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Urea interactions with protein groups: A volumetric study
Author(s) -
Lee Soyoung,
Shek Yuen Lai,
Chalikian Tigran V.
Publication year - 2010
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.21478
Subject(s) - chemistry , urea , amino acid , thermodynamics , solubility , solvent , adiabatic process , stereochemistry , crystallography , organic chemistry , biochemistry , physics
We determined the partial molar volumes and adiabatic compressibilities of N ‐acetyl amino acid amides, N ‐acetyl amino acid methylamides, N ‐acetyl amino acids, and short oligoglycines as a function of urea concentration. We analyze these data within the framework of a statistical thermodynamic formalism to determine the association constants for the reaction in which urea binds to the glycyl unit and each of the naturally occurring amino acid side chains replacing two waters of hydration. Our determined association constants, k , range from 0.04 to 0.39 M . We derive a general equation that links k with changes in free energy, Δ G tr , accompanying the transfer of functional groups from water to urea. In this equation, Δ G tr is the sum of a change in the free energy of cavity formation, ΔΔ G C , and the differential free energy of solute–solvent interactions, ΔΔ G I , in urea and water. The observed range of affinity coefficients, k , corresponds to the values of ΔΔ G I ranging from highly favorable to slightly unfavorable. Taken together, our data support a direct interaction model in which urea denatures a protein by concerted action via favorable interactions with a wide range of protein groups. Our derived equation linking k to Δ G tr suggests that ΔΔ G I and, hence, the net transfer free energy, Δ G tr , are both strongly influenced by the concentration of a solute used in the experiment. We emphasize the need to exercise caution when two solutes differing in solubility are compared to determine the Δ G tr contribution of a particular functional group. © 2010 Wiley Periodicals, Inc. Biopolymers 93: 866–879, 2010.