Avidin-adsorbed peptide–calcium phosphate composites exhibiting high biotin-binding activity
Techniques harnessing the strong interactions of the avidin–biotin system are frequently used in biosensing and protein separation. Hydroxyapatite (Ca10(PO4)6(OH)2, HAp) is a calcium phosphate compound known to have a strong affinity for various proteins. In this study, the adsorption ability of avidin on composites and its binding activity to biotin were investigated using two types of poly(L-glutamic acid)-containing calcium phosphate composites (pGlu-HAp). The morphologies of pGlu-HAp including poly(α-glutamic acid) and poly(γ-glutamic acid) showed plate-like and sheet-like particles, respectively, due to the effects of the peptide structure. Furthermore, these particles exhibited the high adsorption capacity of avidin; the maximum adsorption amounts for poly(α-glutamic acid)-HAp and poly(γ-glutamic acid)-HAp were 776 and 302 μg mg−1 HAp, respectively. In addition, the equilibrium data for the adsorption of avidin on two pGlu-HAp-fitted Langmuir isotherm models correspond to a monolayer. The difference in the amount of adsorbed avidin on the two pGlu-HAp composites was influenced by the density of the carboxyl groups in pGlu-HAp. This revealed that avidin adsorbed on pGlu-HAp was able to bind four biotin molecules, and biotinylated protein was separated from protein mixtures by avidin-adsorbed pGlu-HAp. These results indicate that these particles would be appropriate in the development of biosensing and bioseparation tools.