All chemicals and solvents were of a reagent or HPLC grade and used without further purification. Amino acid monomers N-α-Fmoc-L-aspartic acid β-t-butyl ester (Fmoc-Asp(OtBu)-OH), N-α-Fmoc-L-glutamic acid γ-t-butyl ester (Fmoc-Glu(OtBu)-OH), and N-α-Fmoc-O-t-butyl-L-serine (Fmoc-Ser(tBu)-OH) were used. The amino acids, Wang resin, N,N ′diisopropylcarbodiimide (DIPCI), 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU), 1-hydroxy benzotriazole monohydrate (HOBt), N,Ndiisopropylethylamine (DIEA), trifluoroacetic acid (TFA), and piperidine were purchased from Watanabe Chemical Industries, Ltd. (Watanabe, Hiroshima, Japan). N,N-Dimethyl-4aminopyridine (DMAP) was purchased from Tokyo Chemical Industry Co., Ltd. (TCI, Tokyo, Japan). N-Methylpyrrolidone (NMP), triisopropylsilane, thioanisole, and diethyl ether were purchased from FUJIFILM Wako Pure Chemical Corporation (Wako, Osaka, Japan).