Proteins can also be cross-linked artificially using small-molecule crosslinkers.  This approach has been used to elucidate protein–protein interactions. Crosslinkers bind only surface residues in relatively close proximity in the native state. Common crosslinkers include the imidoester crosslinker dimethyl suberimidate, the N-Hydroxysuccinimide-ester crosslinker BS3 and formaldehyde. Each of these crosslinkers induces nucleophilic attack of the amino group of lysine and subsequent covalent bonding via the crosslinker. The zero-length carbodiimide crosslinker EDC functions by converting carboxyls into amine-reactive isourea intermediates that bind to lysine residues or other available primary amines. SMCC or its water-soluble analog, Sulfo-SMCC, is commonly used to prepare antibody-hapten conjugates for antibody development. An in-vitro cross-linking method is PICUP (photo-induced cross-linking of unmodified proteins). Typical reagents are ammonium persulfate (APS), an electron acceptor, the photosensitizer tris-bipyridylruthenium (II) cation (). In in-vivo crosslinking of protein complexes, cells are grown with [[photoreactive diazirine analogs to leucine and methionine, which are incorporated into proteins. Upon exposure to ultraviolet light, the diazirines are activated and bind to interacting proteins that are within a few ångströms of the photo-reactive amino acid analog (UV cross-linking).