Isoelectric Point: Exploring the Definition and Importance of pH Balance

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In summary, the isoelectric point is the pH at which there is no net charge, where the concentrations of positively and negatively charged forms of an amino acid are equal. This results in no net migration of the molecule when an electric field is applied. At other pH values, there will be a net shift in the positions of the molecule due to the presence of positively and negatively charged forms. Arginine has pKa values of 2.17, 9.04, and 12.48, with a net charge of +0.5 at pH 9.04 and 0 at pH 12.48. However, at pH 9.04 and 12.48, the amino acid is only half protonated
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leopard
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The isoelectric point is define as the pH t which there is no net charge.

If the carbonyl group has pKa = 2 and the protonated amino group pKa = 9 the amino acid will exist as a zwitteron in solutions with pH between 2 and 9, right? A zwitteron has no net charge, right? So why is it called the isoelectric point, and not the isoelectric region?
 
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Zwitterion has no net charge, that's OK, but in the region you mentioned not all aminoacid is in the form of zwitterion - there is an equilibrium mixture of all possible forms - positively charged, negatively charged, neutral. There is only one exact pH value at which concentrations of positively charged and negatively charged forms is identical. At this pH value even if you add electric field there will be no net migration of aminoacid - neutral molecules will stay at their place, while migrations of positively charged and negatively charged will cancel out. At other pH values there will be net shift of the molecule positions.
 
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Arginin has pKa values 2.17, 9.04 and 12.48. Am I then right to say that the net charge of arginin at pH 9.04 and 12.48 is +0.5 and 0 respectively?
 
  • #4
leopard said:
Arginin has pKa values 2.17, 9.04 and 12.48. Am I then right to say that the net charge of arginin at pH 9.04 and 12.48 is +0.5 and 0 respectively?

I can see the reasoning that got you to that first conclusion, for the first answer that is the right kind of reasoning, but
1) think you have made a mistake about the charges, there is one that can be - and two that can be +
2) the second answer is not so much along the right lines.
When the pH = the pK of a group it is half protonated and half unprotonated (check back you understand why).
 

What is the isoelectric point?

The isoelectric point (pI) is the pH at which a molecule or particle carries no net electrical charge. It is the point at which the number of positive and negative charges are equal, resulting in a neutral overall charge.

Why is the isoelectric point important?

The isoelectric point is important because it can affect the solubility, stability, and function of molecules and particles. It is also used in various techniques such as electrophoresis and chromatography to separate and analyze substances based on their charge at different pH levels.

How is the isoelectric point calculated?

The isoelectric point can be calculated using the pKa values of the molecule's acidic and basic functional groups. The pI is equal to the average of the pKa values of the two ionizable groups that are closest in value.

What factors can influence the isoelectric point?

The isoelectric point can be influenced by the presence of different functional groups, the pH of the surrounding environment, and the ionic strength of the solution. Other factors such as temperature and chemical modifications can also affect the pI.

What are the implications of an isoelectric point outside of the physiological range?

An isoelectric point outside of the physiological range can lead to changes in the structure and function of proteins and other biomolecules. This can affect their ability to carry out their biological roles and can have implications in diseases and medical treatments.

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