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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.

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Employing a sequential quantum mechanical/molecular mechanical approach for polar protic solvents, we study the absorption spectrum of eumelanin building blocks including monomers, dimers, and tetramers in pure water and methanol and three water–methanol binary mixtures having water molar fractions (Xw = 0.25, 0.50, and 0.75). The binary mixture of solvents is a common situation in experiments, but theoretical studies are limited to the use of continuum models. Here, we use explicit solvent molecules, and specific solute–solvent interaction is analyzed and seen to play an important role. Effects of the electronic polarization of solute by the environment were included using a reliable iterative scheme. The results illustrate that the monomers, dimers, and tetramers are preferably solvated by methanol, but the composition of the mixture in the vicinity of the solute molecules is different from the bulk composition with a preferential microsolvation (hydrogen bonds) in water for most species considered. It is observed that the short-range electrostatic polarization effects of the hydrogen bonds lead to a slight blue shift of the excitation energies when the concentration of water in the mixture is enhanced. For the same species, there is an enhancement of the higher-energy absorption intensity caused by long-range electrostatic interactions with the environment and that the behavior of the experimental spectrum, which is characterized by a nearly monotonic decay from the ultraviolet to the infrared, is qualitatively reproduced by the superposition of the absorption spectra of monomers, dimers, and tetramers in the liquid phase.

We investigate the nuclear isotropic shielding constants and spin-spin coupling for oxygen and carbons atoms of isomers of tartaric acid in gas phase and water solutions by Monte Carlo simulation and quantum mechanics calculations using the GIAO-B3LYP approach.Solute polarization effects are included iteratively and play an important role in the quantitative determination of shielding constants. Our MP2/aug-cc-pVTZ results show substantial increases of the dipole moment in solution as compared with the gas phase results (61-221%). The solvent effects on the ( !! ) [J(C-C)] values are in general small.More appreciable solvent effects can be seen on the ( !" ) and J(C-O).