Abstract
Melanins, the ubiquitous hetero-polymer pigments found widely dispersed among various life forms, are usually dark brown/black in colour. Although melanins have variety of biological functions, including protection against ultraviolet radiation of sunlight and are used in medicine, cosmetics, extraction of melanin from the animal and plant kingdoms is not an easy task. Using complementary physicochemical techniques (i.e. MALDI-TOF, FTIR absorption and cross-polarization magic angle spinning solid-state 13C NMR), we report here the characterization of melanins extracted from the nitrogen-fixing non-virulent bacterium Azotobacter chroococcum, a safe viable source. Moreover, considering dihydroxyindole moiety as the main constituent, an effort is made to propose the putative molecular structure of the melanin hetero-polymer extracted from the bacterium. Characterization of the melanin obtained from Azotobacter chroococcum would provide an inspiration in extending research activities on these hetero-polymers and their use as protective agent against UV radiation.
Highlights
Melanins are found widely dispersed in the animal and plant kingdoms
The strong band at,3435 cm21 in the FTIR spectrum can be assigned to the vibration of nonhydrogen bonded NH groups [16,29,30] and the broad band observed between 3200–2000 cm21 may be related to O-H stretching vibrations associated to intra/intermolecular hydrogen bonds [25,26]
Existence of the OH functional group and the aromatic system indicated by the FTIR absorption spectrum along with the presence of aromatic/ indole system as evidenced from solid state 13C NMR and the concomitant loss of CO from the immediate precursor observed in MS spectrum clearly indicates that the OH-functionality exists as phenolic OH group(s) in the system [32]
Summary
Melanins are found widely dispersed in the animal and plant kingdoms. They have a variety of biological functions, including protection against the UV radiation of the sunlight and energy transduction [1]. Melanins can protect microorganisms, such as bacteria and fungi, against thermal as well as chemical (e.g. heavy metals and oxidizing agents) and biochemical (e.g. host defenses against invading microbes) stresses [2] that involve cell damage by the solar UV radiation through generation of reactive oxygen species. Organisms of the genus Azotobacter are free-living, non-virulent, nitrogen-fixing obligate aerobes [5]. Among various species of this genus, Azotobacter chroococcum has been most commonly isolated from the soils worldwide. The presence of iron and copper ions in the medium significantly increases the Azotobacter melanization process [9]
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