Biomimetic Mineralization And Growth Mechanism Of Calcium Carbonate Under Proteins Monolayers

Nature has ingeniously succeeded in producing an impressive variety of inorganic functional structures with designed shape and size on specific sites through a biologically controlled mineralization process. In biological environments, the deposition of inorganic crystalline materials and the formation of organic-inorganic hybrid materials are facilitated by biomacromolecules (proteins, polysaccharides etc.) which are classified as "control" and "template" macromolecules. These macromolecules are considered to play a focal role in the biomineral fabrication through controlling crystal morphology, orientation, shape and habit. Current knowledge predicts that the nucleation of biominerals takes place on the cell surface or on extracellular matrix (ECM) mainly composed of biomacromolecules.Inspired by the fabrication mechanism of these materials, many methods have been established to study and mimic the biomineralization process with the aim to synthesizing superstructures that mimic natural biominerals and to gaining an insight into the biomineralization mechanism. Among these methods, Langmuir monolayer has been often used as a tool to mimic biological membrane systems such as cell membrane, and it usually serves as a model system for simulating and studying biomacromolecules and biomacromolecule-controlled mineralization at the air-water interface. At the same time, Langmuir monolayer usully involves the interfacial molecular recognition at organic-inorganic interface. This makes it feasible for Langmuir monolayer to be employed as a synthetic model of biomacromolecules so as to better understand the interface nature of organic-mineral interface and what occurs at the interface between organic molecules and inorganic materials. For example, Mann and co-workers have examined the effect of small molecule models on the growth of calcite crystal. However, small molecules are significantly different from biomacromolecules in many aspects. Therefore, in recent years, interest has been grown in mimicking biological self assembly with peptides or proteins designed in vitro. Typicaly, self-assembled two-dimensional layers of proteins at the air-water interface, the so-called Langmuir monolayers, are of particular interest for the fabrication of biomaterials and have a number of potential applications. And in particular, Langmuir monolayers, as substrates in biological mineralization, play important roles in relation to controlling the size, orientation, and morphology of inorganic crystals and directed crystal growth at the surface of protein layer. Unfortunately, due to the complexity in chemical composition and structure of the matrix protein in biomineralization process and complex interactions between them, the specific role of matrix protein at the air-water interface still remains unclear.In this dissertation, three proteins, bovine serum albumin, pepsin, type-â… collagen, have been used as templates for controlling the inorganic materials to crystallize under Langmuir monolayers. The main work included three sections as followed:1. A new superstructure of spindle-like calcite crystals consisting of nanocrystals has been synthesized via a transformation process under the Langmuir monolayer of bovine serum albumin (BSA) at room temperature. The superstructures are composed of hundreds of well-stacked calcite nanoneedles consisting of oriented and aggregated nanocrystals transformed from amorphous phase. The evidence of the phase transformation process has been observed in detail by measuring the structure of the products at different reaction stages. It has been found that the products are evolved from amorphous particles to spindle-like particles which crystallize into superstructures. The mineralization process and the interaction between the inorganic and bioorganic components are discussed in relation to relevant protein-mediated nucleation models of biomineralization. Hopefully, the present research is to help understanding the protein-directed formation of complex and highly-ordered structures as well as biomineralization mechanism.2. Crystalline flower-shaped superstructures of calcium carbonate, synthesized at the air-water interface through pepsin Langmuir monolayer at room temperature, are shown to be assembled by amorphous calcium carbonate nanoparticles, and evolved from monodisperse nanoparticles to the aggregations of nanoparticle and to flower-shaped superstructures. The phases, morphologies, and structure of the products acquired at the interface were characterized by X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and high resolution electron microscopy and atom force microscopy. The results suggest that pepsin Langmuir monolayer is responsible for the morphologies and structures of calcium carbonate minerals by first stabilizing their nanosized amorphous precursors, which then transform into amorphous aggregates via nonoriented aggregation of nanoparticles. This provides a novel and facile way for the study of biomineralization mechanisms and crystal growth modification. Moreover, the observed results may be of relevance for a better understanding of the role of proteins in the process of mineralization.3. Controlled deposition of rod-like single crystal calcite can be obtained by "copying" the symmetry and dimensionalities of collagen fiber, The calcite crystallites are found on the surface of the collagen fiber with consistent orientation along the longitudinal axis of collagen fiber; the results indicate that the combination of the ordered surface structure on the collagen fiber is the key factor in the oriented nucleation and growth process of the mineral. The mineralized collagen will combine the good mechanical properties of the collagen fiber and the biocompatibility of calcium carbonate and may be assembled into ideal biomaterials as bone implants.