Enthalpic interactions of N,N-dimethylformamide in aqueous glucose and sucrose solutions at 298.15 K

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Abstract

Enthalpies of dilution of N,N-dimethylformamide (DMF) in aqueous sucrose and glucose solutions have been determined using an isothermal calorimeter (4400 IMC) at 298.15 K. The values of dilution enthalpy were used to determine homogeneous enthalpic interaction coefficients which characterize the interactions of examined DMF in aqueous sugar solutions. The results show that enthalpic pair interaction coefficients h2 of DMF are all positive in aqueous sugar solutions and pass through a maximum, respectively, at m  0.5 mol kg−1 of sucrose and at m  0.55 mol kg−1 of glucose in mixed solvents. In the meantime the h2 coefficients of DMF in aqueous sucrose solutions are more positive than that in aqueous glucose solutions. The variations of the enthalpic pairwise interaction coefficients with the mass fraction of sugar in mixtures are interpreted in terms of solute–solute and solute–solvent interactions.

Introduction

The folding, structural stability, and dynamics of globular proteins are thought to be extensively controlled by the interactions of the macromolecule with water. Various added substances affect these interactions and consequently alter the structural stability of proteins. It is well known that sugar and polyhydric alcohols can increase the thermal stability of proteins or reduce the extent of their denaturation by other reagents [1], [2], [3], [4]. Some trends correlating the stabilizing potency of sugars and polyols with the number or configuration of the hydroxy groups have also been noted [5]. However, there are numerous exceptions [6], [7] and not all proteins respond equally to a given compound. Thus our understanding of the stabilization mechanism of proteins is still incomplete.

Among various physical parameters, these thermodynamic parameters have been recognized as being sensitive to structural changes occurring in solutions. The thermodynamic properties associated with the stabilization process in the presence of a large amount of sugars are difficult to interpret, because of the large number of interactions that can occur and which contribute to the overall thermodynamic properties of the protein in each state. Studies on simple compounds that model some specific aspect of a protein can provide estimates of the contributions from particular functional groups on the protein. N,N-Dimethylformamide (DMF) can serve as a model compound of peptides to obtain information on protein systems. As a continuation of that work [8], [9], [10], [11], the present study is aimed at examining the homogenous enthalpic interaction coefficients of DMF in aqueous glucose and sucrose solutions of different compositions.

Section snippets

Materials and sample preparation

DMF (analytical grade, >99.5 mass%) were stored over activated 4Å° molecular sieves in order to keep them dry. Glucose and sucrose (analytical reagents, >99 mass%) were dried over P2O5 in a vacuum desiccator for 72 h at room temperature before use. Both of them were used without further purification. The water used for the preparation of solutions was deionized and distilled using a quartz sub-boiling purifier.

Both the aqueous solutions, which were used as mixed solvents (water + sugar), and the DMF

Results and discussion

An excess thermodynamic property can be expressed as a virial expansion of pairwise, triplet and quadruplet, etc. interaction coefficients, which account for all the variations of the solute–solute and solute–solvent interactions according to the McMillan–Mayer theory [16], [17]. If aqueous sugar solution is regarded as solvent, the excess enthalpy per kg of solvent (HE) of a solution containing DMF at molality m is given byHE=h2m2+h3m3+h4m4+where h2, h3, h4, etc. are enthalpic pairwise,

Conclusion

Enthalpies of dilution of DMF in aqueous sucrose and glucose solutions have been determined using an isothermal calorimeter (4400 IMC) at 298.15 K. Experiential enthalpies of dilution varying with the concentration of sugar 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. Hydrophobic–hydrophobic interaction is stronger in

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (No. 20273061) and to the National Natural Science Foundation of Huzhou (No. 2008YZ04) for financial support.

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