= 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.<Use of dimethyl suberimidate, a cross-linking reagent, in studying the subunit structure of oligomeric proteins by G. E. Davies and G. R. Stark>[Abstract: bifunctional reagent and cross-linked oligomers]{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.<Use of dimethyl suberimidate, a cross-linking reagent, in studying the subunit structure of oligomeric proteins by G. E. Davies and G. R. Stark>[Abstract: detergent electrophoresis and integral-mass product series]{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.<Use of dimethyl suberimidate, a cross-linking reagent, in studying the subunit structure of oligomeric proteins by G. E. Davies and G. R. Stark>[Abstract: dissimilar-protomer comparisons]{ref}
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