 Protein cross-linking

Protein cross-linking is formation of covalent bonds that connect sites within a protein or between different protein chains. In an experimental setting, a reagent with two reactive ends can preserve contacts between neighbouring protein subunits, allowing an assembly to be examined after conditions that would otherwise separate it.^[ref]

In a 1970 experiment, dimethyl suberimidate joined components of several oligomeric proteins. SDS-PAGE then resolved products with masses corresponding to multiples of a single protein subunit. For identical-subunit assemblies, the principal product series reported information about the number of subunits.^[ref]

For an invented example, four identical $25\mathrm{kDa}$ protein subunits can produce linked species near $25$, $50$, $75$ and $100\mathrm{kDa}$. A band near $50\mathrm{kDa}$ alone does not prove that the native assembly has only two subunits: incomplete linking can leave pairs within a larger assembly. Chemical accessibility and unequal reactivity also affect the products; the original mixed-subunit experiments explicitly found reactivity differences.^[ref]