Hydration effects on the electronic properties of eumelanin building blocks

Abstract

Theoretical results for the electronic properties of eumelanin building blocks in the gas phase and water are presented. The building blocks presently investigated include the monomeric species DHI (5,6-dihydroxyindole) or hydroquinone (HQ), DHICA (5,6-dihydroxyindole-2-carboxylic acid), indolequinone (IQ), quinone methide (MQ), two covalently bonded dimers [HM ≡ HQ + MQ and IM ≡ IQ + MQ], and two tetramers [HMIM ≡ HQ + IM, IMIM ≡ IM + IM]. The electronic properties in water were determined by carrying out sequential Monte Carlo/time dependent density functional theory calculations. The results illustrate the role played by hydrogen bonding and electrostatic interactions in the electronic properties of eumelanin building blocks in a polar environment. In water, the dipole moments of monomeric species are significantly increased ([54-79]%) relative to their gas phase values. Recently, it has been proposed that the observed enhancement of the higher-energy absorption intensity in eumelanin can be explained by excitonic coupling among eumelanin protomolecules [C.-T. Chen et al., Nat. Commun. 5, 3859 (2014)]. Here, we are providing evidence that for DHICA, IQ, and HMIM, the electronic absorption toward the higher-energy end of the spectrum ([180-220] nm) is enhanced by long-range Coulombic interactions with the water environment. It was verified that by superposing the absorption spectra of different eumelanin building blocks corresponding to the monomers, dimers, and tetramers in liquid water, the behaviour of the experimental spectrum, which is characterised by a nearly monotonic decay from the ultraviolet to the infrared, is qualitatively reproduced. This result is in keeping with a "chemical disorder model," where the broadband absorption of eumelanin pigments is determined by the superposition of the spectra associated with the monomeric and oligomeric building blocks.

Eumelanin broadband absorption develops from aggregation-modulated chromophore interactions under structural and redox control

“…Very recently, several computational studies pointed out the importance of taking such extrinsic contribution into account 15 16 17 18 . It was suggested that excitonic coupling effects from the interplay of geometric order and disorder of aggregate structures broaden and enhance the visible absorption spectrum of eumelanin 15 .…”

15N NMR Shifts of Eumelanin Building Blocks in Water: A Combined Quantum Mechanics/Statistical Mechanics Approach

“…Following our previous work [32], we have considered for this study monomeric structures composed of fundamental building blocks including DHI (5,6-dihydroxyindole) or hydroquinone (HQ) and the redox and tautomeric species, indolequinone (IQ) and quinone methide (MQ). Two dimers defined by covalent bonding of HQ and MQ (named HM) and of IQ and MQ (named IM).…”

“…In protic solvents, however, the structural and electronic properties of eumelanin pigments can be particularly affected by electrostatic interactions, and the role played by hydrogen bonding and long-range polarization effects in the spectroscopic properties of monomeric and oligomeric building blocks certainly deserve special attention. Indeed, we have shown that for some eumelanin building blocks in liquid water, the light absorption at higher energies is enhanced as a result of the long-range electrostatic interactions with the water environment [32]. It is also shown that the broadening of electronic absorption spectrum with a nearly monotonic decay from the ultraviolet to the near infrared can be obtained by the superposition of the electronic spectra of different monomeric and oligomeric building blocks, in line with the predictions of the chemical disorder model proposed to explain the eumelanin absorption spectrum [33,34].…”

Probing pheomelanin synthesis using thioflavin T fluorescence

“…Early descriptions included that of a semiconductor, distinct states and scattering, 7 and more recently excitonic coupling 10 and long-range water-induced coulombic interaction. 11 Melanin's intrinsic fluorescence has so far been similarly unrevealing as it is weak and complicated by overlapping bands, 12 thus offering little structural insight although the presence of copper ions can simplify the usual complex intrinsic fluorescence, producing only one fluorescence decay component, suggesting a single excited state and a more uniform structure. 13,14 Although the constituents of melanin are known melanin's poor solubility and unsuitability for crystallography have defied conventional approaches to finding how they are organized, with conflicting models continuously emerging.…”