Acidic amino acids carry a negative charge at physiological pH because of their unique side chains. These side chains contain carboxylic acid groups (-COOH). At a neutral pH (around 7), these groups readily lose a proton (H+), becoming carboxylate ions (-COO⁻) and thus negatively charged.
Understanding the Chemistry
- Carboxylic Acid Groups: The key to understanding the negative charge lies in the chemical properties of carboxylic acid groups. These groups are acidic, meaning they tend to donate a proton (H⁺) to their surroundings.
- pKa Values: The tendency to donate a proton is quantified by the pKa value. Acidic amino acids like aspartate (Asp) and glutamate (Glu) have low pKa values for their side chain carboxylic acid groups. This means that at physiological pH (around 7), these groups are significantly more likely to lose a proton than to retain it.
- Proton Dissociation: When the proton is released, the carboxyl group (–COOH) transforms into a carboxylate ion (–COO⁻), resulting in a negative charge on the amino acid side chain. This process is crucial for the overall negative charge of acidic amino acids.
Examples of Acidic Amino Acids
The two main acidic amino acids are:
- Aspartate (Asp or D): Its side chain contains a carboxylic acid group that readily loses a proton at physiological pH.
- Glutamate (Glu or E): Similar to aspartate, glutamate also possesses a carboxylic acid group in its side chain that becomes negatively charged at physiological pH.
These negatively charged side chains play significant roles in protein structure and function, influencing interactions with other molecules and participating in various biochemical processes. For instance, they can interact with positively charged residues or metal ions.
Several sources confirm this explanation, including ScienceDirect Topics, Chemistry LibreTexts, and numerous other scientific articles and websites. The low pKa values of the carboxylic acid groups in the side chains are the determining factor.
