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Dimethyl suberimidate is a bifunctional chemical reagent used for protein cross-linking. Its two imidoester ends can react with amino groups to form amidine linkages, allowing a single reagent molecule to connect two accessible protein groups.^[ref]

In 1970, G. E. Davies and G. R. Stark used the reagent to investigate subunit organization in oligomeric proteins. After cross-linking, detergent gel electrophoresis resolved species with apparent masses corresponding to integral multiples of the individual subunit mass.^[ref]

For an illustrative $20\mathrm{kDa}$ subunit, linked pairs and triples would appear near $40$ and $60\mathrm{kDa}$ before accounting for linker mass and electrophoretic behaviour. A covalent bridge can preserve an association through conditions that disrupt the original noncovalent interactions.

The observation establishes chemical proximity under the reaction conditions. It does not automatically locate a unique native interface or prove that every detected pair is a stable complex in a living cell. Reagent accessibility, possible links within one chain and collisions between molecules all affect the products. Comparing treated and untreated preparations, and identifying the protein species, makes the structural inference more informative.

An imidoester is an ester of an imidic acid. A common carbon-centred pattern is $\mathrm{R}-\mathrm{C}(=\mathrm{NH})-\mathrm{O}{\mathrm{R}}^{\prime }$, with an imino nitrogen in place of the carbonyl oxygen of an ordinary ester. Imidoesters react with amines to form amidines, which makes them useful for modifying protein amino groups.^[ref]

A formal net substitution illustrates the group change:The product has a carbon bonded to two nitrogens rather than an alkoxy group. This equation is structural bookkeeping, not a claim about every protonation state, kinetic pathway or experimental yield.

A bifunctional reagent such as dimethyl suberimidate has two reactive ends and can connect two amino groups. Those groups can lie on different proteins or within one folded protein.^[ref] Thus a detected cross-link constrains which reactive sites could be connected under the assay conditions; it is not automatically proof of direct atom-to-atom contact or a unique unperturbed protein arrangement.