Anhydrous 1 hydroxybenzotriazole hobt

All reagents were used without further purification unless otherwise stated. Optimark^™ was purchased from Mallinckrodt Pharmaceuticals (St. Louis, MO, USA), and saline was obtained from Hospira, Inc. (Lake Forest, IL, USA). The Fmoc-protected amino acids, 2-chlorotrityl chloride resin, and benzotriazol-1-yl-oxy-tris-(pyrrolidino) phosphonium hexafluorophosphate (PyBOP) used for peptide synthesis were purchased from Chem-Impex International, Inc. (Wood Dale, IL, USA), along with anhydrous N,N-diisopropylethyl amine (DIPEA), trifluoroacetic acid (TFA), 1,2-diethanethiol, triisopropylsilane and piperidine from Sigma-Aldrich (St. Louis, MO, USA). N,N-dimethylformamide (DMF), and dichloromethane were purchased from Fisher Scientific (Pittsburgh, PA, USA). Anhydrous 1-hydroxybenzotriazole (HOBt) was obtained from Apex Bio Technology (Houston, TX, USA). The detailed syntheses of maleimido-tris-propargyl [9 (link)] and azido-(Gd-DOTA) [10 (link)] have been described previously. Gadolinium (III) acetate tetrahydrate was from Strem Chemicals (Newburyport, MA, USA).