The Synthesis Of Poly(Amino Acid)S And Their Application In Biomineralization

Synthesis of inorganic/organic hybrid materials with controllable morphology and size in mild conditions via a biomimetic mineralization process has become a hotspot in modern material science. In nature, a few kinds of biomacromolecules, such as polysaccharides, peptides and proteins, were natural additives that mediate the mineralization process of biominerals. As a protein-mimic material, polypeptide or poly amino acid, which has great potential in biomedical application, is being widely taken as an organic additive to mediate the growth of inorganic particles due to its controllable molecular structure and easy preparation. This dissertation starts with the synthesis of polypeptide. Firstly, well-defined homopolypeptide and block copolypeptides were synthesized through an n-hexylamine initiated ring opening polymerization of a-amino acid N-carboxyanhydrides (NCAs) at0℃. Then, the synthesized amphiphilic polypeptide was used to mediate the mineralization of CaCO3and special asymmetrical particles were obtained at the air/water interface. Moreover, superhydrophobic surfaces with micro-and nano-structures were fabricated via a similar polypeptide-mediated mineralization process. Finally, a simple method of preparation of polypeptide nano-vesicles through a deprotection process was reported and the self-assembly mechanism was investigated. The main researches in the dissertation are listed as follows.The ring opening polymerization of NCAs was a traditional and easy way to synthesize polypeptide. However, the large-scale production and application of polypeptide was greatly limited due to the uncontrollable polymerization of NCAs. In recent years, a few of living polymerizations of NCAs were realized by some new initiators and technologies such as HVT (high vacuum techniques). In this thesis, we tried to synthesize the (co-)polypeptide with controllable molecular structure and low PDI through the simplest way, i.e. lowering the reaction temperature to0℃. The MALDI-TOF mass spectrometer and1-NMR were used to analyze the poly(y-benzy-L-glutamate)(PBLG) synthesized at25℃and0℃, respectively. It was found that there was much less by-products when the polymerization was carried out at0℃compared with that at25℃. Thus, amino-terminated PBLG was obtained at0℃, which was then used to initiate the polymerization of NCAs of alanine, phenylalanine and leucine. This part of work provided the well defined block copolypeptides for the control mineralization of CaCO3and fabrication of superhydrophobic surface.In natural biominerals, the matrix proteins typically contain a large amount of aspartic acid (D) or glutamic acid (E) residues. These acidic residues are considered to play key roles in formation of biominerals such as calcium carbonate and phosphates because of their specific interaction with calcium ions. Amphiphilic molecules, however, are special bifunctional additives due to their self-assembly property at the air/water interface and in the bulk solution. In this research, the synthetic amphiphilic PGlu22-b-PAla8was used as organic additive to mediate the mineralization of CaCO3at the air/water interface. Asymmetrical hemispherical and polyhedral calcite particles were prepared through control the supersaturation of mineralization solution. After the time-dependant observation of the growing process of CaCO3at air/water interface by SEM and TEM, an "aggregation-crystallization" mechanism based on polypeptide-stabilized gelatinous precursor was proposed and the bifunctional property of the polypeptide was proved. On one hand, the PGlu22-b-PAlag formed a loose LB film at the air/water interface, which induced the heterogeneous nucleation of CaCO3at the interface. On the other hand, the PGlu22-b-PAla8acted as a process-directing agent in the mineralization process, which stabilized the gelatinous precursor and changed the crystallization process from normal "ions aggregation" to precursor-based "aggregation-crystallization". This combination of amphiphilic polypeptide with air/water interface to mediate mineralization of inorganic particles may become a new approach for the preparation of complex and asymmetrical particles.Although inorganic minerals with multiscale topology can be easily prepared via additive controlled in vitro mineralization, the application of these artificial materials to fabricate superhydrophobic surfaces was rarely reported. After preparation of CaCO3particles in an amphiphilic polypeptide mediated mineralization process, we tried the fabrication of superhydrophobic surfaces via a similar polypeptide mediated mineralization way. Artificial superhydrophobic surfaces were mainly prepared by enhancing the roughness of the surface and reducing the surface free energy. In this work, the normal glass plate and CaCO3was used as model substrate and mineral, respectively. After chemical modification, polypeptide induced superhydrophobic surfaces with micro-and nano-structures were prepared by in-situ mineralization process. It was found that the pre-treatment of the substrate and the organic additive greatly affected the in-situ growth of inorganic minerals on substrate, while the surface morphology determined the superhydrophobic state. In our case, the pre-treatment of glass plate by H2SO4and the addition of PGlu11were both essential for the fabrication of hierarchical mineral structure on surface of substrate. Superhydrophobic surfaces in different states were obtained by simply changing the initial supersaturation of mineralization solution.Poly(γ-benzy-L-glutamate)(PBLG) is a model polypeptide due to its easy preparation and good solubility in common solvents. Moreover, the side groups of PBLG make it possible for chemical modification. PBLG is usually copolymerized with other amino acid to be block polypeptide so as to control the aggregative state in different solvents. It is obviously difficult to synthesize block copolypeptides due to the uncontrollable NCA polymerization process under normal conditions. Thus, in the last part of this research, we took advantage of the side protective groups of PBLG and prepared an amphiphilic PBLG through partly removing the benzyl group of PBLG. The relationship between the aggregation morphology and the deprotection level together with the molecular weight of PBLG was investigated. It was found that partly-deprotected PBLG could form nano-vesicles in aqueous solution and the vesicles formed by low molecular weight PBLG were more stable than those formed by high molecular weight PBLG. These nano-vesicles may be used in some biomedical application such as drug delivery.