distortion of the cyclooctyne helps to speed up the reaction by both reducing the activation strain and enhancing the interactions, thereby enabling it to be used in physiological conditions without the need for the catalyst. For instance, Ting et al. introduced an azido functionality onto specific proteins on the cell surface using a ligase enzyme. The azide-tagged protein is then labeled with cyclooctyne-fluorophore conjugate to yield a fluorescently labeled protein. References Nitrogen heterocycle forming reactions Cycloadditions