Sulfilimine bond
= Sulfilimine bond
A <sulfilimine bond> is a covalent sulfur–nitrogen linkage conventionally represented with an $\mathrm{S=N}$ 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.<A Sulfilimine Bond Identified in Collagen IV by Roberto Vanacore and others>[Abstract and structural assignment: methionine-93–hydroxylysine-211 crosslink]{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.<A Sulfilimine Bond Identified in Collagen IV by Roberto Vanacore and others>[Abstract, Table 1 and Figures 1–3: exact-mass and NMR analysis of crosslinked peptides]{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.