Solubility of DNP-amino acids and their partitioning in biodegradable ATPS: Experimental and ePC-SAFT modeling

Abstract

Predicting the behavior of dinitrophenylated amino acids (DNP-AA) in aqueous solutions requires an understanding and accurate description of interactions that can occur in such systems. In this work, some properties of DNP-AA (DNP-glycine, DNP-alanine, DNP-valine, DNP-leucine) have been determined experimentally. These were liquid densities obtained at T=298.15 – 318.15 K, p=1 bar, and pH-dependent solubility data measured at T=298.15 K, p=1 bar. It was observed that the solubility order for DNP-AA does not follow the same sequence as for aliphatic amino acids. The thermodynamic model ePC-SAFT has been applied to predict the properties density and solubility, and additionally to estimate partition coefficients of DNP-AA in PEG (PEG 4000, PEG 6000, PEG 8000) - organic salt (sodium citrate, potassium citrate, potassium sodium tartrate) aqueous two-phase systems (ATPS). ePC-SAFT pure-component and binary interaction parameters for neutral DNP-AA were acquired using the joinzt joint-parameter method, namely by combining the parameters for dinitrobenzene with parameters for amino acids (glycine, L-alanine, L-valine, L-leucine) from literature. The pure-component parameters for charged DNP-AA- were inherited from their parent neutral DNP-AA. This work shows that ePC-SAFT allows predicting liquid densities and solubilities of neutral DNP-AA with good agreement to experimental results. Moreover, adjusting in sum six binary parameters between charged DNP-AA- and phase-forming species allowed modeling partition coefficients of four DNP-AA in nine different PEG – organic salt ATPS, each at four different ATPS compositions. This can be considered an excellent modeling result and proofs the suitability of ePC-SAFT for such systems.