Investigation of Amino Acid–Polymer Aqueous Biphasic Systems

Abstract

Aiming at gathering further information to evaluate the recently proposed1,2 mechanism of the salt effect in aqueous polymer solutions, experimental vapor-liquid equilibria (VLE), liquid-liquid equilibria (LLE), and volumetric-compressibility measurements were carried out for several polymer-amino acid aqueous systems. The constant water activity lines (obtained through the isopiestic method at 298.15 K) of aqueous polypropylene glycol 400 (PPG400) + alanine or glycine systems, which form aqueous biphasic systems (salting-out effect), have a concave and convex slope, respectively, in the one-phase and two-phase regions. However, all the investigated polyethylene glycols (PEG400, PEG2000, PEG6000, and PEG10000) do not form aqueous biphasic systems with alanine or glycine (salting-in effect) and their constant water activity lines have a convex slope. In the second part of this work, the apparent molar volume and isentropic compressibility of transfer of alanine and glycine from water to aqueous solutions of PEG200, PEG2000, PEG10000, and PPG400 were studied at different temperatures. The third part of this work is concerned with the determination of LLE phase diagrams for several ternary polymer-amino acid aqueous systems containing polymers PPG400 and PPG725 and amino acids alanine, glycine, serine, and proline at different temperatures. On the basis of the obtained cloud point values of aqueous solutions of PPG725 in the absence and presence of various amino acids, it was found that all the investigated amino acids have a salting-out effect on PPG725 in aqueous solutions and entropy is the driving force for biphasic formation.

Volumetric and compressibility behaviour of poly(propylene glycol) – Amino acid aqueous solutions at different temperatures

“…According to the observed trend, it can be said that the effectiveness of the investigated amino acids in decreasing the φ V of PPG725 in aqueous solutions increases with the hydrophobic nature of the amino acid. In fact the ability of the amino acids to reduce φ V of PPG725 follows a reverse order of the strength of these amino acids in salting-out of PPG, which previously observed as serine ≈ glycine > alanine > proline according to the binodal curves [4]. Figure 1B shows that the values of φ V of glycine and proline in aqueous polymer solution, respectively, are larger and smaller than those in pure water.…”

“…On the other hand, in our previous work [4] we showed that poly(propylene glycol) (PPG) and amino acids can induce phase separation in aqueous media over part of the composition space. In other word, they form aqueous biphasic systems (ABS) consist of a PPG-rich and an amino acid-rich phase with water as solvent in both phases.…”

“…(c) Some of the ternary PPG-amino acid aqueous solutions can form ABS over specific threshold concentrations [4]. In aqueous solutions of solute 1-solute 2, which can form ABS, the solute1-water interactions become more stable in the presence of solute 2.…”

“…Volumetric and acoustical studies of polymer-amino acid-water ternary systems can provide some unique information to extend our knowledge about the different interactions between their constituents. Despite this fact, as far as we know, only four articles [4,[8][9][10] have been published regarding to this subject. Among these articles, except one of them [4] which survey the volumetric and compressibility properties of alanine and glycine in aqueous solutions of poly(ethylene glycol)s (PEGs) and PPG400, other works [8][9][10] are devoted to the volumetric properties of PEG-amino acid-water systems.…”

“…Despite this fact, as far as we know, only four articles [4,[8][9][10] have been published regarding to this subject. Among these articles, except one of them [4] which survey the volumetric and compressibility properties of alanine and glycine in aqueous solutions of poly(ethylene glycol)s (PEGs) and PPG400, other works [8][9][10] are devoted to the volumetric properties of PEG-amino acid-water systems. Sasahara et al [8,9] determined the volume and compressibility changes on mixing of aqueous solutions of various amino acids and PEG4000, and Ayranci et al [10] studied the volumetric and acoustic properties of glycine in aqueous PEG400 solutions.…”