Reactive crystallization of selected enantiomers: Chemo-enzymatic stereoinversion of amino acids at supersaturated conditions

A new route for resolution of amino acids that form racemic compounds was developed using chemo-enzymatic stereoinversion reactions in saturated or supersaturated solutions: i.e. at high solute concentrations. The reactions enrich the solution in the desired enantiomer and move the solution composition to a thermodynamic region where the pure desired enantiomer can be crystallized. The reaction sequence first oxidizes the undesired d-enantiomer with d-amino acid oxidase, and the resulting intermediate imino acid is reduced into a racemic mixture of the enantiomers by ammonia borane. The result of this reaction network is completely enantioselective because the first step is enantioselective. The concept has been demonstrated by resolving racemic mixtures of phenylalanine and methionine, which allowed recovery of l-phenylalanine and l-methionine crystals with chemical and enantiomeric purities greater than 99%. Finally, reaction and crystallization kinetics were determined and combined to model this reactive-crystallization system. These results were used to demonstrate the potential of combining chemo-enzymatic reactions and crystallization to enhance the productivity of the process.