chaotropic: what's it mean?

Greg Appleyard greg.appleyard at usask.ca
Sat Aug 29 12:44:10 EST 1998


In article <mwcrepeau-2808981757410001 at curt.ucdavis.edu>, 
mwcrepeau at ucdavis.edu says...
>
>What does this word mean?  Nobody I've asked seems to know.  I am
>referring to chaotropic as in the "chaotropic salts" which are used in
>Qiagen DNA purification kits.
>
>
>Marc Crepeau
><mwcrepeau at ucdavis.edu>

You are right, I couldn't find it in the dictionary either.
To me the word means "promoting disorder". Chaotropic salts such a 
Guanidine thiocyanate and ammonium isothiocyanate cause associations 
between biomolecules to be disrupted. Theory has it that these salts bond 
strongly with water molecules, reducing the availablilty of water in 
solution to participate in hydrophobic/phillic interactions. This reducing 
stresses associated with hydrophobic interactions and these interactions 
are weakened beyond the point where two molecules are helpd together by 
hydrophobic interactions.

Whats really cool is that this is not al all or nothing event. Take 
antigen-antibody (polyclonal) binding for example. Antigen and antibody 
are primarily held together by hydrophobic interactions. If you add a 
small amount of a chaotropic agent, the weakly held antibodies will 
dissociate from the antigen. If you add more chaotropic agent, antibodies 
with increasingly stronger binding affinities will be disrupted. You can 
therefore comapre the average affininty (more correctly-avidity) of 
polyclonal antisera in terms of the amount of chaotropic agent required to 
remove a given fraction (say 50%) of the bound antibody.

Hope this helps.
Greg 
 




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