Zwitterion and isoelectric point

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SUMMARY

Zwitterions are neutral molecules possessing both positive and negative charges at different locations, while the isoelectric point (pI) is the pH at which a molecule carries no net charge. At the isoelectric point, amino acids exist as zwitterions, neutralizing themselves through the bonding of H+ ions from the carboxyl group to the amino group. Below the pI, amino acids become positively charged due to an excess of H+ ions, while above the pI, they acquire a negative charge from the loss of protons. The protonation of proteins can occur at multiple sites, leading to an average positive charge at specific pH levels.

PREREQUISITES
  • Understanding of zwitterions and their properties
  • Knowledge of isoelectric point (pI) and its significance
  • Familiarity with amino acid structure and behavior in different pH environments
  • Basic principles of protein protonation and charge distribution
NEXT STEPS
  • Research the concept of amino acid titration curves
  • Learn about protein solubility at various pH levels
  • Explore the role of pH in protein folding and stability
  • Investigate techniques for measuring isoelectric points of proteins
USEFUL FOR

Biochemists, molecular biologists, and students studying protein chemistry will benefit from this discussion, particularly those interested in the behavior of amino acids and proteins in varying pH environments.

weng cheong
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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.

while

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?
 
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Yep.
 
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.
 
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?
 
physicsn00bie said:
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?

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".
 
Thaaaaanks
 

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