diode illuminators, in the spectral range of 369-733 nm, were used to irradiate melanin. Melanin radicals were measured by EPR spectroscopy, while melanin temperature was monitored by measuring of infrared radiation emission. RESULTS: The effect of light on melanin EPR signal strongly decreased with increasing wavelength. This decrease was significantly more pronounced for pheomelanin than for eumelanin. On the other hand, dependence of the temperature increase on wavelength was rather moderate for both kinds. Also, the temperature increase did not differentiate them significantly. Interestingly, wavelength-dependent heating of both types of melanins is negatively correlated with increase of EPR signal, especially above 500 nm. CONCLUSIONS: In the studied wavelength range, irradiation of both melanins induced similar heating. On the other hand, different effects of long-wavelength light on the melanin EPR signal may potentially be used to distinguish eumelanin from pheomelanin in biological samples. This observation may serve as a basis for developing a new noninvasive method for the diagnosis of skin melanoma by assessing the ratio of eumelanin to pheomelanin concentration.