Does water suppress the racemization and decomposition of amino acids?

Abstract

Several neutral free amino acids, phenylglycine, leucine, phenylalanine and tryptophan, are heated at 100–130 °C under alkaline or acidic conditions in water or various polar organic solvents. Although serious racemization and decomposition occur in polar organic solvents such as DMF, DMSO, or ethylene glycol under alkaline conditions (K₂CO₃, KOH, Et₃N), these phenomena do not occur, or are largely decreased, in water or water-containing organic solvents under the same alkaline conditions. Serious racemization and decomposition of free phenylglycine are also observed in AcOH, while water or aqueous AcOH (<90%) largely suppress the racemization and decomposition. We discuss the possible reasons for this suppression effect of water in terms of differential solvation of bases and states of dissociation of amino acids in aqueous and organic solvents.