A spectroscopic and computational investigation of the vanadomolybdate local structure in the lyonsite phase Mg(2.5)VMoO(8).

The vibrational spectrum of Mg2.5VMoO8 obtained by quantum mechanical simulation is compared with the experimentally observed Raman spectrum. This simulation suggests that the observed band at 1016 cm(-1) is attributed to the Mo=O-Mg stretching from two-coordinate oxygen atoms that are adjacent to Mg2+ cation vacancies. Extended X-ray absorption fine structure spectroscopy supports the structural model used to simulate the vibrational modes in Mg2.5VMoO8 that match the observed Raman data.

Effect of stacking and redox state on optical absorption spectra of melanins -- comparison of theoretical and experimental results.

In this work the effect of aggregation and oxidation on the optical absorption of eumelanin oligomeric sheets is investigated by applying quantum mechanics and atomistic simulation studies to a simplified eumelanin structural model that includes 1-3 sheets of hexameric oligomer sheets. The oligomeric hypothesis is supported by AFM characterizations of synthetic eumelanins, formed by auto-oxidation or electrochemical oxidation of dihydroxyindole (DHI). Comparison of calculated absorption spectra to experimental spectra demonstrates a red shift in absorption with oxidation and stacking of the eumelanin and validates the theoretical results.

Structural studies of bleached melanin by synchrotron small-angle X-ray scattering.

Small-angle X-ray scattering was used to measure the effects of chemical bleaching on the size and morphology of tyrosine-derived synthetic melanin dispersed in aqueous media. The average size as measured by the radius of gyration of the melanin particles in solution, at neutral to mildly basic pH, decreases from 16.5 to 12.5 angstroms with increased bleaching. The melanin particles exhibit scattering characteristic of sheet-like structures with a thickness of approximately 11 angstroms at all but the highest levels of bleaching. The scattering data are well described by the form factor for scattering from a pancake-like circular cylinder. These data are consistent with the hypothesis that unbleached melanin, at neutral to mildly basic pH, is a planar aggregate of 6- to 10-nm-sized melanin protomolecules, hydrogen bonded through their quinone and phenolic perimeters. The observed decrease in melanin particle size with increased bleaching is interpreted as evidence for deaggregation, most probably the result of oxidative disruption of hydrogen bonds and an increase in the number of charged, carboxylic acid groups, whereby the melanin aggregates disassociate into units composed of decreasing numbers of protomolecules.