α lactalbumin from bovine milk

Albumin from chicken egg white, α-lactalbumin from bovine milk and κ-casein from bovine milk were obtained from Sigma (Zwijndrecht, the Netherlands). Papain enzyme from Carica papaya (10 mg/mL) and TFA was purchased from Merck (Hohenbrunn, Germany). The protein standard solutions were prepared by dissolving 1 mg of the respective protein in 1 mL demineralized water (Millipore, Amsterdam, the Netherlands) in individual Eppendorf tubes. Standard solutions were pre-incubated with 30 µL of papain solution (1 mg/mL) at 65 °C for 5 min. The enzyme-to-protein ratio was 1:33. The sample was incubated overnight (16 h) at 37 °C. Enzyme deactivation was then performed by heating the solutions to 95 °C for 5 min.

Cytochrome c from equine heart ≥ 95%, α-Lactalbumin from bovine milk, acetonitrile, ethanol, acetone, dimethylformamide (DMF), reduced glutathione ethyl ester, N,N-diisopropylethylamine (DIEA), tributhylphosphine (PBu3), and 3-nitro-2-pyridinesulfenyl chloride (Npys-Cl) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Carboxylic PEG acid, mPEG-COOH (2000 Da), Folic acid PEG thiol, Folate-PEG-SH (2000 Da), and Folic acid-PEG-FITC (MW 3400 Da) were purchased from NanoCS (Boston, MA, USA). Folate-poly(ethylene glycol)-carboxylic acid FA-PEG-COOH (MW 3000 Da) and Poly(lactide-co-glycolide)-thiol end cap (Mn: 10,000–30,000 Da) were purchased from Akina, Inc. (West Lafayette, IN, USA). Succinimidyl-3-(2-pyridyldithio) propionate (SPDP) was obtained from Thermo Fisher Scientific (Waltham, MA, USA). Fluorescein isothiocyanate isomer I (FITC) was purchased from Sigma-Aldrich (St. Louis, MO, USA).