Identification of Common Resonant Lines in the EPR Spectra of Melanins

Abstract

Melanins are natural pigments with promising bioelectronic applications. Among their unique properties, the existence of a persistent paramagnetic signal can be considered one of the most intriguing and controversial features. Additionally, the possible influence of such centers on charge transport accentuates the need for a better understanding of their origin and specific characteristics. In this report, electron paramagnetic resonance spectra of melanin samples obtained at different experimental conditions were systematically studied. From the fitting procedure, three distinct resonant lines are proposed, two associated with carbon-centered radicals and one with semiquinone free radicals.

Forensic determination of hair deposition time in crime scenes using electron paramagnetic resonance

“…The EPR spectrum of black hairs (5 strands) revealed a slightly asymmetric singlet signal placed around g = 2.0037 (Figure 1B1), consistent with values reported by others [34,39]. This singlet had a similar shape to the signal of synthetic eumelanin standard (Figure 1B2; [81]), but with broader linewidths. The signal obtained from black hair also showed another one (indicated with arrows), albeit of smaller intensity, similar to that of synthetic pheomelanin standard (Figure 1B3).…”

Engineering proton conductivity in melanin using metal doping

“…17 25 36 44 45 46 In the solid-state, the dry melanin CW EPR signal is dominated by an apparent carbon-centered radical (CCR), the exact nature of which is still to be determined. 46 Hence, the data in Figure 6A suggests that by increasing the amount of Cu(II), the CCR is being eliminated. Furthermore, given that once incorporated, Cu(II) is reduced to Cu(I) (as discussed earlier) a very simple reaction may be inferred:…”

Melanin, the What, the Why and the How: An Introductory Review for Materials Scientists Interested in Flexible and Versatile Polymers

“…Furthermore, the signal can be decomposed into several smaller signals [26,85,90,91], with support experimentally for qualitatively two different radicals when using a combination of pH, microwave power, hydration, and photo excitation with one signal being identified as the anionic semiquinone radical and another whose g-value is consistent with a carbon centered radical [92]. The two main parts of the signal may be a superposition of the phenoxyl form and the anionic form, as possibly indicated by the solution work mentioned above [17], but the complexity of the signal and computational work indicates potentially even more radicals [85,90]. There are also differences between synthetic and natural samples and the proportion of DHI to DHICA also affects the signal [54].…”