Journal club
Urea, the water soluble organic compound found in mammalian urine, has been known for its ability to denature or unfold proteins for more than 100 years. To this day, it is among the most widely used protein denaturants. So one could be forgiven for taking it for granted that we know in gory detail what happens when we pour urea into a protein solution. But, alas, nailing down the individual molecular interactions between urea and the chemical groups at a protein's surface is exceedingly difficult. Experiments and simulations suggest that urea interacts primarily with amide groups in the protein backbone, but every such group in a given protein has its own local environment, leading to fuzzy signals in spectroscopic studies. Paul Cremer's group at Texas A&M University came up with a good means by which to address the problem. They employed a popular protein proxy, poly(N isopropylacrylamide) in which all of the amide groups are chemically equivalent (L. B. Sagle et al. J. Am. Chem. Soc. 131, 9304 9310; 2009) Using infrared spectroscopy combined with measurements of hydrophobic collapse, they showed that urea interacts only weakly with this polymer. Essentially, Cremer and colleagues' measurements suggest that one needs buckets of urea to see any effect. This is exactly the same situation as that observed for proteins, in which high concentrations of urea are necessary for denaturation. Thus one of the most common denaturants is actually a shockingly weak one. In fact, the strength of its interactions with the protein is little greater than those of harmless water molecules. In the end, the key to the denaturating mechanism may be the fact that urea is a larger molecule than water which has subtle entropic consequences rather than that the two have different hydrogen binding abilities.
Nature