Adsorption and conformational change of myoglobin on biomimetic hydroxyapatite nanocrystals functionalized with alendronate

The chemical conjugation of bisphosphonates (BP's), specifically alendronate, to hydroxyapatite could be an effective means to impart to it fine-tuned bioactivity. Horse heart myoglobin (Mb), a well-characterized protein, has been adsorbed onto biomimetic hydroxyapatite nanocrystals (nHA) and onto the nHA/alendronate conjugate powdered samples. The obtained materials have potential use in bone implantation and as prospective drug-delivery devices. The kinetic absorption of Mb onto nHA is dramatically affected by its functionalization with alendronate. The covering of the nHA surface by alendronate inhibits the adsorption of myoglobin. The adsorption mechanisms of the protein were studied by spectroscopic techniques (UV-vis and surface-enhanced Raman spectroscopy). The results indicate that the protein changes conformation upon adsorption on the inorganic substrate. In particular, the interaction with nHA alters the coordination state of the iron in the heme through the formation of a hexacoordinated low-spin Mb heme, possibly involving the distal histidine. Instead, the covering of the nHA surface by alendronate does not adsorb the protein but preserves the coordination state of the heme moiety. This study could be of significance either in the field of biomaterials science, in particular, to fine tune a bone-specific drug delivery device and to test nHA as a new support for heterogeneous catalysis, improving the understating of enzyme immobilization.