Model Studies of the Effect of Orthophospho-l-serine on Biological Mineralization

Abstract

The effect of phospho-l-serine in supersaturated solutions of calcium phosphate was investigated at plethostatic conditions. Phospho-l-serine inhibited the crystal growth of hydroxyapatite {Ca5OH(PO4)3, HAP} by blocking the crystal growth centers through adsorption. Complete inhibition was attained with a phospho-l-serine concentration of 4 × 10-4 M. Analysis of the kinetics and adsorption isotherm results showed a Langmuir-type adsorption. The amount of phospho-l-serine adsorbed on HAP decreased as pH increased from 7.4 to 10.4. The adsorbed species of phospho-l-serine, which in the pH range 7.4−10.4 are mainly the monoprotonated species (HL2-), are located in the inner Helmholtz plane (IHP) of the double layer at the HAP/electrolyte interface, interacting with the surface of HAP with both electrostatic and chemical interactions. The location of the negatively charged HL2- species at the IHP caused a shift of the ζ-potential to more negative values. Finally, a model was established for the adsorption of phospho-l-serine onto the HAP surface at pH 7.4, according to which an ion pair forms between the protonated amino group of one HL2- species and one negative site of the surface of HAP, i.e., the phosphate or hydroxyl group. Due to electrostatic repulsion, the negatively charged deprotonated carboxyl and phosphate groups of the adsorbed HL2- species are oriented in a manner ensuring the maximum possible distance from the negatively charged surface of HAP.

Surface-grafting of phosphates onto a polymer for potential biomimetic functionalization of biomaterials

“…XPS analyzes a surface depth of greater than 10Å [28], whereas the molecular length of surface-grafted O -phospho L-serine and O -phosphoethanolamine is ca. 5 Å if extended into a straight line [29]. …”

Adsorption monitoring of phospho-l-serine on hydroxyapatite

“…As may be seen from the kinetics of adsorption curves, adsorption of PLS on HAP reached a plateau value at about 3.5 μmol/m 2 , with the exception of the first concentration (0.5 mM), which gave a plateau value below that corresponding to the calculated surface concentration needed for a monolayer coverage. Assuming that the PLS molecules cover a circular area corresponding to an equivalent radius r00.4 nm, a value, equal to 3.32 μmol/m 2 was calculated for the maximum monolayer coverage of HAP by PLS [16]. It may therefore be concluded that in the case of the three higher initial concentrations, the first plateau of the kinetic curves corresponds to the maximum monolayer surface coverage of HAP by PLS.…”

“…The inhibitory activity of some salivary proteins as well as their adsorption onto HAP is associated with the presence of phospho-L-serine (PLS) in their amino acid sequence [9][10][11][12][13]. It has been suggested that PLS adsorbs on the surface of HAP, blocking the active crystal growth sites thus resulting in the inhibition of HAP crystal growth [14][15][16]. A similar inhibitory effect on the crystal growth of HAP has been reported for the amino acid L-serine at significantly higher concentrations than those of PLS [17].…”

Dependence of the interaction mechanisms between l-serine and O-phospho-l-serine with calcium hydroxyapatite and copper modified hydroxyapatite in relation with the acidity of aqueous medium

“…Particular interest among other AA has been paid on L-Serine (L-Ser) and O-phospho-L-serine (O-Ph-L-Ser) adsorption, which were characterized by Langmuir-type adsorption on apatite [6][7][8][9][10]. The studies carried out at pH 7-10 with L-Ser led to the conclusion that L-Ser adsorbs on the surface of hydroxyapatite (HAp) through electrostatic attractions exerted between one negative site of the HAp surface, i.e., phosphate or hydroxyl ion, and the positively charged protonated amino group of one Ser molecule, forming an ion pair surface complex [6,7].…”

“…The monoprotonated negatively charged (HL 2− ) O-Ph-L-Ser species were found adsorbed forming surface complexes with the positively charged ≡CaOH 2+ sites on the surface of HAp at pH 7 [8]. The negatively charged deprotonated carboxyl and phosphate groups of the adsorbed HL 2- species 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 orient themselves at the maximum possible distance from the negatively charged surface of HAp because of electrostatic repulsions [9].…”