The question "What is pI or pH?" asks for definitions of two distinct but related concepts in chemistry and biochemistry: the isoelectric point (pI) and pH.
Understanding pH
pH is a measure of how acidic or basic a solution is. Specifically, it quantifies the concentration of hydrogen ions (H+) in a solution.
- The pH scale ranges from 0 to 14.
- A pH of 7 is neutral (e.g., pure water).
- pH values less than 7 indicate acidity.
- pH values greater than 7 indicate alkalinity or basicity.
Understanding pI (Isoelectric Point)
The isoelectric point (pI) is the pH value at which a molecule, like a protein, carries no net electrical charge. According to the provided reference:
The isoelectric point (pI) of a protein is defined as the pH at which the net charge of a protein molecule is zero. Accordingly, proteins are positively charged at a pH below their pI and negatively charged at a pH above their pI.
- Each protein has a unique pI value based on its amino acid composition.
- At a pH below the pI, the protein has a net positive charge.
- At a pH above the pI, the protein has a net negative charge.
- At the pI, the protein is neutral and tends to be least soluble.
Example: Protein Charge and pH
Imagine a protein with a pI of 6.0:
- At pH 5.0 (below pI): The protein will carry a net positive charge.
- At pH 6.0 (at pI): The protein will have no net charge.
- At pH 7.0 (above pI): The protein will carry a net negative charge.
In summary, pH measures the acidity or basicity of a solution, while pI represents the pH at which a molecule (particularly proteins) has a net zero charge.
