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Zwitterion and isoelectric point

  1. Aug 22, 2011 #1
    zwitterion is defined as is a neutral molecule with a positive and a negative electrical charge (n.b. not dipoles) at different locations within that molecule.


    isoelectric point means pH at which a particular molecule or surface carries no net electrical charge.

    does it mean that amino acid exist as zwitterion at isoelectric point?
  2. jcsd
  3. Aug 23, 2011 #2


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  4. Aug 25, 2011 #3
    Yeah at the isoelectric point, amino acids neutralise themselves because H+ ion from the carboxyl group bonds to the amino group. Below the IEP there is an excess of H+ ions floating around so the carboxyl group gets its proton back leaving the molecule positively charged. Above the IEP there is a deficiency of H+ ions so the acid group loses its proton leaving the molecule negatively charged.
  5. Apr 17, 2012 #4
    So if the protein is in an environment that is below its pI, it will be positive, but the more acidic the environment becomes does the protein continue to get 'more positive'? Or does it not work like that?
  6. Apr 17, 2012 #5


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    To some extent it works this way. Note that protein (especially a large one) can be often protonated in more than one position. Also note that while it is impossible for a single protein to have a fractional number of protons attached, at equilibrium some proteins would be protonated and some would be not, so we can say things like "because of protonation at this pH XYZ protein has on average a positive charge of 2.35".
  7. Apr 17, 2012 #6
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