The Benzyl-PEG-NHS ester is synthesized by Benzyl-PEG-acid and HUSO. It is sensitive to acid and higly temperature. Stored in 0-4℃. The reagent’s NHS ester reacts with primary amines at pH 7-9 to form stable amide bonds, and the bromacetyl reacts with sulfhydryl groups at pH >7.5 to form stable thioether bonds. The reagent is useful for preparing cyclic peptides and peptide conjugates because the spacer maintains peptide-like character in the crosslinked species.