Preferential solvation and optical properties of eumelanin building blocks in binary mixture of methanol and water

Abstract

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.

First-Principles Investigation of the Optical Properties of Eumelanin Protomolecules

“…Indeed, such simulations are extremely demanding due to the size of the molecules (56 and 58 atoms) and the amount of solvent molecules required to obtain an acceptable first solvation shell. As an example, it has been estimated that, for the cyclic porphyrine-like geometry, at least 400 solvent molecules are needed for a correct solvation. , This would represent an extremely heavy computational load in particular within the BSE that scales like O ( N 4 ) . For this reason, we resorted to the SMD-PCM , implicit solvent scheme, despite the fact that such models may have limitations in identifying the correct optical transitions in some organic pigments .…”

Solvent effects on catechol's binding affinity: investigating the role of the intra-molecular hydrogen bond through a multi-level computational approach

“…1,2 Phenol, hydroquinone and resorcinol share with catechol the hydroxyl functional group, yet, the proximity of its two substituents, which can interchangeably act as both hydrogen donor or acceptor in an intra-molecular hydrogen bond (HB), and the consequent possibility to behave as a chelating agent, make catechol's chemistry more versatile, as witnessed by the continuously growing attention in the last decade. [1][2][3][4][5][6][7][8] Indeed, the plethora of chemical interactions exhibited by catechol containing species has recently stimulated a continuous research, aimed to the design of novel materials in several fields, including opto-electronic devices, 9 biomedical nano-engineering, [10][11][12][13] smart polymers and functional materials, 6,14,15 and, preeminently, bio-inspired underwater adhesives [16][17][18][19][20][21][22][23] In this latter field in particular, the strong yet versatile capacity of catechol to interact with almost any kind of surface, and the possibility to tune such interactions by simply acting on extrinsic factors, as for instance the solvent 24 or the presence of ions, 8,25 is pivotal to its success in wet adhesion or water replacement. 2 In fact, recent studies have evidenced that the key role of catechol in adhesive materials 8,11,20,21,23 is rooted in its exceptional ability to adapt to the absorbing surface, exploiting a network of very different chemical interactions, which may include covalent bonds, HB's, π-π stacking and/or weaker cation-π interactions.…”

“…[1][2][3][4][5][6][7][8] Indeed, a plethora of chemical interactions exhibited by catechol containing species has recently stimulated continuous research aimed at the design of novel materials for application in several fields, including opto-electronic devices, 9 biomedical nano-engineering, [10][11][12][13] smart polymers and functional materials, 6,14,15 and, preeminently, bio-inspired underwater adhesives. [16][17][18][19][20][21][22][23] In this latter field in particular, the strong yet versatile capability of catechol to interact with almost any kind of surface, and the possibility to tune such interactions by simply acting on extrinsic factors, as for instance the solvent 24 or the presence of ions, 8,25 is pivotal to its success in wet adhesion or water replacement. 2 In fact, recent studies have evidenced that the key role of catechol in adhesive materials 8,11,20,21,23 is rooted in its exceptional ability to adapt to the absorbing surface, exploiting a network of very different chemical interactions, which may include covalent bonds, HBs, p-p stacking, and/or weaker cation-p interactions.…”

Complexes of Alkaline and Ammonium Cations with Dopamine and Eumelanin Precursors: Dissecting the Role of Noncovalent Cation−π and Cation–Lone Pair (σ-Type) Interactions

“…55 Finally, with respect to the simpler phenol and catechol molecules considered in our previous work, the conclusions of the present investigation can be more reliably extended to systems of interest in material design, o-benzoquinone being the result of the oxidation of catechol, which may occur in DOPA at a given pH during byssus formation and DHI and 22Q are among the species often considered as the eumelanin precursor. 25,49,56…”