Use of neutral polystyrene resin for rapid desalting and fractionation of non-polar amino acids and non-polar oligopeptides

Abstract

Rapid desalting and chromatographic fractionation of nonpolar amino acids and nonpolar oligopeptides is possible by adsorption on the neutral polystyrene resin Porapak Q. Elution occurs with distilled water and mixtures of water with methanol, ethanol or acetone. The adsorption coefficients, K, of the amino acids in 0.1 N HCl decrease in the following order: ▪ Adsorption of an aliphatic CH 2-group is stronger than that of an aromatic CH-group. Within the homologous series of aliphatic amino acids each CH 2-group increases the K value by a constant factor of 5.1. Adsorption of neutral or acidic oligopeptides in 0.1 N HCl is stronger than, or at least equal to, that of the corresponding most nonpolar amino acid. The repeating unit -NH-CH 2-CO- in the series of oligoglycines increases K by a constant factor of 1.3. The isomers Gly-Pro and Pro-Gly are adsorbed differently. The effect of pH was investigated for Phe, Trp and Tyr. At low salt concentration, a minimum of adsorption was found near pH 7 and loss of adsorption of tyrosine was observed above pH 10 (ionization of the phenolic hydroxyl). Addition of salt increases adsorption. The effect of salting-out onto the adsorbent depends on the pH; it is minimal near pH 6 for Phe, Trp and Tyr, and is postulated to be minimal at the iso-electric point. The salting-out effect can be described for Phe at pH 10.9 by log K/ K 0 = a C 8 , where K and K 0 are the adsorption coefficients in salt-containing and salt-free solution, respectively, a is a constant and C s is the salt concentration