Water-urea binary mixture

There are some small organic molecules with very simple chemical formulas that have proved to be important for biological applications. Urea or carbamide is such an organic compound, with the chemical formula (NH2)2CO. The molecule has two amine (-NH2) groups joined by a carbonyl (C=0) functional group. 

Urea is an important cosolvent generally used in the study of denaturation processes in several proteins. Inspired by the need to understand the microscopic basis of the mechanism of protein denaturation, researchers have performed MD simulations to assess the effects of urea on the hydrophobic interaction between two methane molecules. 

There are some counterintuitive but interesting results obtained on the rate of catalysis in aqueous urea solution. The rate increases at a very low concentration of urea. This has been understood as follows addition of urea can lead to denaturation of the protein. However, to get a complete denaturation one needs to have a sufficient concentration of urea. At low concentration, protein becomes partially denaturated, making it more flexible than in water while staying closer to the native structure. Now, in most catalytic reactions, the change in conformation occurs during the cycle. At a low concentration limit such cycling becomes faster as 

Many studies reveal that urea is either a poor kosmotrope or a chaotrope. However, urea is a popular denaturant. The urea-water HB strength is slightly weaker than water to itself and this seems to increase the water-water interactions favorably. It is preferred over water for binding to the protein backbone, leading to protein denaturation. 

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The hydrophobic effect plays an important role in chemistry. It fosters the formation of micelles and reverse micelles and many other structures and gives rise to the unique solvation properties of aqueous binary mixtures (such as water-urea, water-DMSO, water-ethanol, to name just a few). The hydrophobic effect is also centrally important in biological systems. It is partly responsible for protein folding, micellar aggregation, lipid bilayer formation, cell membrane formation, the assembly of proteins into functional complexes, etc. 

Usually, the Flory—Huggins interaction parameters %ij are positive quantities smaller than unity. " " " There are, however, cases in which %ij has negative values. Because the interaction of a protein with water or urea is exothermic, the above parameters are expected to be negative. Indeed, according to the van Laar expression for the heat of mixing in a two component system, the Flory—Huggins interaction parameter 7 is proportional to the heat of mixing in the binary system i—j. Furthermore, as demonstrated in a previous section, for the water + lysozyme + urea mixture J2i > 0, and eq 22 in which 2.5 leads to... 

Dudley, G. J. and Tyrell, H. J. V., Transport Processes in Binary and Ternary Mixtures Containing Water, Triethylamine and Urea, J. Chem. Soc. Faraday Trans., 69, 2188-2199, 2200-2208 (1973). 

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