Biomimetic deposition of hydroxyapatite on a synthetic polypeptide with β sheet structure in a solution mimicking body fluid

Deposition of a hydroxyapatite layer with similar structure to bone mineral is an attractive approach to the fabrication of bioactive coating layers to achieve direct bonding to living bone. To get successful coating of a hydroxyapatite layer on an organic polymer using a biomimetic solution, it is essential to find organic substrates that can effectively induce heterogeneous nucleation of hydroxyapatite after exposure to the body environment. Our previous study showed that sericin, a type of silk protein, has the ability to induce hydroxyapatite nucleation in a biomimetic solution when the sericin has a β sheet structure. To confirm the effectiveness of the β sheet structure in hydroxyapatite nucleation, we focused on investigating hydroxyapatite deposition on a synthetic polypeptide with a β sheet structure in a biomimetic solution. The β sheet forming polypeptides with and without carboxyl groups, poly(FE)₃FG, poly(FQ)₃FG, poly(LE)₃LG and poly(LQ)₃LG, were synthesized in this study. All the polypeptides had mainly β sheet structure. After soaking the polypeptide films in 1.5SBF, which has 1.5 times the inorganic ion concentrations of human blood plasma, hydroxyapatite formed on the surfaces of the polypeptides with carboxyl groups, poly(FE)₃FG and poly(LE) ₃LG, within 2 days, but not on those without carboxyl groups, poly(FQ)₃FG and poly(LQ)₃LG. We confirmed that the β sheet structure was effective for hydroxyapatite nucleation even in the synthetic polypeptide. This finding is useful for the future design of organic polymers that can effectively induce nucleation of hydroxyapatite.