Syntheses of (2S,3R)- and (2S,3R)[3- 2h]- 3-methylaspartic acid: slow substrates for a syn-elimination reaction catalysed by methylaspartase.

Abstract

Methylaspartase catalyses the slow syn-elimination of ammonia from the (2S,3R)-[L- erythro]-diastereomer of the natural substrate, (2S,3S)-3-methylaspartic acid, to give mesaconic acid. To provide material of sufficient stereochemical purity to probe the mechanism of the reaction, two synthetic routes to (2S,3R)- and (2S,3R)[3- 2H]- 3-methylaspartic acid were devised. The use of these (2S,3R)-3-methylaspartic adds revealed that the enzymic reaction does not involve C-3 epimerisation followed by normal anti-elimination, ruling-out the possibility of a carbanion intermediate. Conversely, the substrate displayed very large primary deuterium isotope effects indicating rate-limiting CH bond cleavage.