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CaCO3 precipitation is physical-chemical basis of biomineral formation of hard tissue (shells, skeletons) in marine calcifying organisms (=biomineralization). Processes controlling biomineralization are still not fully clarified, so the study of influence of pH on basic processes of CaCO3 precipitation should contribute to better understanding of biomineralization under climate change. This paper reports on the effect of initial pH (pH0) and type of stirring (mechanical and magnetical) on spontaneous precipitation and phase composition, size and morphology of spontaneously precipitated CaCO3 formed at the identical initial supersaturation, ionic strength and a(Ca2+)/a(CO32−) ratio. The initial pH varied in a range 8.50 ≤ pH0 ≤ 10.50 and included values relevant for mimicking the conditions related to biomineralization in marine organisms. In all systems two CaCO3 polymorphs were found: calcite and/or vaterite. The increase of pH0 favoured the formation of rhombohedral calcite no matter the type of stirring. This was exclusively influenced by the systems’ pH0 (other relevant initial parameters were identical). Furthermore, increase of pH0 caused change of vaterite morphology from cauliflower-like spheroids to regular spherulites. The mechanically stirred systems produced larger calcite and vaterite particles and higher content of calcite.

Poly-L-glutamic (pGlu) and poly-L-aspartic (pAsp) acids, as analogues of naturally occurring soluble acidic proteins, were used to investigate specific interactions between the acidic polypeptides and calcite as a mineral surface. A basic polypeptide (poly-L-lysine, pLys) served as a negative control for testing the role of conformity between the substrate and the adsorbed polypeptides in biomineralization processes. The kinetics of calcite crystal growth was determined in a simplified precipitation model system by inoculating well defined calcite seed crystals into a moderately supersaturated solution containing one of the polypeptides. From the kinetic data, the mode and extent of the calcite/polypeptide interactions were determined. The parabolic rate law was found to be valid for the calcite crystal growth, the integration of ions into the spiral steps at the calcite crystal surface being the rate-determining mechanism. Small amounts of pGlu or pAsp caused an inhibition of calcite crystal growth, the effect being pAsp > pGlu, and the exponential dependence of the growth rate on supersaturation confirmed that surface nucleation was the growth controlling mechanism in the presence of the two acidic polypeptides. The pLys non-selective, weak, electrostatic adsorption at the crystal surface was probably responsible for increasing the calcite growth rate at low concentrations and for inhibiting it at higher concentrations. The strongest interactions between the crystal surfaces and the polypeptides were observed for the calcite/pAsp systems. They could be accounted for by coordinative interactions between the side chain carboxylic groups of the predominantly planar arrangement of the pAsp structure (β -pleated sheet) and Ca2+ ions from the calcite surface.

This paper reports on the precipitation of CaCO3 polymorphs, having various crystal morphologies under different conditions. In particular, systems that were subject to ultrasonic irradiation were compared to the corresponding reference systems in the absence of such a treatment. The application of ultrasonic irradiation predominantly resulted in a change of particle size distribution and polymorphic composition of the precipitate, in comparison to the reference systems. Thus, it was found that the supersaturation and temperature influenced the size distribution, in both the reference and sonicated systems. A mixture of calcite, vaterite and aragonite was obtained in all reference systems, at 25 °C. At this temperature, the sonication caused the vaterite content to increase, while aragonite was not detected. In reference and sonicated systems at 80 °C, only aragonite precipitated. The results also indicate that the principle parameter responsible for the morphology of vaterite was the initial supersaturation: at higher supersaturation spherical vaterite particles precipitated, while at lower supersaturation hexagonal platelets were obtained. The morphological investigations also indicated different mechanisms of vaterite formation in the systems in which precipitation was initiated at higher supersaturation: spherulitic growth of vaterite was observed in sonicated systems, while the aggregation of primary particles was predominant in the reference systems. At lower supersaturation, the effect of c(Ca2+)/c(CO32-) on the morphology of hexagonal platelets of vaterite was observed as well. By varying the c(Ca2+)/c(CO32-), significant changes of the polymorphic composition were observed only in the sonicated systems, at 25 ˚C.

Calcium carbonate is main inorganic component of biomineralization in different invertebrate organisms, either in a form of certain polymorph (calcite, vaterite and aragonite) or hydrated modification.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 mineralized 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, the formation of specific crystal modification can be influenced by parameters like temperature, pH or initial supersaturation, as well as by the presence of inorganic or organic additives. The aim of this research is to investigate 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 biomineralizing systems. For that purpose the amino acids having asparagine (Asn), aspartic acid (Asp) and lysine (Lys) were chosen because of differently charged side chains, while the tyrosine (Tyr) and phenylalanine (Phe), as well as serine (Ser) and alanine (Ala) have different polarity. The results of PXRD and EPR techniques as well as growth kinetic data indicate that amino acids interaction with calcite surface increase in the order: Asp > Tyr > Asn > Lys > Ser > Phe > Ala.

A need for novel and advanced materials, as well as for their environmentally friendly synthesis is constantly growing. Biomaterials and processes of their formation in organisms (biomineralization) are good pattern for production of such materials. Calcium carbonate (CaCO3), main inorganic component of invertebrate’s hard tissues, is considered to be a relevant biomineralization model. CaCO3 can precipitate (crystallize), either as a polymorph (calcite, vaterite and aragonite), or hydrated and amorphous phase. The calcitic skeletal elements regularly contain small amounts of macromolecules (glycoproteins) which are either, incorporated or adsorbed on the single crystals of calcite. It has been shown previously that the 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. The aim of this research is to investigate a role of the selected amino acids as a simple models of macromolecules supposed to be responsible for precipitation of specific calcium carbonate polymorph or different crystal morphology, during the biomineralization processes. Thus, the aspartic acid (Asp), asparagine (Asn) and lysine (Lys) are selected because of different charge of their side chains, while tyrosine (Tyr) and phenylalanine (Phe), as well as serine (Ser) and alanine (Ala), are chosen because of different polarity. The hypothesis is that not only the acidic, but also the hydrogen bonding amino acid can specifically interact with selected CaCO3 surfaces. Therefore, the Langmuir adsorption constants are calculated from growth kinetic data and used as an indication of extent of organic/inorganic interaction.

Calcite plays an important role in mineralized tissues of many organisms and is very important component in the natural environment as rocks and sediments. The calcitic skeletal elements of many organisms contain small amounts of proteins, which are incorporated within single crystals of calcite. For all of them the crystal size distribution and morphology of the crystals are of great significance and impurities of both inorganic and organic nature play here an important role. The aim of this study is to obtain initial information on the mechanisms of calcitic biominerals formation, in the simple model systems that could be relevant for the initial stages of mineralization. Several amino acids (Tyr, Asp, Lys) were used as models of soluble part of organic matrix that is supposed to be relevant for formation of specific calcium carbonate polymorph and morphology. In order to study changes of local environment of calcite crystal lattice regarding to incorporation of amino acids in calcite, by EPR spectroscopy, Mn2+ was added to systems as structural probe for EPR analysis, along with other analytical methods and technics (SEM, PXRD, FT-IR). The obtained results may shed light on the understanding of protein interaction with mineral phase.

Calcium carbonate is the main inorganic component of biominerals found in many invertebrate organisms and is present either as a specific polymorph (calcite or aragonite), hydrated or in 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. The aim of this research is to investigate a role of the selected amino acids as a simple models of biomacromolecules supposed to be responsible for specific precipitation of calcium carbonates in biomineralising systems. In addition, possible crystallographic distortions of the calcite lattice will be investigated. For that purpose amino acids having distinct chemical and physical properties were selected: asparagine, aspartic acid and lysine were chosen because of differently charged side chains, while tyrosine and phenylalanine, as well as serine and alanine have different polarity. The results of structural (EPR) and kinetic analyses indicated an overall inhibition of calcite growth 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.

Many living organisms form mineral phases through biologically controlled processes, known as biomineralization. Thus created materials are composites of both, mineral and organic components. The shell of the gastropod mollusc red abalone (Haliotis rufescens) consists of calcite and aragonite layers, each of them containing characteristic biopolymers responsible for biomineralization. In this work, the effect of interstitial green sheet polypeptide, GP, extracted from the green layer of the mollusc red abalone shell, on the process of spontaneous precipitation of calcium carbonate polymorphs, was investigated. Three precipitation systems, in which the initial mineralogical composition of the precipitate was different, have been studied. Thus, in system (1) calcite appeared, in system (2) a mixture of calcite and vaterite was found, while in system (3), ASW, only aragonite precipitated. However, the precipitation kinetic measurements, X-ray diffraction, FT-IR spectroscopy, and light and scanning electron microscopy indicated that the addition of GP in the model systems caused the inhibition of precipitation and change of morphology of crystals as a consequence of GP adsorption on the crystal surfaces and its entrapment into the mineral structure. (doi: 10.5562/cca2197)

Biomineralization processes are the subject of numerous investigations. This article gives a review of the study on interactions between the charged polypeptides and the mineral surfaces involved in biomineralization, with an additional kinetic approach. The influence of polypeptides on two types of precipitation processes is discussed: the spontaneous precipitation from supersaturated solution and the growth kinetics of calcite seed crystals. In the first case the phenomenon of the formation and stabilization of metastable phases was found while in the second case the influence of the applied polypeptides on the kinetics and mechanisms of calcite crystal growth was investigated. Calcium carbonate polymorphs, calcite and vaterite, were used as biomineral substrates and acidic polypeptides, poly-L-aspartic (pAsp) acid and poly-L-glutamic (pGlu) acid, as simplified models of naturally occurring soluble acidic proteins. A basic polypeptide, poly-L-lysine (pLys), was also used in experiments in order to find out whether conformity between the crystal surface and the adsorbed polypeptide, or just the electrostatic interactions, have a decisive role in these processes. The addition of a particular polypeptide into the precipitation system caused a significant inhibition of nucleation and growth of vaterite, the extent of inhibition being in the order InhpAsp > InhpGlu >> InhpLys. In addition to the inhibition of precipitation, the change of the polymorphic composition and the crystal morphology of the precipitate were also achieved. The explanation of such acidic polypeptide behaviour is a consequence of kinetic constraints through the diverse efficiency of inhibition of both calcite nucleation and vaterite growth caused by adsorption of acidic polypeptides. The acidic polypeptides also caused the inhibition of calcite crystal growth, the effect being pAsp > pGlu, and changed the observed mechanism of growth controlled by the integration of ions into the spiral steps, as found for the model systems, to the surface nucleation rate-determining mechanism. Nonselective, weak and electrostatic adsorption of pLys at the crystal surface was probably responsible for increasing the calcite crystal growth rate when pLys was present at low concentrations and for inhibiting it at pLys higher concentrations. The strongest interactions between the crystal surfaces and the polypeptides, observed for the calcite/pAsp system, can account for coordinative interactions between the side chain carboxylic groups of the predominantly planar arrangement of the pAsp structure (β-pleated sheet) and Ca2+ ions from the crystal surface. (doi: 10.5562/cca1809)

A method for preparation of significant amount of hollow rhombohedral calcite nanoparticles, based on carbonation of calcium hydroxide suspension, is described. The mineralogical and morphological analyses of the precipitate confirmed the existence of exclusively stable polymorphic modification, calcite, with the mean particle size of about 100 nm and the diameter of the holes observed at the surfaces that are about 50 nm. The analysis of carbonation kinetics pointed out to a complex mechanism of hollow particles formation at high initial supersaturation, that assumed nucleation of amorphous precursor calcium carbonate phase and its solution mediated transformation into nanosized crystalline calcite. The holes obtained at the calcite surfaces are most probably the imprints remained after the dissolution of amorphous calcium carbonate particles.

Spontaneous precipitation of calcium carbonate was investigated in two precipitation systems: (1) with initial supersaturation lower than that corresponding to the solubility of amorphous calcium carbonate (ACC), at which vaterite precipitated, and (2) with initial supersaturation higher than that of ACC solubility, at which a mixture of calcite and vaterite was formed. After the addition of an acidic polypeptide, poly-L-glutamic acid (pGlu) or poly-L-aspartic acid (pAsp), into (1) a significant inhibition of nucleation, expressed as an increase in induction time, and growth of vaterite, perceived as a dead zone, was observed. Extent of inhibition decreased in the order: Inh(pAps)>Inh(pGlu)>>Inh(pLys). The addition of a polypeptide into (2) caused the inhibition of precipitation and changed the morphology and polymorphic composition of the precipitate; only vaterite appeared at approximately c(pAsp)=3 ppm, c(pGlu)=6 ppm, or c(pLys)=7 ppm. This finding is explained as a consequence of kinetic constraints through the inhibition of calcite nucleation and stronger binding of acidic polypeptide by the calcite surfaces than by the vaterite surfaces. Laboratory precipitation studies using conditions that resemble those in living organism should be run at an initial supersaturation corresponding to the solubility of ACC as a limiting condition.

Calcium carbonate polymorphs (calcite, aragonite and vaterite) are probably the most common biominerals, at least the two more stable ones (calcite and aragonite). In the case of mollusc shells, regular calcite and/or aragonite mineral layers are associated with organic matrix, thus resulting in hybrid materials having remarkable mechanical properties. Water-soluble components of organic matrix, mostly acidic proteins rich in aspartic and glutamic acid residues, are supposed to be responsible for the formation of a particular polymorph and for arranging its growth into desired patterns by controlling the shape, size and orientation of single crystals. In this work, poly-L-glutamic (pGlu) and poly-L-aspartic acid (pAsp) were chosen as simple analogues to the mentioned soluble acidic proteins. Although both polypeptides have similar chemical composition, they differ in structure when in solution containing calcium ions: pAsp is predominantly in the form of  sheet and pGlu is predominantly in random coil conformation. In order to study the specific polypeptide/mineral surface interactions, the kinetics of calcite seed crystal growth was determined in the precipitation systems containing different concentrations of polypeptides. It was found that, for the whole range of the investigated initial supersaturations, the crystal growth rate of calcite was a parabolic function of supersaturation, indicating that the integration of constituent ions into the spiral step was the rate determining process. Both, pAsp and pGlu inhibited the crystal growth rate of calcite, the effect being more pronounced in the case of pAsp. Moreover, the exponential relationship between the crystal growth rate and supersaturation was found when either pGlu or pAsp was present in the solution. This indicated that the surface nucleation was the controlling mechanism. The obtained results suggest the possible role of the polypeptide structure on the extent and the type of their interactions with the calcite surface.