Examples include a method of making a protein-PEG conjugate. The method may include providing an aqueous protein solution. The aqueous protein solution may include a protein, a pH buffer, and a chelating agent. The chelating agent may be chosen from the group consisting of an aminopolycarboxylic acid, a hydroxyaminocarboxylic acid, an N-substituted glycine, 2-(2-amino-2-oxocthyl) aminoethane sulfonic acid (BES), and deferoxamine (DEF). The method may also include introducing sodium cyanoborohydride and a methoxy polyethylene glycol aldehyde to the aqueous protein solution. The sodium cyanoborohydride in the methoxy polyethylene glycol aldehyde may have a molar ratio ranging from about 5:1 to about 1.5:1. The method may further include reacting the methoxy polyethylene glycol aldehyde with the protein to form the protein-PEG conjugate. The pH buffer may maintain a pH of the aqueous protein solution ranging from 4.0 to 4.4 during the reaction.
