Structural dynamics of melanin investigated by Rayleigh scattering of Mössbauer radiation

Abstract

SynopsisMeasurements of Rayleigh scattering of Mossbauer radiation (RSMR) have been performed on synthetic L-Dopa melanin as a function of the scattering vector, Q, and temperature, T, and are compared with the results of x-ray diffraction analyses. The data obtained allow us to derive a radial distribution function for the atomic positions not affected by the dynamic disorder and determined only by the distribution of equilibrium interatomic distances. The mean-square atomic displacements, < u2 > , derived from the RSMR measurements, evidence a strong anisotropy of the atomic vibrations: even at room temperature, < u2 > reaches fairly high values for off-plane displacements, as compared with those for motions within the planes of the structure. A connection is suggested between the observed dynamics and the biological screening action of melanins.

The physical and chemical properties of eumelanin

“…Jastrzebska et al (1998) have performed one of the most recent and complete dielectric studies. They rationalize the dielectric response of eumelanin in terms of a cluster model with mobile water [as per the Albanese et al (1984) Mossbauer radiation study of 1984]. The authors conclude that charge hopping is indeed the main transport mechanism.…”

“…In this context, Jastrzebska et al (1995) report a total surface area of 250 m 2 /g for a powdered eumelanin sample. Water is thought to exist in two forms within the materials -so-called 'weakly bound surface water' which is present within the pore structure, and 'strongly bound water' intimately associated with the macromolecule (Albanese et al, 1984). The weakly bound water can be easily removed by extended drying of the sample above 60°C, but several hundred degrees are required to remove the strongly bound water.…”

“…They postulate that these protons are trapped by the 2-carboxylate group as the system is heated, and this causes a decrease in electrical conductivity. However, they still maintain that the dominant processes are electronic (via the type of hopping described by Albanese et al (1984) and Jastrzebska et al (2002b), and that hydration controls the degree of chain protonation -an effect similar to that seen in conducting polymers such as polyaniline. Given the available evidence, this would seem a complex, yet plausible model.…”

Application of semiconductor detector for recording the Rayleigh scattering of Mössbauer radiation

“…The dependence of the scattering intensity on both the scattering vector (0.2 to 9.4 Å −1 ) and temperature (at room temperature and after heating to 145 • C) in the L-Dopa melanine has been investigated [10]. Analogous measurements were also performed by X-ray diffraction.…”

“…Thus, the traditional Mössbauer spectroscopy is difficult to apply for the investigation of the dynamics of biomolecules and proteins. As a result, a new method based on the same Mössbauer effect but not requiring Mössbauer isotopes to be in the samples, the Rayleigh scattering of Mössbauer radiation (RSMR), has been developed and used [5][6][7][8][9][10]. In this method, the Möss-bauer gamma-radiation is scattered by the electron cloud of atoms of the investigated materials.…”