Ion effects on Protein Model Compounds in Aqueous Systems: Experimental and Computational Studies

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Salt effects on the solubility of aromatic and dicarboxylic amino acids in water

Publication . Aliyeva, Mehriban; Brandão, Paula; Gomes, José R.B.; Coutinho, João A.P.; Held, Christoph; Ferreira, Olga; Pinho, Simão

The salt effect on the solubility of the amino acids L-aspartic acid, L-glutamic acid, L-tryptophan, and L-tyrosine, seldomly found in the literature, was studied at 298.2 K, in aqueous solutions of KCl and (NH4)2SO4, for salt concentrations up to 2.0 mol⋅kg -1. In this concentration range, both salts are salting-in agents for glutamic acid and aspartic acid, with a stronger effect induced by (NH4)2SO4. Regarding the two aromatic amino acids, a slight increase in the solubility was obtained at low salt concentrations, followed by a stronger salting-out effect, more pronounced by (NH4)2SO4 than by KCl. The relative solubility data obtained in this work were compared to literature data for other amino acids in the same electrolyte solutions to establish a relative solubility ranking connected to their structure. Finally, the solubility data were modeled using the electrolyte Perturbed-Chain Statistical Association Theory (ePC-SAFT). The modeling requires parameters for the amino acids and ions as well as melting properties of the amino acids. All these parameters and properties were obtained from previous works. To quantitatively describe the solubility of amino acids upon salt addition, binary interaction parameters (kij) between any amino acid and anions were determined, while between any amino acid and the cations were fixed to kij = 0.08. The kij parameters between amino acid and the inorganic anions show very similar values for amino acids of the same chemical class (e.g. kij between anion and amino acid with apolar side chains), which may be used to systematically reduce the number of adjustable parameters in future work.

2023Journal article

Open access

Electrolyte effects on the Amino acid solubility in water: solubilities of Glycine, l-Leucine, l-Phenylalanine, and l-Aspartic acid in salt solutions of (Na+, K+, NH4+)/(Cl–, NO3–)

Publication . Aliyeva, Mehriban; Brandão, Paula; Gomes, José R.B.; Coutinho, João A.P.; Ferreira, Olga; Pinho, Simão

The solubilities of glycine, L-leucine, L-phenylalanine, and L-aspartic acid in aqueous solutions of the salts composed by combining Na+, K+, and NH4 + cations and Cl− and NO3 − anions were measured up to 2.0 salt molality at 298.2 K by the analytical gravimetric method. Using these data along with a review of literature information, encompassing all amino acids for which solubility is available in the studied aqueous electrolyte solutions, allowed us to interpret the effect of the functional groups of amino acids on their solubility. The four amino acids studied here showed higher solubility in aqueous solutions of salts with the nitrate anion. Except for L-aspartic acid with a polar side chain, amino acids with apolar side chains presented the highest salting-in effect in aqueous salt solutions with NH4 +. The cations Na+ and K+ did not seem to establish relevant interactions with the amino acids and had little impact on their aqueous solubility.

2022Journal article

Open access

Solubilities of amino acids in aqueous solutions of chloride or nitrate salts of divalent (Mg2+ or Ca2+) cations

Publication . Aliyeva, Mehriban; Brandão, Paula; Gomes, José R.B.; Coutinho, João A.P.; Ferreira, Olga; Pinho, Simão

The solubilities of glycine, L-leucine, L-phenylalanine, and L-aspartic acid were measured in aqueous MgCl2, Mg(NO3)2, CaCl2,, and Ca(NO3)2 solutions with concentrations ranging from 0 to 2 mol/kg at 298.2 K. The isothermal analytical method was used combined with the refractive index measurements for composition analysis guaranteeing good accuracy. All salts induced a salting-in effect with a higher magnitude for those containing the Ca2+ cation. The nitrate anions also showed stronger binding with the amino acids, thus increasing their relative solubility more than the chloride anions. In particular, calcium nitrate induces an increase in the amino acid solubility from 2.4 (glycine) to 4.6 fold (L-aspartic acid) compared to the corresponding value in water. Amino acid solubility data in aqueous MgCl2 and CaCl2 solutions collected from the open literature were combined with that from this work, allowing us to analyze the relations between the amino acid structure and the salting-in magnitude.

2022Journal article

Restricted access