The relationship between isoelectric point (pI) and pH is that the pI represents the specific pH at which a molecule, particularly a protein, carries no net electrical charge. This relationship is critical in understanding protein behavior in solutions.
Understanding pI and pH
What is pI?
The isoelectric point (pI) is a unique property of proteins and other amphoteric molecules, which means they can act as both acids and bases. The pI is the pH value where the number of positive and negative charges on the molecule are equal, resulting in a net zero charge.
What is pH?
pH measures the acidity or basicity of a solution. A pH of 7 is neutral, below 7 is acidic, and above 7 is basic.
How pI and pH Interact
The pI and pH relationship can be summarized as follows:
- pI = pH: At this point, the protein has a net charge of zero. It's electrically neutral.
- pH < pI: In a solution where the pH is lower than the pI (more acidic), the protein will carry a net positive charge because it will have more protonated (positively charged) groups than deprotonated (negatively charged) groups.
- pH > pI: When the pH is higher than the pI (more basic), the protein will have a net negative charge, as it has more deprotonated groups.
Table summarizing pI and pH interaction
| Condition | pH compared to pI | Net Charge on Protein |
|---|---|---|
| Isoelectric Point | pH = pI | Zero |
| Acidic Solution | pH < pI | Positive |
| Basic Solution | pH > pI | Negative |
Practical Implications
The relationship between pI and pH affects how proteins behave in different environments. For instance:
- Protein Solubility: Proteins are often least soluble at their pI because they have a neutral charge and are more likely to aggregate.
- Electrophoresis: Proteins can be separated using electrophoresis based on their charge at a given pH. If the pH of the buffer solution is above the pI, the protein will migrate towards the positive electrode, and vice-versa. At the pI, the protein will not move.
- Chromatography: pI is essential for techniques like ion exchange chromatography, where proteins are separated based on their charge at a specific pH.
Reference Information
The provided reference states that:
The isoelectric point (pI) is the pH of a solution at which the net charge of a protein becomes zero. At solution pH that is above the pI, the surface of the protein is predominantly negatively charged, and therefore like-charged molecules will exhibit repulsive forces.
This information reinforces the understanding that pI is specifically the pH of a solution where the net charge is zero, and that at higher pH values than the pI, proteins become negatively charged.
