Preparation of methionine-free natural leucine

In a recent note in Science, Mueller pointed out that commercial I-leucine is contaminated with methionine. The presence of a sulfur-containing impurity was recognized by Emil Fischer in 1900. Unsuccessful attempts to purify natural I-leucine have included: the amino-acid ester method, repeated recrystallization from water, electrodialysis, precipitation of methionine as mercury complex and recrystallization from acidified butyl alcohol. Recrystallization of the formylated amino-acid has now been found, however, to yield a sulfur-free leucine without appreciable racemization.