**Suzuka Kojima, Hitomi Nakamura, Sungho Lee, Fukue Nagata and Katsuya Kato \***

National Institute of Advanced Industrial Science and Technology, 2266-98, Anagahora, Shimo-Shidami, Moriyama-ku, Nagoya, Aichi 463-8560, Japan; suzuka-kojima@aist.go.jp (S.K.); hi.nakamura@aist.go.jp (H.N.); sungho.lee@aist.go.jp (S.L.); f.nagata@aist.go.jp (F.N.)

**\*** Correspondence: katsuya-kato@aist.go.jp; Tel.: +81-52-736-7551; Fax: +81-52-736-7405

Received: 19 July 2019; Accepted: 17 September 2019; Published: 19 September 2019

**Abstract:** Self-assembling peptides have been employed as biotemplates for biomineralization, as the morphologies and sizes of the inorganic materials can be easily controlled. We synthesized two types of highly ordered self-assembling peptides with different secondary structures and investigated the effects of secondary structures on hydroxyapatite (HAp) biomineralization of peptide templates. All as-synthesized HAp-peptides have a selective protein adsorption capacity for basic protein (e.g., cytochrome c and lysozyme). Moreover, the selectivity was improved as peptide amounts increased. In particular, peptide–HAp templated on β-sheet peptides adsorbed more cytochrome c than peptide–HAp with α-helix structures, due to the greater than 2-times carboxyl group density at their surfaces. It can be expected that self-assembled peptide-templated HAp may be used as carriers for protein immobilization in biosensing and bioseparation applications and as enzyme-stabilizing agents.

**Keywords:** solid-phase peptide synthesis; hydroxyapatite; peptide; secondary structure; selective protein adsorption; biotemplate
