Structural Model of Eumelanin

Melanin is a ubiquitous pigment in living organisms with multiple important functions, yet its structure is not well understood. We propose a structural model for eumelanin protomolecules, consisting of 4 or 5 of the basic molecular units (hydroquinone, indolequinone, and its tautomers), in arrangements that contain an inner porphyrin ring. We use time-dependent density functional theory to calculate the optical absorption spectrum of the structural model, which reproduces convincingly the main features of the experimental spectrum of eumelanin. Our model also reproduces accurately other important properties of eumelanin, including x-ray scattering data, its ability to capture and release metal ions, and the characteristic size of the protomolecules.

Figure 1

A tetramer composed of IQ-QI2-HQ-QI2 ($\mathrm{\text{gold}}=\mathrm{C}$, $\mathrm{\text{red}}=\mathrm{O}$, $\mathrm{\text{blue}}=\mathrm{N}$, and $\mathrm{\text{white}}=\mathrm{H}$). Atoms in the QI2 monomer are numbered in the standard notation.

Figure 2

Formation energies of the various tetramers labeled by the first letter of the constituent quinones, I for IQ, H for HQ, and Q for either QI1 or QI2. The bars correspond to results for the two different forms of QI, as obtained from the SIESTA code (first two, red-shaded bars) or from the VASP code (second two, green-shaded bars).

Figure 3

Calculated structure factors $S(q)$ and radial distribution functions (RDF) of the DHI monomer, the tetramers, and the three-layer model. Experimental data are presented for comparison (from 14). The tetramer curves are averaged over the dominant structures (see text).

Figure 4

Absorption spectrum of the superposition of dominant tetramers (thick black line, shifted up by 1.5 units for clarity). Individual tetramer contributions are given by the thin colored lines and monomer spectra are also shown (shifted down by 1 unit for clarity).