Aspartate, also known as aspartic acid, is an amino acid with a side chain that contains a carboxylic acid group. The ionization states of aspartate can change based on the pH of the solution it is in. At different pH levels, aspartate can gain or lose protons, resulting in different charged forms. Here’s how aspartate behaves at pH 3, 7, and 12:
At pH 3:
At this low pH, aspartate primarily exists in its protonated form. The carboxylic acid group on the side chain is protonated, giving it a neutral charge, while the α-carboxylic acid group is also protonated (R-COOH). Therefore, it has a net positive charge:
- α-Carboxyl group: COOH (neutral)
- α-Amino group: NH3+ (positive charge)
- Side-chain carboxyl group: COOH (neutral)
Overall, the ionization state at pH 3 is: [NH3+ – R – COOH – COO-]
At pH 7:
At physiological pH (around 7), aspartate is typically in a zwitterionic form. The α-carboxylic group loses a proton, converting it to its deprotonated form, while the α-amino group remains protonated. The side-chain carboxylic acid group loses a proton, resulting in a negative charge:
- α-Carboxyl group: COO- (negative charge)
- α-Amino group: NH3+ (positive charge)
- Side-chain carboxyl group: COO- (negative charge)
So, the ionization state at pH 7 is: [NH3+ – R – COO- – COO-]
At pH 12:
At a high pH of 12, aspartate is negatively charged overall. Both the α-carboxylic acid group and the side-chain carboxylic group are deprotonated, while the α-amino group is still protonated but becoming less favorable:
- α-Carboxyl group: COO- (negative charge)
- α-Amino group: NH2 (neutral)
- Side-chain carboxyl group: COO- (negative charge)
The resulting ionization state at pH 12 is: [NH2 – R – COO- – COO-]
In summary, aspartate can have three distinct forms depending on the pH of the solution, reflecting its ability to gain or lose protons in response to its environment.