Alternative Route To Dehydroalanine

Dehydroalanine (Dha), which can be formed by oxidative elimination of alkylated derivatives of cysteine (Cys) or selenocysteine, provides a chemical handle for selectively modifying proteins and thereby studying the process of posttranslational modifications. The usual strategies for forming Dha rely on peroxide-induced oxidative elimination, which can unintentionally affect methionine as well. Benjamin G. Davis and coworkers at the University of Oxford instead use O-mesitylenesulfonylhydroxylamine (MSH) for the oxidative elimination reaction (J. Am. Chem. Soc., DOI: 10.1021/ja800800p). When the researchers treated Boc-protected Cys methyl ester with excess MSH (shown), the Dha derivative formed at almost quantitative yield in just a few minutes at room temperature in open air. They used the strategy to convert the single exposed Cys on the surface of subtilisin from the bacterium Bacillus lentus without oxidizing any of the three methionine units also present. The researchers then tethered various posttranslational modifications to the protein by adding thiols to the Dha. In one case, they regenerated Dha from the resulting thioether by a similar oxidative elimination using MSH, allowing a "functional switch" on the protein surface.