Um estudo teorico das propriedades estruturais e oticas de derivados de Eumelanina

AUTOR(ES)
DATA DE PUBLICAÇÃO

1997

RESUMO

Melanins are a class of biological pigments widely spread over in nature from fungi to man. They are roughly classified in three great subclass: eumelanins, pheomelanins and allomelanins. In this work we restrict ourselves to investigate the subclass of eumelanins, the pigment present in most pigmented animal tissues. It is believed that the main biological function associated to melanins is photoprotection against the biochemical devastation induced by exposure to ultraviolet radiation. However the presence of melanins in nonilluminated areas in organism, such as in the substancia nigra in the brain and a preferential destruction of melanized cells in patients with Parkinson s disease led to speculations of other biological functions besides simple photoprotection. In this work we have carried out theoretical studies at semi-empirical level (Parametric Method 3 - PM3 and Zerner Intermediate Neglect of Differential Overlap -ZINDO including Configuration Interaction) on the electronic structure of molecules believed to be structurally related to eumelanins. Our results show that these structure are electron acceptors and probably form amorphous structures. We propose a mechanism for cellular protection against free radicals, potentially cytotoxic, based on this physical property. Our results for the simulation of optical absorption spectra also indicate data compatible with the semiconductor model earlier proposed for these organic pigments. We have modified this methodology to analyze two other problems involving conducting polymers and acetylene-like polymer deposited by plasma polymerization. The results for oligothiophenes can explain the anomalous behavior of the fluorescence in terms of the character of the electronic states involved in the optical processes and not in terms of conformational disorder as previously suggested in the literature. Similarly it is also possible to explain the optical data, at least at qualitatively level, of the thin films plasma polymerized in terms of the relative movement of active molecular electronic levels involved in the optical transitions. We believe this kind of methodology can be a very efficient tool to the analysis of the electronic behavior of many classes of organic materiais

ASSUNTO(S)

metodos semi-empiricos

ACESSO AO ARTIGO

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