Temperature-enhanced synthesis of DMSO-Melanin

• The influence of temperature on d-Melanin synthesis and its properties is studied.
• Increasing the synthesis temperature, synthesis time can be decreased down to 7 times.
• Increase in temperature causes a decarboxylation of d-Melanin monomers.
• Decarboxylation processes facilitating the polymerization of d-Melanin.

Melanins are a class of pigmentary conjugated macromolecules found in many biological systems. Functionalization of synthetic melanin provides interesting new properties like the greater solubility of melanin synthesized in dimethyl sulfoxide, d-Melanin. In this work we have studied the influence of temperature on d-Melanin synthesis and its properties. To this end, UV–Vis, Fourier-transform infrared (FTIR) spectroscopy and Nuclear Magnetic Resonance (NMR) techniques have been employed to analyze d-Melanin synthesized within the range of 25–100 °C. Our results reveal that by increasing the synthesis temperature up to 100 °C, the synthesis time can be decreased by a factor of 7 when compared to room temperature. From FTIR and 13C CP/MAS NMR analyses the increase in temperature causes a decrease in the number of carbonyl groups from carboxylic acid and from ionized carboxylic acid. The decarboxylation of d-Melanin monomers at higher temperatures shows that the use of higher synthesis temperatures influences the elimination of carbonyls present in the precursor molecules, thus facilitating the polymerization of d-Melanin.

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