Synthesis of mPEG-SS-NH2Under a nitrogen atmosphere, N-hydroxy succinimide (NHS, 0.07 g, 0.6 mmol), N,N -dicyclohexylcarbodiimide (DCC, 0.12 g, 0.6 mmol) and mPEG-COOH (1.0 g, 0.5 mmol) were suspended in dichloromethane (30 mL) at 0°C for 5 h. Then a dry solution of cysteamine (0.5 g, 3.5 mmol) in 5 mL dichloromethane was dropwise added, and the reaction mixture was maintained at room temperature for another 24 h. The resulting product was isolated by filtration to remove insoluble byproducts, precipitated in cold diethyl ether twice and then purified by dialysis using a membrane (MWCO 1000 Da) for 24 h.
Synthesis of PHIS-PEG2000
10 mg Poly (L-histidine) was dissolved in 5 mL of 10 mM acetic acid followed by adjustment of pH to 6.5 with 0.1 M NaOH. An excess amount of mPEG-NHS (1.2:1, mol/mol) was added to the above solution and the reaction was carried out under the nitrogen gas for 8 h at room temperature. The reaction mixture was monitored by the TLC and dialyzed with cellulose ester membranes with a molecular weight cut-off of 5 kDa (Spectrum Medical Industries, Rancho Dominguez, CA) against deionized water for 72 h to remove the unreacted mPEG-NHS. The solution was lyophilized after filtering through a 0.45 mm syringe filter. About 8.5 mg of white fluffy product were obtained. The synthetic scheme is shown in Fig. 2. The freeze- dried powder (2 mg) was dissolved in the deuterated (d)-chloroform (CDCl3, 1 mL) and analyzed by the 1H NMR using Varian 400 mHz (Bruker, Avance).