Inorganic and organic ions, such as Na+, PO43-, and acetate, are essential to metabolism in all living organisms. Previous studies have shown that ions and zwitterions containing the carboxylate group, like betaine, do not sense any obvious difference in their solvation shell environment when dissolved in either pure water or sodium chloride solutions. The observation opens questions about the solvation of the carboxylate group and its relation to the biological mechanism of osmotic protection. In this project, we study the solvation dynamics of the acetate ion because it shares the structural similarity with the carboxylate group of betaine and many other amino acids. The solvation dynamics of acetate was studied by measuring the two-dimensional infrared spectra of solutions of the ion in pure D2O and a 6M NaCl solution in the infrared region of the carboxylate asymmetric stretch. Our experiments showed that the motions of the acetate solvation shell do not seem to be affected by the presence of sodium and chloride ions at high concentration; i.e., 6M; even when a similar experiment using azide ions reveals a strong slowdown in the solvation shell dynamics around the azide ion at high salt concentration. The motive for the curious behavior of the acetate ion is uncovered from ab initio molecular dynamics simulation. Simulations demonstrate that the solvation dynamics of acetate is isolated from the influence of Na+ and Cl- because the methyl group of acetate undergoes thermal rotations, which periodically disrupt the solvation shell environment. The results suggest that the ions and zwitterions containing the carboxylate group can undergo similar motions to avoid the formation of ion pairs and maintain their high water solvation number, even in solutions with high salt concentrations.

Acetate ion and its interesting solvation shell

255th National Meeting (2018)

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