Densities of aqueous solutions containing model compounds of amino acids and ionic salts at T = 298.15 K

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Abstract

Densities of amino acids in aqueous and in aqueous electrolyte solutions have been measured by a high precision vibrating tube digital densitometer at T = 298.15 K under atmospheric pressure. The investigated systems contained amino acids of zwitterionic glycine peptides: glycine (Gly), diglycine (Gly2), triglycine (Gly3), and tetraglycine (Gly4) and cyclic glycylglycine (c(GG)) with electrolytes of potassium chloride (KCl), potassium bromide (KBr) and potassium acetate (KAc). In this series of measurements, the aqueous samples were prepared with various concentrations of the amino acids, up to saturated conditions, and over salt concentrations from 1 to 4 M. The density increments resulting from the addition of the different model compounds of amino acids and the ionic salts were investigated, respectively. An empirical linear combination equation with an augmented term to account the interactions between amino acid and ionic salt was used to quantitatively correlate the experimental densities over the entire concentration ranges.

Introduction

The nature and the arrangement of the amino acid side chain along the protein backbone are responsible for the individual characteristics of the macromolecules, and it has been recognized that all of the information pertaining to the proteins is implicit in the amino acid sequence. From the chemical point of view, proteins are linear, heterogeneous polymers genetically mandated from 20 different building blocks or amino acid residues linked by covalent peptide bonds (–CO–NH–) into the polypeptide chain. In physiological conditions, the two terminals of amino acids are charged both positive charge (amino group, NH3+) and negative charge (carboxyl group, COO), and therefore the molecule has the properties of a “zwitterion”. The peptide bond is very stable and has unusual conformational properties. The peptide bond is rigid and planar that three bonds separate sequential α carbons in a polypeptide chain. The peptide C–N bonds are unable to rotate freely because of their partial double bond nature. Rotation is permitted about the N–Cα and C–Cα bonds. However, other single bonds in the backbone may also be hindered rotationally, depending on the size and charge of the R groups. On the other hand, the influence of aqueous solutions on peptide backbone unit targets these structures, it is important to have a clear idea on the solubility and thermodynamic properties of these molecules. Their structural and chemical properties have been extensively investigated by both theoretical and experimental techniques thereby providing valuable insights into the chemistries of peptides [1], [2]. There have been notable numerical simulation studies to delineate the amino acid side chains based on all atom empirical force field calculations, which were applied to zwitterionic peptide and also performing free energy simulations based on molecular dynamics of atomic models [3], [4], [5].

Amino acids have been used extensively as model compounds for specific aspects of the more complex proteins in aqueous solutions as these small solutes incorporate some of the structural features found in globular proteins. The interactions between solvent and the various constituent groups of a protein, such as the amino acid side-chains and the peptide backbone group, play a central role in the structure, the conformation and the function of proteins in aqueous solution [6], [7], [8], [9], [10]. Much thermodynamic evidence was conventionally interpreted by Hedwig and co-workers [11], [12], [13] as indicating the group additivity properties of the partial molar heat capacities and volumes of aqueous solutions of some peptides, which are model side-chain of proteins.

The important tools to investigate the interactions between ionic salts and amino acids are volumetric properties [14], [15], [16], [17], [18], [19] and thermodynamic properties [20], [21], [22], such as free energy, entropy and enthalpy of amino acids in aqueous electrolyte solutions. These results lead to the conclusion that some of electrolytes can stabilize biological important molecules such as proteins. Nevertheless, numerous issues remain to be resolved on amino acids interactions with ionic salts. Salts solutions have significant effects on the structure and the properties of proteins including their solubility, stability, denaturation, and dissociation into subunits. Very little attention has been given recently to the effect of electrolytes on the stability of proteins and polypeptides in aqueous solutions. However, for completely understanding of the influence of electrolytes on amino acids and for revealing the molecular interactions between salts and protein functional groups, more detailed investigations on thermodynamic behaviour are still needed.

In order to improve our understanding of the ionic salt effect on PVT behaviour in the electrolyte aqueous amino acid solutions, the present study focused on the interactions between three selected salts (KCl, KBr, and KAc) and five amino acids of glycine (Gly), diglycine (Gly2), triglycine (Gly3), tetraglycine (Gly4) and cyclic glycylglycine (c(GG)), also known as glycine anhydride or diketopiperazine (DKP), via density measurements, which were measured by a high precision vibrating tube digital densitometer.

Section snippets

Materials

The salts, potassium chloride (KCl, mass fraction purity 0.9999+), and potassium acetate (KAc, mass fraction purity 0.9998+) were purchased from Aldrich Chemical Co. Potassium bromide (KBr, mass fraction purity 0.995+) was obtained from Acros Organics (USA). Gly (mass fraction purity >0.99) was obtained from Acros Organics (USA). Gly2 (mass fraction purity 0.995+), Gly3 (mass fraction purity >0.99), Gly4 (mass fraction purity >0.99) and c(GG) (mass fraction purity >0.99) were purchased from

Results and discussion

Table 1 provides the molar mass (M) of the salts and the densities (ρ) of the aqueous electrolyte solutions at T = 298.15 K. These densities of the aqueous electrolyte solutions are also graphically represented in figure 1 as a function of salt concentration in water. To check the reliability of experimental method, we measured the solubility limits of amino acids in water at T = 298.15 K. The results are compared with the literature values in table 2. From this table, one can clearly see that the

Conclusions

Density data have been measured for various binary and ternary aqueous systems containing amino acid model compounds (glycine, diglycine, triglycine, tetraglycine, and cyclic glycylglycine) and ionic salts (potassium chloride, potassium bromide and potassium acetate) at T = 298.15 K under atmospheric pressure. The density of the binary aqueous systems varies approximately linearly with the concentration of amino acids or salts over the entire experimental conditions. The magnitudes of density

Acknowledgement

Financial support from the National Science Council, Taiwan, through Grant No. NSC 94-2811-E-011-005 is gratefully acknowledged.

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