Enthalpic interaction coefficients of N,N-dimethylformamide in aqueous ethylene glycol and glycerol solutions at 298.15 K

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Abstract

The dilution enthalpies of N,N-dimethylformamide (DMF) in aqueous ethylene glycol and glycerol solutions have been determined using a CSC-4400 isothermal calorimeter at 298.15 K. The experimental data have been analyzed in terms of McMillan–Mayer formalism to obtain the enthalpic virial coefficients for heterotactic interaction. The results have been interpreted from the point of view of solute–solute interactions and solute–solvent interactions.

Introduction

Proteins play a vital role in nearly all chemical and biological processes. However, it is quite difficult to study the interactions between proteins directly because the conformational and configurational factors that affect the structures of proteins in different solvents are very complicated. Therefore, one useful approach is to study their model compounds, such as amino acids, small peptides, and acylamides [1], [2], [3], [4]. The importance of acylamides for structural chemistry and biochemistry is obvious: this is the molecule containing the CONH unit that is characteristic of the peptide bond.

The majority of the proteins exist in aqueous mixed solutions containing many organic substances. Many investigations have shown that polyalcohol can increase the thermal stability of globular proteins or reduce the extent of denaturation by other reagents [5], [6]. Although the diols under investigation are not found in cellular or extracellular fluids of living organisms, they find wide application in pharmacy and cosmetics industry. On the other hand, glycerol occurs as a primary biomolecule in the intestine as a product of hydrolysis of lipids and also in the liver where it participates in the metabolism of glucose.

In spite of large number of studies the mechanism through which polyalcohol affect the stability and solubility of protein remains unresolved. As a continuation of our work [7], [8], [9], [10], the present study reports the dilution enthalpies of N,N-dimethylformamide in aqueous ethylene glycol and glycerol solutions of different compositions at 298.15 K.

Section snippets

Materials and sample preparation

Analytical grade DMF (purity >99.5%, purchased from shanghai Chem. Co.) was dried by storage over molecular sieves of 0.4 nm for 2 days and then was used without further purification. Ethylene glycol (analytical grade, purity >99%, purchased from shanghai Chem. Co.) and glycerol (analytical grade, purity >99%, purchased from shanghai Chem. Co.) were distilled, under atmospheric pressure with middle fraction was collected. All of the products were stored over P2O5 in a vacuum desiccator for 72 h

Results and discussion

Information about the interaction mechanism can be inferred from the pairwise interaction coefficients of the virial expansion of an excess thermodynamic property. According to the McMillan–Mayer theory [15], [16], thermodynamic properties of multicomponent solutions can be expressed using a viral expansion of in molality that relates the nonideal contributions of any total thermodynamic function to a series of interaction parameters. If aqueous polyalcohol solution is regarded as solvent, the

Conclusion

Enthalpies of dilution of DMF in aqueous ethylene glycol and glycerol solutions have been determined using an isothermal calorimeter (4400 IMC) at 298.15 K. Experiential enthalpies of dilution varying with the concentration of polyalcohol were correlated with the virial expansion equation. Enthalpic interaction coefficients h2, h3, and h4 in the equations are obtained, and the values of the pairwise enthalpic interaction coefficient h2 have been discussed. In H2O-rich region in aqueous ethylene

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (No. 20273061) and Zhejiang Provincial Education Department (No. ZC200805553) for financial support.

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