Analysis of hydroxyapatite surface coverage by amelogenin nanospheres following the Langmuir model for protein adsorption

The assembly of amelogenin protein into nanospheres is postulated to be a key factor in the stability of enamel extracellular matrix framework, which provides the scaffolding for the initial enamel apatite crystals to nucleate and grow. Adsorption isotherms ware evaluated in order to investigate the nature of interactions of amelogenin nanospheres with hydroxyapaite crystals in solution, where their assembly status and particle size distribution are defined. We report that the adsorption isotherm of a recombinant mouse amelogenin (rM179) on synthetic hydroxyapatite crystals can be described using a Langmuir model indicating that amelogenin nanospheres adsorb onto the surface of apatite crystals as binding units with defined adsorption sites. The adsorption affinity and the maximum adsorption sites were 19.7 x 10(5) L/mol and 6.09 x 10(-7) mol/m(2), respectively, with an r(2) value of 0.99. Knowing the composition and particle size distribution of amelogenin nanospheres under the condition of adsorption experiments, we have calculated the number of nanospheres and the crystal surface area covered by each population of nanospheres at their maximum adsorption. It was found that total maximum binding covers 64% of the area unit. This observation supports the speculation that amelogenin binding onto apatite surface is selective and occurs only at certain sites.