MPEG-pALD (MW 20000) is an important unilaterally active PEG derivative whose reactive groups can react with amines, peptides and the N-termini of proteins to form imines containing C=N double bonds, which can be further reduced. Polyethylene glycol aldehyde or ketone derivatives can be used for reversible PEGylation by reacting with hydrazine or hydrazine to form hydrolyzed acylhydrazone bonds[1].
