Role of Silicon-Organic Interactions in Silica Biomineralization

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SahaiEAR-0208036There is a growing appreciation of the need to understand organic-inorganic interactions at the molecular-level for processes relevant to biogeochemistry as exemplified by silica biomineralization, and to materials science as in the controlled synthesis of biomimetic mesoporous silica. I propose to investigate the nature of silica interactions with carbohydrates and amines in the aqueous phase combining theoretical and experimental approaches. We will (i) determine whether putative hypercoordinated Si-carbohydrate complexes can play a role in biological silicon uptake, and (ii) identify the role of amines in catalyzing the biopolymerization of dissolved silicon species. The carbohydrates examined will include polyalcohols, sugar-acids and polysaccharides. The amines will be examined for the most likely pathway of biogenic silica nucleation and polymerization. Specifically, we will attempt to determine whether the amines act to catalyze hydrolysis of a starting Si-organic compound via a SN2 mechanism or whether polyamines act to bring about aggregation of inorganically formed silica clusters from a starting compound of silicic acid. For the former mechanism, we will examine amines of different nucleophilicities including methylamine, dimethylamine, pyridine, guanidine, imidazole, lysine, arginine and histidine. The alternative mechanism will be studied using 1,2-diaminoethane, 1,2-diaminopropane, 1,3-diaminopropane, 1,2,3-triaminopropane, and analogous triamines on a butane backbone as the model polyamines. The effect of nitrogen hybridization will be determined by comparing 1,3-diaminopentane to imidazole. Further, the effect of 1,6-diaminohexane versus 1,4-diaminobenzene will permit examination of carbon hybridization effects. Experimental NMR and Attenuated Total Reflectance Fourier Transform Infra-Red (ATR FTIR) spectroscopy will provide independent measurements. Temperature dependence NMR will provide activation enthalpies. The computational method used will be ab initio MO theory to calculate the structure, energy, vibrational frequencies and 29Si Nuclear Magnetic Resonance (NMR) shifts to explain the experimental spectra, and to determine the electronic contributions to the overall nucleophilicities of the amines.