Calcium phosphate mineralization through homogenous enzymatic catalysis: Investigation of the early stages

HYPOTHESIS

The crystallization of calcium phosphate (CaP) is a ubiquitous process that occurs in several mineralized tissues and involves a variety of biochemical and chemical reactions. This issue has been hitherto continuously studied from supersaturated solutions (chemical procedure), i.e. by adding calcium and orthophosphate ions in a homogenous phase. Yet, both in vivo and in vitro investigations have clearly shown the implication of enzymes, namely alkaline phosphatase (ALP), to initiate the mineralization process by generating orthophosphate ions.

EXPERIMENTS

We report a thorough investigation on the mechanism of enzyme-induced mineralization in homogenous phase (enzymatic procedure). For this purpose, ALP is introduced in Ca²⁺/Mg²⁺-containing solution (pH = 7.4; 37 °C), and its activity modulated by the concentration of its substrate.

FINDINGS

Results show that after 24 h of mineralization both chemical and enzymatic procedures lead to the formation of well-crystalline hydroxyapatite nano-objects, however with noticeable impact on their shape and dimensions. Remarkably enough, by combining in situ monitoring and ex situ characterizations, we identify several intermediate phases, including amorphous phase, dicalcium phosphate dehydrate phase (DCPD or brushite) and Whitlockite (WH). Besides, mineralized nano-objects with a core-shell structure is observed, and hydroxyapatite platelets are shown to grow on the surface of their shell.

Supramolecular chiral self-assemblies of Gly–Pro dipeptides on metallic fcc(110) surfaces

Adsorption of the Glycine–Proline (Gly–Pro) dipeptide has been investigated using surface science complementary techniques on Au(110) and Ag(110), showing some interesting differences both in the chemical form and surface organization of the adsorbed peptide. On Au(110), Gly–Pro mainly adsorbs in neutral form (COOH/NH₂), at low coverage or for a short interaction time; the surface species become zwitterionic at a higher coverage or longer interaction time. These changes are accompanied by a complete reorganization of the molecules at the surface. On Ag(110), only anionic molecules (COO⁻/NH₂) were detected on the surface and only one type of arrangement was observed. These results will be compared to some previously obtained on Cu(110), thus providing a unique comparison of the adsorption of the same di-peptide on three different metal surfaces; the great influence of the substrate on both the chemical form and the arrangement of adsorbed di-peptides was made clear.

Salt concentration and pH-dependent adsorption of two polypeptides on planar and divided alumina surfaces. In situ IR investigations

The adsorption of proteins is the first process to take place when a solid is immersed in a biological fluid; though not yet thoroughly understood at a molecular level, this process is also known to be strongly influenced by the presence of salt in solution or by pH changes. In the present work, poly-L-glutamic acid (PG) and poly-L-lysine (PL) were selected to mimic the behavior of some protein fragments. Their adsorption was investigated by infrared spectroscopy in various modes, both on planar and on divided (powder) surfaces of aluminum oxide. These two peptides were shown to have different behaviors when adsorbed from solutions with or without CaCl2 and at various pH values. Polarization modulation-reflection absorption infrared spectroscopy, applied in a special cell designed to characterize the solid surface in contact with the liquid, enabled the observation of the influence of pH and salts upon polypeptide adsorption. At pH values higher than 5 and in the presence of CaCl2 in solution, a net increase of the PG adsorbed amount is observed, whereas no such effect could be detected for PL. Specific interactions between the COO- groups on the side chains and the surface, or between those of two different molecules, was inferred. Interestingly, similar conclusions could be drawn for the surface of alumina powders contacted with solutions of PG and PL and characterized by attenuated total reflectance IR. This work demonstrates the potential for IR investigations of solid oxide-liquid interfaces combining the study of planar and finely divided surfaces.

Synthesis and adsorption on gold surfaces of a functionalized thiol: elaboration and test of a new nitroaromatic gas sensor

A new type of sensor was built by synthesizing a long-chain thiol functionalized with an aromatic head group and grafting it onto a gold surface. The synthesis route is here described, together with the IR, MS, and RMN analysis of the new product. Adsorption of the latter onto gold was assessed by a combination of RAIRS and XPS data. Those reveal that a monolayer of thiol is adsorbed and oriented with the benzene groups toward the external part of the layer. Detection tests were performed in various atmospheres by QCM. The response shows good sensitivity to 2,4-dinitrotrifluoromethoxybenzene as a model of nitroaromatic compound.