Propargyl-PEG5-NHS ester has a propargyl group and an NHS ester group. The terminal NHS ester and is an amine reactive moeity which is useful for derivatizing peptides, antibodies, amine coated surfaces etc. The propargyl group can participate in Click Chemistry reactions with azide compounds, copper is needed for catalyzation. The PEG units can enhance the hydrophilicity of the molecule.