A Model for the Activated Energy Transfer within Eumelanin Aggregates

Abstract

The emission properties of eumelanin from Sepia officinalis are examined following UV-A excitation. The emission decay is nonexponential, exhibiting decay components on the tens of picosecond to several nanosecond time scales. The corresponding depolarization dynamics are also nonexponential and reveal that the emission becomes totally depolarized with an average time constant of ∼80 ps at 20 °C. The depolarization of the emission is found to be activated; a simple Arrhenius fit to the depolarization rate data gives an activation barrier of 21 ± 3 kJ mol-1. The nonexponential emission decay is concluded to be a reflection of the structural disorder of eumelanin. The rapid and nonexponential depolarization dynamics are attributed to energy transfer processes that occur within “spherical” subunits that comprise the eumelanin aggregates.

The physical and chemical properties of eumelanin

“…This is an important contribution because it suggests that UV radiation with wavelengths below 300 nm generates a longer lived excited state with a significant yield, which in turn may result in excited state photochemistry, which does not occur under visible light stimulation. This photocalorimetric study was followed by additional time resolved measurements, which showed the emission to be depolarised within 80 ps of excitation (Forest et al 2000). Such rapid depolarization is indicative of a high degree of chemical and structural disorder.…”

“…Several such studies indicate that the decay is non-exponential with three or four dominant components on the ns and sub-nanosecond time scale. For example, Forest et al (2000) report four components: c. 60 ps (0.52); c. 0.52 ns (0.22); c. 2.5 ns (0.19); c. 6.9 ns (0.08), where the numbers in brackets is the normalized strength of the component. This emitted radiation represents only a tiny fraction of the total energy absorbed.…”

Fluorescence lifetime imaging ophthalmoscopy

“…Eumelanin is more common in brown-eyed people, while pheomelanin is found in blue-and green-eyed population (Kolb, 2007). The emission decay of melanin is complex, as would be expected from its featureless absorption spectra and the lifetime ranges from picoseconds to almost 8 ns (Forest et al, 2000;Ehlers et al, 2007).…”

Polymeric scaffolds for enhanced stability of melanin incorporated in liposomes

“…The fitting is omitted for it coincides with the experimental curves. The pre-exponential factors and lifetime constants are listed in Table 2, which also includes the values of Forest et al [35] for eumelanin from sepia officinalis obtained with time-resolved emission at 520 nm and excitation at 335 nm. The parameters in Table 2 for the dynamics of the DPPG + melanin solution are similar to those of eumelanin [35] and for bovine and human eye melanins [36], which were non-degraded.…”

“…The pre-exponential factors and lifetime constants are listed in Table 2, which also includes the values of Forest et al [35] for eumelanin from sepia officinalis obtained with time-resolved emission at 520 nm and excitation at 335 nm. The parameters in Table 2 for the dynamics of the DPPG + melanin solution are similar to those of eumelanin [35] and for bovine and human eye melanins [36], which were non-degraded. In contrast, for non-incorporated melanin in solution, the parameters differed, with a larger contribution from the fast dynamics (lifetime less than 1 ns) associated with aggregated melanin [36].…”