Interaction of radicals from water radiolysis with melanin

• Published in: Biochimica et biophysica acta Vol. 883; no. 1; pp. 162 - 167
• Main Authors: Sarna, Tadeusz, Pilas, Barbara, Land, Edward J., Truscott, T.George
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 06.08.1986; Elsevier; North-Holland
• Subjects: Biological and medical sciences; Carbon Dioxide - metabolism; Cysteinyldopa - metabolism; Dihydroxyphenylalanine - analogs & derivatives; Dihydroxyphenylalanine - metabolism; Free Radicals; Fundamental and applied biological sciences. Psychology; Hydroxides - metabolism; Hydroxyl Radical; Melanin; Melanins - metabolism; Molecular biophysics; Nitrogen Oxides - metabolism; Oxidation-Reduction; Pule radiolysis; Pulse Radiolysis; Radiation-biomolecule interaction; Radiochemistry; Redox process; Spectrophotometry; Superoxides - metabolism; Water; Water radical; Pule radiolysis; Redox process; Melanin; Water radical; Water; Radiochemistry; Hydrated electron; Pigments; Molecular interaction; Pulse radiolysis; Free radical
• ISSN: 0304-4165; 0006-3002; 1872-8006
• DOI: 10.1016/0304-4165(86)90147-9

Melanins are considered to be natural photoprotectors in the melanocytes and keratinocytes of the skin. These pigments have also been suggested to play an important role in protection of melanin-containing cells against ionising radiation. Various mechanisms have been proposed to explain the protective role of melanin which invoke the radical scavenging properties of the polymer. In the present work the reactions of melanins with radicals generated in aqueous media by pulse radiolysis have been studied. Time-resolved changes in absorbance of the melanin or the radical species were recorded at selected wavelengths. Experiments were carried out on synthetic dopa- and 5- S-cysteinyldopa-melanins and a natural melanin in phosphate buffer (pH 7.4). Under the conditions employed, melanin reacted predominantly with either oxidising (OH ., N 3 .) or reducing (e aq −, CO 2 −) species. We were also able to monitor the interaction of melanin with superoxide radical, which was reducing in this case. Detailed analysis of transient changes in melanin absorbance, detected at different wavelengths, was demonstrated to be a convenient method for studying redox processes of this substance, as shown by model experiments using ferricyanide and dithionite as oxidising and reducing agents, respectively. Among the radicals studied, OH . exhibited the strongest reactivity with melanins. Apparent rate constants for the reactions of radicals with autoxidative dopa-melanin (1.5·10 9 M −1·s −1, 2.6·10 8 M −1·s −1, 1.8·10 8 M −1·s −1, 5·10 5 M −1·s −1, 10 6–10 7 M −1·s −1 for OH ., e aq −, N 3 ., O 2 − and CO 2 −, respectively) are reported. The reactivity of melanins with radicals from water radiolysis and their effect on pigment properties are discussed in terms of the structure and possible biological role of the pigments.