Sulfilimine bond

A sulfilimine bond is a covalent sulfur–nitrogen linkage conventionally represented with an $\mathrm{S}=\mathrm{N}$ unit. In type IV collagen, it links the sulfur of a methionine side chain to the nitrogen of a hydroxylysine side chain from a neighboring protein unit. It is chemically different from a disulfide bond, which links two sulfur atoms.^[ref]

The collagen example makes atom identity important. Although hydroxylysine contains a hydroxyl group, the identified interprotein linkage involves its nitrogen, not that oxygen. Linking separate protein units reinforces the type IV collagen network of the basement membrane.

The 2009 identification combined two kinds of evidence. High-accuracy mass spectrometry constrained the composition of isolated crosslinked peptides, while nuclear magnetic resonance supplied information about the local chemical environment. Together these supported the sulfur–nitrogen assignment; mass alone would not uniquely specify how atoms of a given composition are connected.^[ref]

This is a useful example of why a protein crosslink needs a chemical description as well as a mechanical one. Observing that two protein units remain connected can establish a covalent association under the measurement conditions, but identifying the bond requires evidence about the participating atoms and their bonding environment.