Distribution Behavior of L-Tryptophane by Extraction with Di(2-ethylhexyl) Phosphoric Acid

Abstract

To demonstrate that the proton-transfer reaction occurs in the extraction of α-amino acid and to explain the complicated phenomenon present in the extraction of L-tryptophane, a series of extraction equilibrium experiment for L-tryptophane with di(2-ethylhexyl) phosphoric acid (D2EHPA) dissolved in n-octane (System A) and a mixture of n-octane and n-octanol (System B) using L-tryptophane (L-Trp) as a model amino acid were carried out. The effects of L-tryptophane (L-Trp) concentration, D2EHPA concentration, n-octanol concentration, and pH on the extraction distribution ratio are discussed in detail. In System A, in the 1.0 < pH < 3.5 range, distribution ratio D increases with an increase of pH, but changes slightly in the 3.5 < pH < 5.0 range. The data indicate the formation of both (1:1) and (1:2) L-tryptophane–D2EHPA (dimeric) complexes, and the complexes tend to cluster together, away from the low-polarity bulk solvent. The infrared spectrogram of the organic-phase-loaded solute illustrates that the pH condition has little effect on the structure of complex. In System B, both the (1:1) and (1:2) L-tryptophane-D2EHPA (dimeric) complex form, but they do not cluster together because of the diluent that improves the polarity of the bulk solvent. The proton-transfer reaction is again demonstrated in the extraction of α-amino acid with D2EHPA. Expressions of the equilibrium distribution are proposed.