Abstract
Melanin and melanoma tumors are two fields of increasing interest in biomedical research. Melanins are ubiquitous biopigments with adaptive value and multiple functions, and occur in the malignant melanoma. Although several chemical structures have been proposed for eumelanin, molecular modeling and orbitals indicate that a planar or spiral benzoquinone-porphycene polymer would be the model that better explains the broad-band light and ultrasound absorption, electric conductivity, and graphite-like organization shown by X-ray crystallography and electron microscopy. Lysosomes and melanosomes are selectively labeled by vital probes, and melanin also binds to metal cations, colorants, and drugs, with important consequences in pharmacology, pathology, and melanoma therapy. In addition to traditional and recent oncologic treatments, photodynamic, photothermal, and ultrasound protocols represent novel modalities for melanoma therapy. Since eumelanin is practically the ideal photothermal and ultrasound sensitizer, the vibrational decay from photo-excited electrons after NIR irradiation, or the electrochemical production of ROS and radicals after ultrasound absorption, induce an efficient heating or oxidative response, resulting in the damage and death of tumor cells. This allows repetitive treatments due to the remaining melanin contained in tumoral melanophages. Given that evolution and prognosis of the advanced melanoma is still a concern, new biophysical procedures based on melanin properties can now be developed and applied.
Highlights
The study of interactions between small molecular ligands and macromolecular substrates is a biomedical field of steadily growing interest and involves the chemical structure and properties of both components
Detailed physico-chemical features of small dye molecules are better known than those of biopolymers, and often the knowledge of dye-substrate binding mechanisms allows a deeper understanding of macromolecular structure
Regarding the BQPo model (Figure 5A), the formation of ether bridges [45] leads to a chain curvature and potential spiral organization of the poly-BQPoe model [33]
Summary
The study of interactions between small molecular ligands (colorants, drugs) and macromolecular substrates is a biomedical field of steadily growing interest and involves the chemical structure and properties of both components. Quinone compounds containing the vicinal carbonyls (=HC−CO−CO−CH=) from benzoquinones (eumelanin, allomelanin) show a longitudinal LUMOs pattern In these graphitic materials (melanins, carbon pigments), the massive dissipation of the electronic energy from excited MOs as vibrational energy produces a great amount of heat, namely, a photothermal response [32]. Regarding the BQPo model (Figure 5A), the formation of ether bridges [45] leads to a chain curvature and potential spiral organization of the poly-BQPoe model [33] In this case, the excited state LUMO+1 of dodeca-BQPoe (Figure 5B) shows an extended longitudinal π *–electron conjugation, as well as fused MOs, whereas the ground HOMO-0 has Colorants 2022, 1, FOR PEER REVIEWa transversal conjugation (not shown). Eumelanin was claimed to be composed of onion-like concentric circles [26], which agrees with the idea that the eumelanin proto-molecules could assume fullerene-like closed forms [45,46], or spiral globular structures [33]
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