Sergey E. Kruchinin

Hydration and Ion Binding of the Osmolyte Ectoine

Ectoine is a widespread osmolyte enabling halophilic bacteria to withstand high osmotic stress that has many potential applications ranging from cosmetics to its use as a therapeutic agent. In this contribution, combining experiment and theory, the hydration and ion-binding of this zwitterionic compound was studied to gain information on the functioning of ectoine in particular and of osmolytes in general. Dielectric relaxation spectroscopy was used to determine the effective hydration number of ectoine and its effective dipole moment in aqueous solutions with and without added NaCl. The obtained experimental data were compared with structural results from 1D-RISM and 3D-RISM calculations. It was found that ectoine is strongly hydrated, even in the presence of high salt concentrations. Upon addition of NaCl, ions are bound to ectoine but the formed complexes are not very stable. Interestingly, this osmolyte strongly rises the static relative permittivity of its solutions, shielding thus effectively long-range Coulomb interactions among ions in ectoine-containing solutions. We believe that via this effect, which should be common to all zwitterionic osmolytes, ectoine protects against excessive ions within the cell in addition to its strong osmotic activity protecting against ions outside.

Ion-binding of glycine zwitterion with inorganic ions in biologically relevant aqueous electrolyte solutions.

The ion-binding between inorganic ions and charged functional groups of glycine zwitter-ion in NaCl(aq), KCl(aq), MgCl2(aq), and CaCl2(aq) has been investigated over a wide salt concentration range by using integral equation theory in the 3D-RISM approach. These systems mimic biological systems where binding of ions to charged residues at protein surfaces is relevant. It has been found that the stability of ion pairs formed by the carboxylate group and added inorganic cations decreases in the sequence Mg(2+)>Ca(2+)>Na(+)>K(+). However, all formed ion pairs are weak and decrease in stability with increasing salt concentration. On the other hand, at a given salt concentration the stability of (-NH3(+):Cl(-))aq ion pairs is similar in all studied systems. The features of ion-binding and the salt concentration effect on this process are discussed.

Mobility and association of ions in aqueous solutions: the case of imidazolium based ionic liquids

The mobility and the mechanism of ion pairing of 1,1 electrolytes in aqueous solutions were investigated systematically on nine imidazolium based ionic liquids (ILs) from 1-methylimidazolium chloride, [MIM][Cl], to 1-dodecyl-3-methylimidazolium chloride, [1,3-DoMIM][Cl], with two isomers 1,2-dimethylimidazolium chloride, [1,2-MMIM][Cl], and 1,3-dimethylimidazolium chloride, [1,3-MMIM][Cl]. Molecular dynamics (MD) simulations, statistical mechanics calculations in the framework of the integral equation theory using one-dimensional (1D-) and three-dimensional (3D-) reference interaction site model (RISM) approaches as well as conductivity measurements were applied. From experiment and MD simulations it was found that the mobility/diffusion coefficients of cations in the limit of infinite dilution decrease with an increasing length of the cation alkyl chain, but not linearly. The aggregation tendency of cations with long alkyl chains at higher IL concentrations impedes their diffusivity. Binding free energies of imidazolium cations with the chloride anion estimated by RISM calculations, MD simulations and experiments reveal that the association of investigated ILs as model 1,1 electrolytes in water solutions is weak but evidently dependent on the molecular structure (alkyl chain length), which also strongly affects the mobility of cations.

Hydration structure of osmolyte TMAO: concentration/pressure-induced response

Despite the well-known fact that the natural osmolyte trimethylamine-N-oxide (TMAO) is able to prevent protein denaturation and to stabilize the folded state of proteins in living cells under abiotic stress, much of its molecular mechanism of action remains elusive. At the moment, there is some evidence that osmolytes, including TMAO, do not interact with proteins directly but only through a water layer. It is supposed that their protective mechanism should be mediated and determined largely by their hydration, i.e. by osmolyte interactions with surrounding water. However, to date the details of these interactions are far from being fully understood. To gain further insight into the mechanism behind the protecting effect of osmolytes statistical mechanics calculations in the framework of 1D- and 3D-RISM (reference interaction site model) approaches were performed to yield information on the impact of solute concentration and pressure on the hydration structure of TMAO. An attempt was made to link the structural features of TMAO hydration to its biological role.