Mid-infrared vibrational spectroscopic characterization of 5,6-dihydroxyindole and eumelanin derived from it

► Mid-infrared absorption spectra of DHI and DHI-derived eumelanin have been measured. ► Dominated bands of the DHI spectrum have been assigned using ab initio calculations. ► The spectrum of DHI-derived eumelanin suggests the existence of oxidized form of DHI.