Guanidinium urea complexes

Buschmann, H. J., Dong, H., Mutihac, L., and Schollmeyer, E. (1999) Complex formation between crown ethers and urea and some guanidinium derivatives in methanol, J. Thermal Anal. Color. 57, 487-491. 

The effect of chemicals is more complex and depends on the denaturant (guanidinium chloride, urea, acids). In this case, we have to deal with an additional component to the protein. For example, acids break salt bridges of protein while urea contributes to break H-bonds and P-mercaptoethanol and dithiotreitol reduce S-S bonds. (For a review see reference )... 

Although uronium behaves very similarly to guanidinium in the com-plexation discussed, the binding and extraction of urea are a challenging point. Actually, the conversion of urea to uronium requires acidification with, e.g., perchloric acid, which is inconvenient for practical applications. The problem with urea itself is that this neutral molecule forms only weak complexes with various hosts [132,133]. Some initial attempts to overcome the difficulty involved crowns with intra-anular acidic groups and (proton-ated) pyridino crowns [126,134,135], designed to protonate urea upon complexation. However, it was found that these hosts are not efficient in extraction and membrane transport the pyridine compounds failed to protonate urea and have an unfavorable tendency toward self-complexation [136]. Acidic functionalities help to bind urea strongly, but their nature simultaneously manifested itself in unfavorably low host lipophilicity. 

A similar behaviour is reported (see Figure 35) for the more complex case of BSA - SDS interaction [199], Ionic surfactants have the peculiarity to denature proteins at millimolar concentrations (molar concentrations are necessary for other denaturants, like urea and guanidinium chloride), provided that the surfactant/protein molar ratio can be higher than the number of the strongly binding sites. 

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