Four different initial orientations of BMP-2 above the graphite surface (indicated by 2 gray lines). Nonpolar residues shown in blue, neutral residues in green and polar residues in red.The N-terminus is shown in yellow and C-terminus in orange.

Biocompatibility of implants is mainly governed by nonspecific protein adsorption processes and the subsequent protein–cell interactions. For instance ideas to improve osteointegration consider functionalizing implant surfaces with bone growth factors such as BMP–2. Yet the molecular mechanisms of the adsorption process are worth considering since the protein structure has to stay intact in order to remain its biological function, e. g. the stimulation of bone growth.

It‘s convenient to investigate adsorption processes on graphite surfaces since pyrolytic graphite is widely used as an implant material due to its high biocompatibility, low coefficient of friction against bone and cartilage, and stiffness match with bone.

Christian Mücksch and Herbert M. Urbassek

Since the temperature is controlled via the velocity–rescaling thermostat all excess kinetic energy is removed from the protein because potential energy is influenced by the attractive Lennard–Jones interactions. That causes the total energy to sink eventually. The perpendicular orientations 3 and 4 show a less strong initial adsorption, thus allowing more flexibility for spreading which is indicated by the radius of gyration. Also, the secondary structure dissolves upon approaching the surface and some of the amino acids with aromatic rings (His, Phe,Tyr,Tryp) tend to orient parallel to the graphite surface.

Total energy of the system, averaged over 100 ps.

Time evolution of the radius of gyration of BMP-2 (component parallel to the graphite surface).

  1. results indicate that both proteins unfold on a hydrophobic graphite surface but there is a certain dependency on the initial orientation

  2. simulations employing implicit inviscid water might be one way to analyze protein adsorption given that adsorption processes in experiments typically happen on a time scale ranging from milliseconds to hours

  3. in case of BMP-2 following interactions with cell receptors tend to be unlikely which emphasizes the need of spacer groups between graphite and protein