*3.2. Preparation of Peptide–HAp Particles*

Two peptides (Ac-(LELL)5-PEG70 and Ac-(VEVV)5-PEG70) were prepared via solid-phase peptide synthesis according to our previous reports [30–33]. One or 3 mg of Ac-(LELL)5-PEG70 or Ac-(VEVV)5-PEG70 was added to a 20 mL solution of dissolved (CH3COO)2Ca (52.8 mg), followed by stirring for 30 min at 20 ◦C. After addition of 20 mL (NH4)2HPO4 solution (23.8 mg), the mixture was heated to 60 ◦C at a heating rate of 1 ◦C min−1. The temperature was maintained for 3 h with stirring, and then the precipitant was collected by centrifugation at 6000 rpm for 10 min. The final products were washed twice with deionized water (Milli-Q, Merck KGaA, Darmstadt, Germany) and freeze-dried. To compare protein adsorption behavior, non-peptide–HAp was also synthesized using the same process as peptide–HAp.

#### *3.3. Characterization of Synthesized Peptide–HAp*

To analyze the secondary structure of Ac-(LELL)5-PEG70 and Ac-(VEVV)5-PEG70, the CD spectrum was measured using J-820 (JASCO Co., Tokyo, Japan) in a range scanning of 190–260 nm. The morphologies of all peptide–HAp samples were visualized with FE-SEM (S-4300, Hitachi Ltd., Tokyo, Japan) under 10.0 kV accelerating voltage and TEM (JEM-2010, JEOL Ltd., Tokyo, Japan) at 200 kV accelerator voltage. The specific surface area, pore volume, and pore size distribution were calculated on the basis of nitrogen adsorption–desorption isotherms using a TriStar 3000 (Shimadzu Co., Kyoto, Japan) via the BET and BJH models. For the measurement of the Ca/P molar ratio of synthesized peptide–HAp, inductively coupled plasma optical emission spectrometry (ICP-OES; IRIS Advantage, Thermo Fisher Scientific Inc., Waltham, MA, USA) was employed. The calculated peptide amounts of peptide–HAp were analyzed by TG-DTA (Thermo Plus TG 8120, Rigaku Co., Tokyo, Japan) in the operation range of room temperature to 1000 ◦C (heating rate of 10 ◦C min−1). ELSZ-1000 (Otsuka Electronics Co., Tokyo, Japan) was employed to measure the zeta potential of peptide–HAp, with the samples prepared via dispersion in 10 mM phosphate buffer of pH 7.0 with sonication for 3 min. XRD (SmartLab SE/B1, Rigaku Co., Tokyo, Japan) analysis was carried out using CuKα radiation operated at an accelerator voltage of 40 kV and a beam intensity of 30 mA. The XRD patterns were collected at a step size of 2.0◦ min−<sup>1</sup> and a 2θ range between 3◦ and 60◦. FTIR spectra in the 400–4000 cm−<sup>1</sup> range were recorded by FT/IR-4700 (JASCO Co., Tokyo, Japan) with attenuated total reflection. STEM (JEM-2100 Plus, JEOL Ltd., Tokyo, Japan) operated at 200 kV with EDX (Noran System 7, Thermo Fisher Scientific Inc., Waltham, MA, USA) was used to analyze the element distribution of peptide over peptide–HAp.
