Structure Of Shell Nacre And Pearl In Freashwater Hyriopsis Cumingii And Their Biomineralization Mechanism

Calcium carbonate biominerals are the most important and abundant biomineralson the earth and they form spicules, eggshells and the exoskeleton of manyinvertebrates. Thereinto, one of the most striking biomaterials, nacre has inspiredresearchers to investigate the formation mechanisms for decades.Hyriopsis cumingii was chosen as the research model, and shell nacre, aragonitepearls and vaterite pearls originated from this Hyriopsis cumingii were used tosystematic compare their compositions and structures. Subsequently, organic matriceswere extracted from the above mentioned three biominerals and then they wereanalyzed comparatively from the point of materials and biochemistry science. Finally,these organic matrices, accompany by the different microenvironments were used inin-vitro mineralization experiments to study the CaCO3biomieralization mechanismunder the control of the organic matrices and the mineralization microenvironment.Additionally, some kinds of polysaccharides (e.g. alginate, silk fibroin and chitin) werechosen to develop the in vitro study, which could further understand biomineraliationmechanism of nacre.By studying the compositions and structures of these three biominerals, we foundthe protein content in vateriate pearls was slight higher than that in the other twoaragonite biominerals. Shell nacre, aragonite pearls and vaterite pearls are composed ofcalcium carbonate tablets to form a staggered structure of space-filling organic matrices.Rougher surface, more irregular microstructure and thinner thickness of calciumcarbonate tablets are existed in vaterite pearls rather than shell nacre and aragonitepearls.Water soluble matrix (WSM) was extracted by using Milli-Q water and acidsoluble matrix (ASM) and acid insoluble matrix (AIM) were extracted by dilute aceticacid solution. The most obvious structural differences were found between WSM ofaragonite pearls and vaterite pearls. A140kDa protein is the main component of the twoaragonite biominerals including shell nacre and aragonite pearls. Protein content inASM of vaterite pearls is about65%and it is twice as much as that in aragonitebiominerals. The major component of ASM of vaterite pearls is a55kDa hydrophobic protein. AIM contains α helical structure and some kinds of structural proteins rich inglycine and alanine.WSM, ASM and AIM were chosen to carry out calcium carbonate in-vitromineralization experiments. We found WSM of aragonite pearls and ASM of vateritepearl could fine control the corresponding polymorph of CaCO3crystals. While singleAIM membranes had no obvious effect on CaCO3crystallization.In order to better mimic nacre biomineralization process, ion diffusion method wasused in alginate hydrogel and silk fibroin hydrogel mineralization system separately inCaCO3in-vitro study. These studies investigate calcium carbonate crystallizationprocess and provide the possible mechanism of CaCO3crystal growth. In addition,organic matrix or magnesium ions viewed as the additive, and chitin film regarded asthe template, the phase transition process of amorphous calcium carbonate wascomprehensively investigated in the study.