Adsorption of myoglobin onto various synthetic hydroxyapatite particles

The adsorption of myoglobin (MGB) onto various kinds of colloidal synthetic hydroxyapatite[X_10(PO_4)_6(OH)_2, noted CaHap, SrHap and CaSrHap for X = Ca, Sr and Ca + Sr, respectivelyJ particles was investigated at 15 °C in a 1 x 10^(-4) mol dm^(-3) KCl solution of pH 6.0. The adsorption rate of MGB onto CaHap was comparable to that of lysozyme (LSZ) and was faster than that of bovine serum albumin (BSA). This distinction was explained by the difference in the molecular mass of these proteins (MGB: 17800 Da, LSZ: 14600 Da, BSA: 67 200 Da); the diffusion rates of the smaller MGB and LSZ molecules to the CaHap surface are faster than that of the larger BSA. The adsorption isotherms of MGB onto these synthetic hydroxyapatites (Haps) exhibited the Langmuir type and the surface charge of the MGB-covered Haps showed almost constant negative value and was independent of the MGB concentration for all the Haps. The values of the saturated amounts of adsorbed MGB for CaHap were independent of the Ca : PO_4 molar ratio of the materials, supporting less importance of an electrostatic interaction between MGB and Haps because MGB molecules are electrostatically neutral at pH 6. It was presumed that MGB molecules are adsorbed onto phosphate ions exposed on ac and bc crystal faces by the van der Waals attractive force without taking advantage of the electrostatic interaction