Thermogravimetry and differential scanning calorimetry of natural and synthetic melanins

Abstract

The thermal stability of natural melanins from bovine eyes, black human hair and the hard core of banana peel, synthetic melanins obtained enzymatically or by autoxidation of various precursors, and chemically modified synthetic mclanins was studied by DSC and TG analysis. It was shown that the resistance of melanins to thermal degradation depends on their origin. Synthetic melanins were found to be more stable to thermal decomposition than natural melanins. Methylation of melanins caused a significant increase in thermal stability. The DSC curves of melanins reveal typical relaxation phenomena in the temperature range 293-413 K. It is generally believed that melanins

Thermal and mass spectroscopic characterization of a sulphur-containing bacterial melanin from Bacillus subtilis

“…However, other works with different synthetic and natural melanins report a permanence of 12-26% of the initial weight [32]. Some authors have proposed a relationship between melanin resistance to thermal degradation and the melanin origin [25,27,32]. Our results suggest that microbial melanins are thermally more stable than melanins produced by superior organisms.…”

“…Derivatives are as follows: A -Benzene (A, A1, A2 and A3), Cpyridine (C, C1) S -sulphur-containing compounds (S, S′, S1, and S2), Ebenzenenitrile (E), H -Toluene (H) and O -oxygen containing compounds (O). However, our curve also has an extra exothermic peak that L-Dopa and natural melanins, eumelanins mainly, do not have [23,32]. Pheomelanins have sulphur units in their structure, and the decomposition of these molecules could be responsible for the appearance of this new peak.…”

“…Our results suggest that microbial melanins are thermally more stable than melanins produced by superior organisms. However, one study has found that 17.5% of the initial mass of the L-Dopa melanin, obtained by auto-oxidation over 48 h, remains stable up to 500°C [32], which suggests a close relationship between thermal stability and the degree of polymerisation of the melanin.…”

“…In those cases, 40% and 55% of the total mass are retained at temperatures up to 500°C, respectively [23,31]. However, other works with different synthetic and natural melanins report a permanence of 12-26% of the initial weight [32]. Some authors have proposed a relationship between melanin resistance to thermal degradation and the melanin origin [25,27,32].…”

“…3) is similar to those previously reported from melanin pigments, both synthetic and natural. A strong exothermic peak was attributed to the beginning of rapid polymer decomposition and an endothermic peak was specified as weakly bonded water [25,27,32]. Table 2).…”