Adsorption of myoglobin onto various synthetic hydroxyapatite particles

The adsorption of myoglobin (MGB) onto various kinds of colloidal synthetic hydroxyapatite [X-10(PO4)(6)(OH)(2), noted CaHap, SrHap and CaSrHap for X= Ca, Sr and Ca+Sr, respectively] particles was investigated at 15 degrees C in a 1x10(-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 th at of bovine serum albumin (BSA). This distinction was explained by the dif ference in the molecular mass of these proteins (MGB: 17 800 Da, LSZ: 14 60 0 Da, BSA: 67 200 Da); the diffusion rates of the smaller MGB and LSZ molec ules to the CaHap surface are faster than that of the larger BSA. The adsor ption isotherms of MGB onto these synthetic hydroxyapatites (Haps) exhibite d the Langmuir type and the surface charge of the MGB-covered Haps showed a lmost constant negative value and was independent of the MGB concentration for all the Haps. The values of the saturated amounts of adsorbed MGB for C aHap were independent of the Ca:PO4 molar ratio of the materials, supportin g less importance of an electrostatic interaction between MGB and Haps beca use MGB molecules are electrostatically neutral at pH 6. It was presumed th at MGB molecules are adsorbed onto phosphate ions exposed on ac and bc crys tal faces by the van der Waals attractive force without taking advantage of the electrostatic interaction.