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The bifunctional reagent, methyl 4-mercaptobutyrimidate, has been useful in identifying neighboring protein pairs in the Escherichia coli 30S ribosomal subunit. The reagent reacts with protein amino groups and upon oxidation forms intermolecular disulfide bonds. The compound has been characterized further employing the techniques of nuclear magnetic resonance spectroscopy, infrared spectroscopy, ultraviolet spectroscopy, mass spectroscopy, elemental analysis, and sulfhydryl group titration. All of the results indicate that the compound formerly considered to be methyl 4-mercaptobutyrimidate is in fact 2-iminothiolane (2-iminotetrahydrothiophene). Unlike most imidates, 2-iminothiolane is highly stable in solution at acidic and neutral pH. Furthermore, we have not observed any protein:protein cross-links produced withThe bifunctional reagent methyl 4-mercaptobutyrimidate, used previously in this laboratory to cross-link neighboring ribosomal proteins, has been found to have properties indicating its identity with the compound 2-iminothiolane described by Schramm & Diilffer [Schramm, H. J., & Diilffer, T. (1977) Hoppe-Seyler's Z. Physiol. Chem. 358,[137][138][139], The latter name will be used here.

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