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The role of amino acids as simple models of organic matrix molecules participating in calcium carbonate biomineralization

Calcium carbonate is the main inorganic component of biominerals in a wide range of invertebrate organisms and is present either as a specific polymorph (calcite or aragonite), hydrated or in an amorphous form. The calcite skeletal elements regularly contain small amounts of proteins which are either incorporated or adsorbed on the single crystals of calcite. Previously it has been shown that isolated fragments of proteins extracted from mineralised tissue, or their synthetic macromolecular analogues, exert a significant influence on the morphology and crystal structure of calcium carbonate when precipitated in the appropriate model systems. In addition, it is known that the in vitro formation of specific crystal modifications is typically determined by parameters such as temperature or initial supersaturation, as well as by the presence of inorganic or organic additives. The aim of this research is to investigate polymorphism and crystallographic distortions of the calcite lattice due to the influence of amino acids, selected as simple models of biomacromolecules supposed to be responsible for nucleation, growth and transformation of calcium carbonates in biomineralising systems. For that purpose amino acids having distinct chemical and physical properties were selected: asparagine (Asn), aspartic acid (Asp) and lysine (Lys) were chosen because of differently charged side chains, while tyrosine (Tyr) and phenylalanine (Phe), as well as serine (Ser) and alanine (Ala) have different polarity. The results of structural (FT-IR, PXRD, EPR) and morphological (SEM) analyses indicated an overall inhibition of calcite precipitation in the presence of all amino acids. The inhibition is probably caused by a slower transformation of initially formed vaterite into the calcite. Since the selected amino acids are charged under the applied experimental conditions, some surface interactions are assumed to be responsible for the observed effect.

amino acids ; calcium carbonate ; biomineralization

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