Melanin: The Effects of Dimethyl Sulfoxide on the Spectral Properties

Investigation of the spectroscopic properties of melanin dissolved in dimethyl sulfoxide (DMSO) is one part of a more expansive study of melanin. The absorption spectrum of melanin dissolved in DMSO differed from the spectrum in acidic or neutral solution, but was similar to the spectrum when melanin was dissolved in a basic solution. All absorption spectra fit to an amorphous semiconductor model. Infrared absorption spectra of melanin dissolved in DMSO and melanin suspended in a potassium bromide pellet were collected. Changes in the infrared spectrum were observed upon dissolution into DMSO. These changes had spectral features of melanin that had been previously dissolved in basic solution and dried. Fluorescence spectra of melanin when excited at 310 nm in aqueous suspension differed from those collected in 10% (v/v) DMSO. In the presence of DMSO, the fluorescence was quenched by a factor of 4, and the fluorescence maximum shifted from 435 nm to 450 nm. Compared to the fluorescent properties of the water-inaccessible fraction of chromophores obtained from an iodide quenching experiment (fluorescence maximum of 415 nm), DMSO lowers the energy of the excited state more than water. The results indicate that DMSO alters the micro-environment of the chromophore by changing its acid-base properties and altering the solvation of its excited state, perhaps through the formation of a hydrogen bond to melanin.