Mechanisms of Nucleation and Surface Growth Dynamic of Calcium Orthophosphates

Author:Li Meng

Supervisor:wang li jun


Degree Year:2019





Phosphorus is one of the essential nutrients in plants.Plant roots absorb and utilize the available phosphorus in the soil to form organic/inorganic phosphorus components.Animal/human ingests phosphorus-containing substances to form the componets of nucleic acids and important tissues such as bones and teeth.Both in soil or organisms,the most important inorganic form of phosphorus is calcium phosphate and its gradually crystallizes into the thermodynamically stable hydroxyapatite(HAP)through a multi-stage phase transition.Calcium hydrogen phosphate(DCPD,CaHPO4·2H2 O),with relative lower solubility,is mainly crystallized by classic monomer addition.For the kinetic crystallization of DCPD,biomolecules are either disrupting the local microenvironments surrounding crystal-solution interfaces or physically blocking solute molecule attachment.HAP crystallization occur by nonclassical nanoparticle attachment,in which process organic molecules can template the nucleation thermodynamic process.Nevertheless,the research on the mechanism of nucleation and surface growth kinetics of calcium phosphate remain limited.The study quantify the crystallization dynamic utilizing imaging and mechanical test modes of atomic force microscope(AFM)and the main results are summarized as following:1.Mechanism of modulation of DCPD pathological mineralization by Citrate(CA)and hydroxycitrate(HCA)We demonstrate the role of CA and HCA in brushite(DCPD)crystallization over a broad range of both inhibitor concentrations and supersaturations by in situ AFM.We observed that both inhibitors exhibit two distinct actions:control of surface crystallization by the decrease of step density at high supersaturations and the decrease of the[1?00]Cc step velocity at high inhibitor concentration and low supersaturation.The switching of the two distinct modes depends on the terrace lifetime.Molecular modeling shows the strong HCA-crystal interaction by molecular recognition,explaining the AFM observations for the formation of new steps and surface dissolution along the[101]Cc direction due to the introduction of strong localized strain in the crystal lattice.2.HAP crystallization occurs via either classical spiral growth or nonclassical particle-attachmentWe present an in situ study of HAP(100)surface growth with long imaging times by different imaging modes of AFM.We observed that HAP crystallization occurred by either classical spiral growth or nonclassical particle-attachment from various supersaturated solutions at near-physiological conditions,suggesting these mechanisms do not need to be mutually exclusive.We provided,to our knowledge,the first evidence of time-resolved morphology evolution during particle attachment processes,ranging from primary spheroidal particles of different sizes to triangular and hexagonal solids formed by kinetically accessible organized assembly and aggregation.3.We reveal the thermodynamic basis of inhibition of calcium phosphate biomineralization by osteopontin(OPN)We quantify the kinetics of HAP nucleation on lipid raft membrane substrates using in situ AFM.We find that rates are sequence-dependent,and the thermodynamic barrier to nucleation is reduced by minimizing the interfacial free energyγ.Combined with single-molecule determination of the binding energy(ΔGB)of the OPN peptide segments adsorbed to the HAP(100)face,we show a linear relationship ofγandΔGB and reveal fundamental energetic clues of inhibition of calcium phosphate by OPN,suggesting that the increase in the nucleation barriers correlates with strong peptide-crystal nuclei binding.