4-(N-Boc-amino)-1,6-heptanedioic acid is a branched PEG derivative with a Boc protected amino group and two terminal carboxylic acids. The protected amine can be deprotected by acidic conditions. The terminal carboxylic acids can be reacted with primary amino groups in the presence of activators (e.g. EDC, or DCC) to form a stable amide bond.