Spectral properties of two water-soluble melanins

Aim. The work was purposed to study the spectral properties of two types of water-soluble melanins to clarify the nature of the optical absorption and emission centers in them. Materials and Мethods. Studied are the spectral properties of two types of water-soluble melanins: melanin obtained from black yeast-like fungi Pseudonadsoniella brunnea 470 FCKU, dissolved in an aqueous NaOH solution with pH 11, and plant melanin obtained from black tea according to German patent DE102004003801A1, which is added to drinking water for medicinal purposes. In the spectral region of 200–900 nm, optical absorption spectra, spectra, and fluorescence excitation spectra are measured at room temperature. Results. The optical density of both studied samples monotonically decreases with increasing radiation wavelength. Fluorescence spectra represent one broadband, the peak position of which depends on the excitation wavelength. The fluorescence excitation spectrum consists of at least two bands and does not coincide with the absorption spectrum. Conclusions. The shape of the absorption spectra may indicate that the aqueous solutions of melanins under study contain many absorption centers with different resonance frequencies. Fluorescence spectra represent a superposition of several elementary bands with varying spectra of excitation. The different excitation spectra corresponding to fluorescence at different wavelengths most likely indicate that each of the melanin solutions under study contains several of its species. The spectral range of absorption, as well as the low fluorescence intensity, make further studies of water-soluble melanins as components of complex nanosystems for photoacoustics and photothermal therapy promising. The use of producer microorganisms for melanin production provides a cost effective biotechnological process.