Reversible blocking of amino groups with citraconic anhydride.

Butler, Harris, Hartley & Leberman (1967) showed that maleic anhydride could be used for the reversible blocking of amino groups. The maleyl group could be removed because the protonated form of the free carboxyl group catalysed the hydrolysis of the amide bond, presumably by intramolecular general acid catalysis (cf. Bender, Chow & Chloupek, 1958). The present paper reports the effects of introducing methyl groups into the molecule of maleic anhydride. 2,3-Dimethylmaleic anhydride. We had wished to investigate whether the use of maleylation to introduce glyoxyloyl groups (Dixon, 1968) could be extended to the introduction of pyruvoyl groups. We therefore treated arginine with 2,3-dimethylmaleic anhydride (Fluka A.-G., Buchs, Switzerland) as follows. To a solution of arginine hydrochloride (5M) in water was added, with stirring, dimethylmaleic anhydride to a final concentration of 5-5m, and the pH was maintained at 8 by the addition of N-NaOH. Uptake ofbase ceased after about an hour and suitable samples were then applied to paper for high-voltage electrophoresis in the buffer systems used previously (Perham, 1967). Material was detected on the paper by means of the ninhydrincadmium reagent (Heilmann, Barrollier & Watzke, 1957) and also by the Sakaguchi reaction (Jepson & Smith, 1953). It was immediately apparent that arginine alone and no product could be detected after electrophoresis at pH 3 5, but that at pH 6 5