Molecular dynamics simulation of aqueous urea solution: is urea a structure breaker?

Area of Science:

• Biochemistry
• Physical Chemistry
• Chemical Physics

Background:

• Urea is biologically significant as a protein denaturant at high concentrations.
• The effect of urea on water structure has been debated, with theories suggesting it acts as a structure-breaker, maker, or is neutral.
• Previous analyses employed various methods, leading to conflicting conclusions regarding urea's impact on water's structural integrity.

Purpose of the Study:

• To conclusively determine urea's effect on the local structure of water.
• To investigate whether urea disrupts the tetrahedral hydrogen-bonded network of water, particularly at high concentrations.
• To clarify the long-standing debate surrounding urea's influence on water's molecular organization.

Main Methods:

• Employed molecular dynamics simulations to model urea-water interactions.
• Utilized a nearest neighbor approach to analyze the local structure of water molecules.
• Defined reference water molecules and considered urea as a potential neighbor to assess structural changes.

Main Results:

• Conclusively demonstrated that urea does not break the local tetrahedral structure of water, even at elevated concentrations.
• Observed a minor shift in tetrahedral order parameters correlated with changes in n-hydrogen-bonded water molecules.
• Indicated that urea molecules can effectively substitute for water molecules within the hydrogen-bonded network.

Conclusions:

• Urea integrates into the hydrogen-bonded network of water without disrupting its fundamental tetrahedral structure.
• The observed changes in water structure are attributed to the substitution of water by urea, not structural breakdown.
• This finding resolves the debate on urea's role, establishing it as a compatible component within water's structural framework.