When the pH is higher than the pI (isoelectric point) of a molecule, the molecule will carry a net negative charge.
Understanding pI and Charge
The isoelectric point (pI) is the pH value at which a molecule, like a protein or amino acid, has no net electrical charge. This means the total positive charges equal the total negative charges. The charge of a molecule is dependent on the pH of the surrounding solution.
- pI: The pH at which a molecule has no net charge.
- pH < pI: The molecule has a net positive charge. This is because the molecule is in an environment with a higher concentration of protons (H+), which can protonate available functional groups, leading to a positive charge.
- pH > pI: The molecule has a net negative charge. This is because the molecule is in an environment with a lower concentration of protons (H+), which can deprotonate available functional groups, leading to a negative charge.
Why Does This Happen?
Amino acids, the building blocks of proteins, contain both acidic (carboxyl) and basic (amino) groups. At a pH below the pI, the amino groups tend to be protonated (NH3+), resulting in a positive charge. At a pH above the pI, the carboxyl groups tend to be deprotonated (COO-), resulting in a negative charge. When the pH is significantly higher than the pI, the deprotonation of acidic groups outweighs the protonation of basic groups, resulting in the net negative charge.
Example: Glutamic Acid
Glutamic acid is an amino acid with two carboxyl groups and one amino group. Its pI is approximately 3.2.
- At pH 2 (pH < pI), glutamic acid has a net positive charge because the amino group is protonated.
- At pH 3.2 (pH = pI), glutamic acid has no net charge.
- At pH 7 (pH > pI), glutamic acid has a net negative charge because the carboxyl groups are deprotonated.
Implications
Understanding the relationship between pH and pI is crucial in various fields, including:
- Protein Purification: Techniques like isoelectric focusing utilize the pI to separate proteins based on their charge.
- Biochemistry: Enzyme activity and protein-protein interactions can be heavily influenced by the charge state of molecules, which is dependent on pH relative to pI.
- Pharmaceutical Science: Drug solubility and stability can be affected by the charge state of the drug molecule, which depends on the pH of the environment and the drug's pI.
In summary, when the pH exceeds a molecule's pI, the molecule will predominantly exist in a negatively charged state due to the deprotonation of acidic functional groups.
