Quantitative Fluorescence Excitation Spectra of Synthetic Eumelanin

Abstract

Previously reported excitation spectra for eumelanin are sparse and inconsistent. Moreover, these studies have failed to account for probe beam attenuation and emission reabsorption within the samples, making them qualitative at best. We report for the first time quantitative excitation spectra for synthetic eumelanin, acquired for a range of solution concentrations and emission wavelengths. Our data indicate that probe beam attenuation and emission reabsorption significantly affect the spectra even in low-concentration eumelanin solutions and that previously published data do not reflect the true excitation profile. We apply a correction procedure (previously applied to emission spectra) to account for these effects. Application of this procedure reconstructs the expected relationship of signal intensity with concentration, and the normalized spectra show a similarity in form to the absorption profiles. These spectra reveal valuable information regarding the photophysics and photochemistry of eumelanin. Most notably, an excitation peak at 365 nm (3.40 eV), whose position is independent of emission wavelength, is possibly attributable to a 5,6-dihydroxyindole-2-carboxylic acid (DHICA) component singly linked to a polymeric structure.

A quantum yield map for synthetic eumelanin

“…7,8 It is clear from this map that while emission spectra ͑vertical cross sections of the map͒ exhibit only a single peak, excitation spectra ͑horizontal cross sections of the map͒ present multiple peaks, particularly for emission wavelengths between 450 and 550 nm. This is due to the fact that regardless of excitation wavelength, virtually all emission occurs at wavelengths between 385 nm ͑3.2 eV͒ and 585 nm ͑2.2 eV͒.…”

“…Along these lines, we have previously suggested that the 3.4 eV peak in excitation spectra is due to 5,6-dihydroxyindole-2-carboxylic acid ͑DHICA͒ due to the peak's proximity to absorption peaks in the DHICA monomer. 7,17 If this is the case, then the peak near 5 eV may be due to its higher-order transition.…”

“…Since eumelanin absorbs very strongly and its absorbance profile is exponential in nature, these effects distort the spectra significantly. However, a correction method has recently been used to successfully recover the actual eumelanin emission 6 and excitation spectra 7 at several excitation and emission wavelengths, respectively. The first study clearly showed that eumelanin emission is dependent upon the excitation wavelength, as increases in excitation wavelength redshifted the emission peak and reduced its intensity.…”

Quantitative fluorescence spectra and quantum yield map of synthetic pheomelanin

“…Absorbance and fluorescence spectroscopy have proven to be valuable tools in distilling clues to melanin's underlying structure, but virtually all such studies have been applied to eumelanin. [15][16][17] Although pheomelanin fluorescence has been observed in steady-state 18,19 and time-resolved studies, 20 investigations into its emissive characteristics have been sparse and studies of the optical properties of pheomelanin monomers and oligomers are virtually nonexistent. A thorough understanding of the steady-state absorptive and fluorescent properties of pheomelanin and its precursors can contribute significantly to understanding these structures.…”

Chemische und strukturelle Vielfalt der Eumelanine – ein kaum erforschtes optoelektronisches Biopolymer

“…Außerdem haben Nighswander-Rempel und Mitarbeiter gezeigt, [8,9] dass die Emission abhängig von der Energie der Anregungsstrahlung ist. In der Theorie aber ist die Emission für alle Chromophore konstant, wenn die Anregungsenergie die Energielücke des Moleküls übersteigt.…”