Interactions Of Ionic Liquids With Amino Acids Anddipeptides In Aqueous Solution:Volumetric, Conductometric And Spectroscopy Study

Ionic liquids(ILs) are the focus of many scientific researchers due to their extraordinary physicochemical properties and have been widely applied in various areas of industrial production and everyday life. Amino acids and small peptides which are the fundamental part of proteins are the biological activity function basis of proteins. The studies on the interactions between amino acids or small peptides and surface active ILs in aqueous solution is of great significance in understanding the role of amino acids and small peptides in the process of micellization of surface active ILs and in the separation and purification process of biomolecules. In this paper, the the interactions between surface active ILs with different cationic head group(1-methyl-1-octyl pyrrolidinium bromide [C8pyrr]Br, 1-methyl-1- octyl piperidinium bromide C8 PDB, 1-octyl pyridinium bromide [OPy]Br) and amino acids/glycyl dipeptides with different alkyl chain length(glycine, DL-alanine, L-valine, L-leucine, glycylglycine, glycyl-L-valine, glycyl-L-leucine) have been studied systematically by means of density, conductivity and spectroscopy. The major contents and conclusions of the paper are as follows:(1) The densities of ternary systems of ILs + amino acids/dipeptides + water at different temperatures have been measured by using the volumetric method and volume parameters of amino acids and dipeptides in aqueous different concentrations of ILs solutions have been calculated. The results showed at a certain temperature the values of standard partial molar volumes of amino acids and dipeptides in ILs aqueous solution are larger than the corresponding values in water, standard partial molar volumes of transfer are positive and both of them gradually increase as the temperature and concentration of ILs increase. This is due to the predominate interactions of ion-ion and ion-polar group compared with ion-nonpolar and nonpolar-nonpolar group between amino acids/dipeptide and ILs. The hydration numbers decrease as the temperature and concentration of ILs increase, which suggests that the increase of temperature and concentration of ILs can lead to dehydration of amino acids and dipeptides.(2) The partial molar expansibility Eqj and Hepler’s constant of amino acids and dipeptides in ILs aqueous solution gradually increase with the increasing temperature. This may be attributed to the fact that the interactions between amino acids/dipeptides and ILs cause the larger volumetric change of the ternary systems of ILs + amino acids/dipeptides + water than that of binary systems of amino acids/dipeptides + water as temperature and concentration of ILs increasing. At the same time, amino acids and dipeptides behave differently in affecting the structure of ILs aqueous solution compared with that of water.(3) The conductivities of the ternary systems at different temperatures were measured by conductivity technique and the limited molar conductivities, critical micelle concentration(CMC) and Walden Product in aqueous different concentrations of ILs solutions were calculated. The results showed that the limited molar conductivities and CMC of ILs in aqueous amino acid and dipeptide solutions can be influenced by the species of ILs, temperature, as well as the structure and concentration of amino acids and dipeptide. They both decrease with the addition of amino acid and dipeptide and the increasing concentration of them as well as the increase in the alkyl chain length of amino acids and dipeptide. In a certain solution of amino acid and dipeptide, the CMC of ILs decreased to a certain minimum and then increased with the temperature increasing. Walden Product of ILs in aqueous amino acid and dipeptide solutions which are not constant are larger than the corresponding values in water and this is ascribed to the preference solvation of ILs by amino acid and peptide molecules in mixed solvent. The order of the interactions between amino acid/dipeptide and the three ILs is C8PDB>[C8pyrr]Br>[OPy]Br.(4) Thermodynamic parameters of micellization of ILs in aqueous amino acid and dipeptide solutions have been obtained by applying the mass action model. The data showed that free energy of micellization in all cases is negative which indicates that the micellization process is a thermodynamically favorable process. The micellization process is an entropy driven process at the low temperature, but it is an enthalpy driven process at the high temperature. Moreover, the enthalpy-entropy compensation effect is observed.(5) Based on the pyrene fluorescence spectra, the intensity ratio I1/I3 of the first and the third vibronic peaks of the pyrene fluorescence spectrum and aggregation number of ILs in the ternary systems were obtained. The results showed that the micropolarity of the microenvironment and aggregation number of ILs decrease with addition of amino acids and dipeptides and change with the alkyl chain length of amino acids and dipeptides.(6) The binding constants of amino acids and dipeptides with ILs have been determined by UV-vis absorption spectra. The results showed that the strength of the interaction between amino acid/dipeptide and ILs gradually increased with the increase in the length of alkyl chain of amino acids and peptides.