The amine group is the most common target for modifying protein molecules because it is numerous in most proteins either due to the presence of lysine bearing amino side chain
functionality or the N-terminal-amine. Amine-reactive biotinylation reagents can be divided into two groups based on water solubility: NHS-esters and sulfo-NHS-esters.
NHS-esters of biotin are water-insoluble and membrane-permeable because they do not possess a charged group.
They may be used for biotinylating internal as well as external cellular components.
Sulfo-NHS-esters are water soluble and useful for applications that cannot tolerate organic solvents.
Because of the charged sulfonate group, these compounds do not penetrate the plasma membrane, thus sulfo-NHS-esters of biotin are recommended for use as
cell surface biotinylation reagents.