BIOPHYSICAL THEORY AND MODELING |
Molecular Basis of the Apparent Near Ideality of Urea Solutions
Hironori Kokubo 1, Joerg Roesgen 2, David Wayne Bolen 3 and Montgomery Pettitt 1*
1 University of Houston
2 UT Medical Branch, Galveston
3 Univ. of Texas Medical Branch
* To whom correspondence should be addressed. E-mail: pettitt{at}uh.edu.
Submitted on June 10, 2007
Revised on July 3, 2007
Accepted on 16 July 2007
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Abstract |
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Activity coefficients of urea solutions are calculated to explore the mechanism of its solution properties which form the basis for its well known use as a strong protein denaturant. We perform free energy simulations of urea solutions in different urea concentrations using two urea models (OPLS and KBFF models) to calculate and decompose the activity coefficients. For the case of urea, we clarify the concept of the ideal solution in different concentration scales and standard states and its effect on our subsequent analysis. The analytical form of activity coefficients depends on the concentration units and standard states. For both models studied urea displays a weak concentration dependence for excess chemical potential. However, for the OPLS force field model this results from contributions which are independent of concentration to the van der Waals and electrostatic components whereas for the KBFF model those components are nontrivial but oppose each other. The strong ideality of urea solutions in some concentration scales, implying a lack of water perturbation, is discussed in terms of recent data and ideas on the mechanism of urea denaturation of proteins.
Key Words:
Free energy, Simulation, Solution theory, activity coefficients
