engleski

Effect of a nucleating polypeptide extracted from the abalone shell on precipitation of calcium carbonate polymorphs

Many organisms exhibit highly specific control of mineralization by using biomolecules [1]. Molluscs shells, pearls and corals are examples of natural biocomposites in which calcium carbonate (CaCO3) is the principal mineral component. Mechanical and chemical properties of these materials are the results of a controlled crystallization of CaCO3 crystals, that is, essentially, induced by the organic matrix containing proteins reach in acidic amino acid residues. In this work, the effect of a water soluble polypeptide, extracted from green layer of the abalone shell (GP), on the process of spontaneous CaCO3 precipitation was investigated. Although the green sheet is reported to be unique to the abalone shell, it represents a good model for studies of molluscan shell biomineralization, because of observed nucleating activity [2]. Since GP has composition and structure similar to those of the parent green sheet, it can be used as model in nucleation experiments. As the CaCO3 precipitation models, three systems were investigated. Thus, system (1) imitated the sea water composition and only aragonite spontaneously precipitated. System (2) is of lower initial supersaturation and only calcite precipitated in the absence of GP, while the initial supersaturation in system (3) is relatively high and mixture of calcite and vaterite initially precipitated. In systems (2) and (3) the precipitation was initiated by mixing equimolar concentrations of Na2CO3 and CaCl2. All experiments were conducted at t = 21 °C and the progress of precipitation was followed by recording pH as a function of time. Samples were characterized by means of FT-IR spectroscopy, X-ray diffraction and optical and scanning electron microscopy. It was found that GP inhibited precipitation in all model systems, while the significant morphological changes were observed on calcite crystals only. It could be concluded that adsorption of GP onto the surface of the growing crystals is responsible for the inhibition effects.

biomineralization; calcium carbonate; green layer peptide

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