= Imidoester
An <imidoester> is an ester of an imidic acid. A common carbon-centred pattern is $\mathrm{R-C(=NH)-OR'}$, 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.<Imidoesters in NLM MeSH>[Descriptor D007096, Scope Note]{ref}
A formal net substitution illustrates the group change:
$$
\mathrm{R-C(=NH)-OR'+H_2N-R''\longrightarrow R-C(=NH)-NH-R''+R'OH}.
$$
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>.<Dimethyl Suberimidate in MeSH>[D004120, Scope Note; bifunctional imidoester and amidine formation]{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.
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