Conformational modifications of serum albumins adsorbed on different kinds of biomimetic hydroxyapatite nanocrystals

Nanosized carbonate-hydroxyapatite represents a suitable material for bone substitution and delivery of biomolecules. Its interaction with serum proteins plays a central role in the process of implantation of a device. Herein, surface interactions of human (HSA) and bovine (BSA) serum albumin with two biomimetic carbonate-hydroxyapatites (CHA) differing for size, surface area, crystallinity degree and surface properties have been investigated. BSA reached a different maximum coverage, being higher with the CHA nanocrystals with the higher surface area, the smaller size, the lower crystallinity degree and the plate shape morphology. On the contrary, HSA did not show significant differences of maximum coverage on the two substrates. The adsorption isotherms have been fitted by the Langmuir and Freundlich models, showing that the biomimetic hydroxyapatite matrix behaves as a mixture of energetically homogeneous and heterogeneous sites due to its surface disorder. By using Fourier Transform Infrared Spectroscopy (FT-IR) and Circular Dichroism (CD) it has been found that CHAs affect the conformation of the adsorbed proteins. FT-IR spectroscopy shows that protein adhesion on CHA surface results in a significant reduction in helicity for both proteins on both substrates, as function of the coating extent as well as modification in the β-structures. CD spectroscopy of the HSA and BSA released in solution after desorption from the matrices shows that, while both proteins partially regain their helical structure, they show a distinct behaviour in their tertiary structure.