Ion solvation scenario in an aqueous solution mixture of counteracting osmolytes: Urea and trimethylamine-N-oxide (TMAO)

Abstract

In this study, we investigate that how the ion solvation scenario of different charged species (Na+, K+, Cl−, Cl0, NH4+, NMe4+ and CH3COO−) in highly concentrated urea solution (8 M) gets modified on addition of counteractive osmolyte, TMAO (upto 5.6 M). It is observed that, unlike Na+, K+ prefers to interact with oxygen of TMAO with increasing TMAO concentration. Ammonium-ion exhibits stronger hydrogen bonding interaction with water as well as with TMAO. In case of tetramethylammonium-ion, addition of TMAO seems to hinder preferable Me-Ourea interaction by Me-OTMAO interaction. Acetate-ion shows stronger hydrogen bonding interaction with water as compared to urea which increases with addition of TMAO. Whereas methyl-group of acetate ion prefers to interact with methyl-carbon of TMAO though there exists a weak hydrogen bonding between oxygen of acetate ion and methyl-hydrogen of TMAO. Neutral solute (Cl0) prefers to interact with methyl-carbon of TMAO with increasing of TMAO concentration in the solution. Ion (ammonium/acetate)-urea hydrogen bond lifetimes have negligible effect when TMAO is added to the solution. TMAO mainly strengthens the ion-water and water-water hydrogen bond lifetimes, and also forms stronger hydrogen bonds with water molecules. We have also verified the effects of change of L-J combination rule along with another TMAO model (Shea model) in case of a NH4+and CH3COO−, as these ions resemble the zwitter-ionic form of amino acids. It was found that at intermediate concentration of TMAO, Shea model does not show appreciable difference with Kast (AM), whereas at higher concentration, Shea model shows higher preference to interact with ammonium ion.