The amine group is the most common target for modifying protein molecules because it is abundant in the majority of 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

Sulfo-NHS-esters are water soluble and useful for applications that cannot tolerate organic solvents. Sulfo-NHS-esters of biotin are recommended for use as cell surface biotinylation reagents. Because of the charged sulfonate group, these compounds do not penetrate the plasma membrane, thus sulfo-NHS-esters biotinylation is restricted to the cell surface.

EZ-Link™ Sulfo-NHS Biotinylation Kits: Complete kit with an optimized protocol for labeling a protein and determining how much biotin has been attached. Fast and efficient labeling procedure.
EZ-Link™ Sulfo-NHS-LC Biotinylation Kits: Complete kit with an optimized protocol for labeling a protein and determining how much biotin has been attached. Fast and efficient labeling procedure.
EZ-Link™ NHS-PEG₄ Biotinylation Kits: PEG₄-Biotin transfers water-solubility to biotinylated molecules.
EZ-Link™ Sulfo-NHS-SS Biotinylation Kits: Cleavable biotinylation reagent allows for later removal of the biotin label.

They may be used for biotinylating internal as well as external cellular components. Since NHS-esters of biotin are water-insoluble, for reactions in aqueous solution they must first be dissolved in an organic solvent, such as DMSO and DMF.