Universal Charge Quenching and Stability of Proteins in 1-Methyl-3-alkyl (Hexyl/Octyl) Imidazolium Chloride Ionic Liquid Solutions

作者：Rawat Kamla; Bohidar H B^*

来源：Journal of Physical Chemistry B, 2012, 116(36): 11065-11074.

DOI：10.1021/jp3049108

This study reports pH dependent stability of protein dispersions of five common proteins, bovine serum albumin (BSA), human serum albumin (HSA), immunoglobulin (IgG), beta-lactoglobulin (beta-Lg), and gelatin-B (Gel-B), all having isoelectric pH, pI approximate to S, in room temperature ionic liquid solutions of 1-methyl-3-alkyl (hexyl/octyl) imidazolium chloride (concentration 0-0.2% w/v). Molecular hydrophobicity index, (H-index = hydrophobicity/hydrophilicity) of these molecules spanned the range 0.43-0.87. Electrophoretic characteristics, surface tension data and hydrodynamic size information revealed that IL solutions provide dispersion stability owing to specific protein-IL binding which did not alter their pI values though their surface charge was considerably screened. Change in maximum (zeta(max)) and minimum (zeta(min)) zeta potential values observed at pH similar to 3 (maximum protonated state) and pH similar to 8 (maximum deprotonated state) could be described universally as function of IL concentration, c as Delta zeta(x) = [1 - exp(-ac)] where Delta zeta(x) is either vertical bar(zeta(max) - zeta(w))vertical bar/zeta(w) or vertical bar(zeta(min) - zeta(w)vertical bar zeta(w), and zeta(w), is the corresponding value in water. Tensiometry data showed two major stages of protein-IL interactions: (i) for c < cmc of IL, the IL molecules selectively bind with imidazolium cation through electrostatic forces forming protein-IL (complex) and (ii) for c> cmc free IL-aggregates begin to form. Similarly, we can define Delta gamma(x) as either vertical bar(gamma(max) -gamma(w))vertical bar/gamma(w) at pH 3 or vertical bar(gamma(min) gamma(w))vertical bar/gamma(w) at pH 8. Both Delta zeta(x) and Delta gamma(x) showed linear dependence with c(1) Delta gamma(min, max) (or Delta zeta(min, max)) = (1 - K-gamma (or K-zeta) H-index), where the slopes K-zeta and K-gamma defined intermolecular interactions. Hydrodynamic radii data revealed protein stabilization, circular dichroism spectra implied retention of secondary structures, and Raman spectra confirmed a marginal increase in water structure. Results concluded that selective binding of IL molecules to protein surface in the form of bilayer screen protein surface charge, thereby, contributing to its dispersion stability.

• 出版日期2012-9-13