Separation of Low Molecular Weight Peptides from Amino-Acids on ‘DEAE-Sephadex’

Abstract

METHODS are now available for the separation of amino-acids from peptides, with a molecular weight greater than 1,000, by molecular sieving on cross-linked dextran gels^1,2. However, the separation of peptides of lower molecular weight from all their constituent amino-acids is often difficult because of their similar physical properties. Recently I described a method for the separation of neutral amino-acids from neutral peptides of low molecular weight on the anion exchanger ‘Dowex-1’ with ammonium acetate buffer pH 8.65 (ref. 3). At this pH the monoaminomonocarboxylic acids, which all have pK values for the amino group greater than 9.0, are predominantly in the neutral form, whereas many peptides of these amino-acids have pK values less than 8.3 and thus are predominantly in the anionic form (pK values from Cohn and Edsall⁴). This method had the disadvantages that the aromatic amino-acids were strongly adsorbed on the resin, and that traces of amino-acids contaminated the peptide fraction.