Partitioning of some combination di‐ and tripeptides in aqueous two‐phase systems of poly(ethylene glycol) and Di‐potassium hydrogen phosphate at 293 K: Experimental results and predictions

Abstract

AbstractAqueous two‐phase systems are effective extraction systems for separating peptides and enzymes from aqueous phases. Such extraction processes are based on the finding that peptides and enzymes show different preferences for the coexisting phases. Methods to calculate and predict the liquid‐liquid phase equilibrium in those systems can only be developed when an extensive experimentally determined data base is available. The present contribution contributes to that goal by presenting experimental data for the partitioning of small amounts (micrograms solute per kilogram of solution) of low molecular‐weight combination peptides of glycine, 1‐glutamic acid, 1‐phenylalanine, and 1‐lysine in aqueous two‐phase systems of di‐potassium hydrogen phosphate and poly(ethylene glycol) of molecular mass of about 6000 and 35000 at about 293 K. A group‐contribution model for the excess Gibbs energy is used to predict liquid‐liquid phase equilibrium. A preliminary set of binary interaction parameters was taken from the literature. These model parameters were determined using experimental results for the partitioning of single amino acids and some of their peptides in the same aqueous two‐phase system. No experimental results for the partitioning of combination peptides had been used for parameter estimation, i.e. those peptides were products of condensation reactions of molecules of a single amino acid. Therefore, partition coefficients calculated for combination peptides are predictions. For some peptides predicted and measured partition coefficients agree within experimental uncertainty. In a few cases predicted partition coefficients deviate from experimental results by a factor of about two. However, the model always correctly predicts which phase (polymer‐rich or salt‐rich) is preferred by the peptide.