ORIGINAL ARTICLE Physicochemical characterization and antioxidant activity of melanin from a novel strain of Aspergillus bridgeri ICTF-201 C. Ganesh Kumar, P. Mongolla, S. Pombala, A. Kamle and J. Joseph Chemical Biology Laboratory, Indian Institute of Chemical Technology, Hyderabad, India Introduction In the recent years, there is a renewed consumer interest towards the use of natural dyes and pigments in food, cosmetic and pharmaceutical industries because synthetic pigments are perceived as undesirable and potentially harmful; some of them are considered as carcinogenic, responsible for allergenic and intolerance reactions (Keymolen 1998; Duran et al. 2002). In the international market, the demand for pigment production is increasing at a rate of 10% every year (Wang et al. 2006). Melanin production is a universal and enigmatic adaptation of liv- ing organisms to the variable conditions on the Earth. The term ‘melanin’ refers to a negatively charged, brown or black pigmented, hydrophobic heterogeneous biopoly- mer of high molecular weight with an undetermined structure (Fitzpatrick 1965; White 1974) that are formed by oxidative polymerization of dihydroxyindole (DIC) and dihydroxyindole carboxylic acid (DHICA) monomers linked by short-distance nonhydrolysable bonds (Cheng et al. 1994). The granule formation of melanin is strongly dependent on pH, wherein a low pH promotes aggrega- tion, while high pH causes breakdown of the granules to smaller oligomers, with a low degree of polymerization. This is because of the polyelectrolyte nature of melanins, which is dependent on the ionization state of melanin groups present which include carboxylic, phenolic and amine groups and on ionic strength of the environment. These characteristics make melanin a very complex light absorbing material (Bridelli 1998). Melanin synthesis is not essential for fungal growth and development, but confers a broad spectrum of biological functions such as increased virulence in many fungi pathogenic to human and plants (Nosanchuk and Keywords antioxidant, Aspergillus bridgeri, EPR spectroscopy, melanin, tricyclazole. Correspondence C. Ganesh Kumar, Chemical Biology Laboratory, Indian Institute of Chemical Technology, Uppal Road, Hyderabad 500607, India. E-mails: cgkumar@iict.res.in; cgkumar1@rediffmail.com 2011 ⁄ 0743: received 30 April 2011, revised 22 June 2011 and accepted 29 June 2011 doi:10.1111/j.1472-765X.2011.03116.x Abstract Aims: The aim of the study is to isolate and characterize a melanin pigment from a new strain of Aspergillus bridgeri isolated from rhizosphere soil of Euca- lyptus tree and to investigate its antioxidant activity. Methods and Results: The extracellular pigment was alkali soluble, acid-resis- tant and insoluble in organic solvents and water. The pigment was precipitated on treatment with FeCl 3 , ammoniacal AgNO 3 and potassium ferricyanide and was bleached in the presence of oxidants and reductants. It was confirmed as melanin based on the Fourier transform infrared and electron paramagnetic resonance spectroscopy techniques apart from chemical analysis. Inhibition of melanin production by inhibitors like tricyclazole, 6-hydroxyflavanone, 4-hydroxy-7-methoxy-3-phenyl-coumarin, 7-hydroxy-4-phenyl-coumarin and 7-hydroxy-3,4,8-trimethylcoumarin confirmed that melanin produced by A. bridgeri is synthesized by 1,8-dihydroxynaphthalene (DHN)-melanin path- way. The melanin showed good free radical scavenging activity by DPPH method with an EC 50 of 54Æ12 lg ml )1 . Conclusions: The results of the study indicate that the melanin produced by the newly isolated A. bridgeri strain is a member of DHN melanin family and exhibited significant free radical scavenging activity. Significance and Impact of the Study: This is the first report on characteriza- tion of DHN melanin produced by a novel strain of A. bridgeri and may find potential application as a natural antioxidant in the cosmetic and pharmaceuti- cal industries. Letters in Applied Microbiology ISSN 0266-8254 ª 2011 The Authors Letters in Applied Microbiology ª 2011 The Society for Applied Microbiology 1