Melanin in art: Photobleaching of melanin-on-paper is accompanied by robust modification of eumelanin structure.

Melanin pigments find a diverse field of practical application including art. One of the authors has developed a method of printing images with genetically modified Escherichia coli cells that produce melanin onto paper using the screen-printing process. The method prints the cell culture and then incubates the paper over 3 days, allowing the cells to deposit the melanin as they reproduce. However, using melanin comes with challenges related to color fading of "melanin-on-paper." The aim of this study was to evaluate how melanin-on-paper was photobleached. First, we characterized E. coli (EC)-melanins by spectrophotometric and chemical degradation methods, which indicated that EC-melanins consisted of 98% eumelanin and 2% pheomelanin with high purity (ca. 80%). Second, we evaluated photomodification of melanin-on-paper during 19 months of exposure to ambient light, showing that the exposure caused photobleaching of the melanin color intensity to a half with cross-linking of 5,6-dihydroxyindole-2-carboxylic acid units of eumelanin structure and photodegradation of 5,6-dihydroxyindole units. These results indicate that melanin-on-paper undergoes gradual bleaching during months of exposure to sunlight with robust changes in melanin structure. To mitigate deterioration of melanin-on-paper by sunlight, one option would be to frame it under UV protective acrylic.