Solubility and Solvation Thermodynamics of a Series of Homologous α-Amino Acids in Nonaqueous Binary Mixtures of Ethylene Glycol and Dimethyl Sulfoxide

Abstract

In this article the standard free energies (ΔG t 0 (i)) and entropies (ΔS t 0 (i)) of transfer of four homologous α-amino acids including glycine (Gly), DL-alanine (DL-Ala), DLα-amino butyric acid (DL-Aba.) and DL-nor-valine (DL-n-Val) from ethylene glycol (EG) to nonaqueous mixtures of ethylene glycol and dimethyl sulfoxide (DMSO) at 298.15 K are reported. The Gibbs energies of solutions have been determined from solubility measurements of each amino acid at different temperatures, that is, from 288.15 to 308.15 K by "formol titrimetry". The chemical parts of free energies (ΔG t,ch 0 (i)) and entropies (TΔS t,ch 0 (i)) of transfer of the homologous α-amino acids have been computed by subtracting the cavity effects and dipole−dipole interaction effects from the total transfer energies. The characteristics of the solvation thermodynamics of α-amino acids in EG-DMF and EG-ACN mixed solvent systems studied earlier are also discussed here for comparison.

Solubility and solvation thermodynamics of dl-nor-valine in aqueous solutions of NaCl and KCl

“…A low-cum-high temperature thermostat was used for all measurements which is capable of registering temperatures having an accuracy of ±0.1 K. A known mass of filtered saturated solution was transferred into a dry conical flask. The saturated solubility of DL-nor-valine was measured by the wellestablished formol titrimetry method [41][42][43]. Four sets of measurements for all the co-solvent mixtures were made at 288.…”

“…It is of vital importance because such studies give valuable information on protein folding and unfolding process [17,18] and the nature of hydrophobic interaction of amino acids. Thus to explore the solvation mechanisms of various amino acids, several studies were carried out by different groups of researchers in different aquo-organic and non-aqueous solvent systems [25,33,41]. However, it is important to note that though various efforts were made on such studies, only a preliminary study on the measurement of solubility of DL-valine in aqueous electrolyte solutions was done by Khoshkbarchi et al [34] at 298.15 K, which lacks a detailed report on the solvation mechanisms of it in terms of the thermodynamic point of view.…”

Chemical Transfer Energetics of a Series of Homologous α-Amino Acids in Quasi-Aprotic 2-Methoxyethanol–Water Mixtures

“…There are several methods [38][39][40][41][42][43] for the determination of the activity coefficients that involve various theoretical aspects and assumptions. But for simplicity, in many earlier studies the activity coefficient was assumed to be unity [11,12,15,17,[19][20][21][22][23][24]44] during the determination of the Gibbs energy of several amino acids in solution.…”

“…Moreover, our main concern in the current study is to determine and characterize the total transfer Gibbs energy (DG 0 t ðiÞ) in such binary solvent mixtures; therefore, we also have assumed here that the contribution of activity coefficient is negligible [11,12,15,17,[19][20][21][22][23][24]44]. DG 0 t ðiÞ is also related as DG 0 t ðiÞ ¼ DG 0 s ðiÞ À DG 0 R ðiÞ, where DG 0 t ðiÞ, DG 0 s ðiÞ and DG 0 R ðiÞ are the Gibbs energies of the amino acid solutions in co-solvent (aqueous ME) and reference solvent, water, respectively.…”

“…A known mass (of about 0.2-0.3 g) of filtered saturated solution was transferred to a dry conical flask from solution thermostated at all experimental temperatures. The saturated solubility analyses of Gly, Ala, Aba and Val were made by the 'formol titrimetric method' [4,11,12,[19][20][21][22][23] using freshly standardized (0.05 molÁL -1 ) NaOH (GR, E Merck; [99.0 %) solution and 1 % alcoholic phenolphthalein solution as indicator. The solubilities were measured at the natural pHs of the solutions.…”