A thoretical study of the unique hydrogen bond ability of osmolyte

Abstract

Trimethylamine oxide and Urea are known as osmolytes. Marine organisms hold these in their intracellular fluid to control osmotic pressure. In this study, we focus on the hydration of TMAO and Urea. We optimized the geometries of TMAO, Urea, TMAO-nH₂O and urea-mH₂O complexes with the theoretical level of MP2/6-31G*. We found TMAO (or Urea) had much larger dipole moment than that of water molecule. From electrostatic potential (ESP) maps, positive and negative charges are separated in TMAO and Urea. We also found there are three spots in TMAO, and two spots in Urea, to which water molecules are bound very strongly. These facts indicate TMAO and urea have the capability to capture water molecules electrostatically and spatially, and there are strong interactions between TMAO (or Urea) and water molecules. We found the hydration stabilization energy of TMAO (or Urea) was much larger than the interaction energy between water molecules. It indicates the interaction between TMAO (or Urea) and water molecule creates very large stabilization. From the above results, we propose why TMAO and Urea play an important role as osmolytes is due to their capability to capture water molecules strongly.