Aspartic acid (Asp, D)
Negatively charged side chain, nonessential in the human diet
What is the charge of aspartic acid at pH 7.4?
At pH 7.4, the pH of blood plasma, aspartic acid carries a net charge of −1.01, and 99% of its molecules are the anion, charge −1. Its β-carboxyl side chain, pKR 3.65, has lost its proton in over 99.9% of molecules. Its isoelectric point is 2.77, from Lehninger’s pKa values of 1.88, 9.60 and 3.65.
Properties of aspartic acid
| Three-letter code | Asp |
|---|---|
| One-letter code | D |
| Side-chain class (Lehninger) | Negatively charged |
| Side-chain group | β-carboxyl |
| Formula (uncharged form) | C₄H₇NO₄ |
| Molar mass | 133.10 g/mol |
| pK₁ (α-carboxyl) | 1.88 |
| pK₂ (α-amino) | 9.60 |
| pKR (side chain) | 3.65 |
| Isoelectric point (pI) | 2.77 |
| Configuration at Cα (L form) | S |
| Hydropathy (Kyte and Doolittle) | −3.5 |
| mRNA codons | GAU, GAC |
| Human diet | Nonessential |
pKa values and pI from Nelson and Cox, Lehninger Principles of Biochemistry, 7th edition, Table 3-1. Formula and molar mass from the wwPDB Chemical Component Dictionary and the IUPAC standard atomic weights; codons from NCBI translation table 1; hydropathy from Kyte and Doolittle (1982); diet from the Institute of Medicine, Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (2005), chapter 10.
The values are facts and free to use; this page’s selection and presentation are © 2026 ScienceQuest.
Charge of aspartic acid at common pH values
| pH | Net charge | Most common form | Share of molecules |
|---|---|---|---|
| 1 | +0.88 | cation (+1) | 88% |
| 3 | −0.11 | zwitterion (0) | 76% |
| 5 | −0.96 | anion (−1) | 96% |
| 7 | −1.00 | anion (−1) | 99.7% |
| 7.4 (blood plasma) | −1.01 | anion (−1) | 99% |
| 9 | −1.20 | anion (−1) | 80% |
| 11 | −1.96 | dianion (−2) | 96% |
| 13 | −2.00 | dianion (−2) | over 99.9% |
The net charge is an average over all the molecules, so it can be a fraction; each molecule carries a whole number of charges, and the most common form is the one protonation state with the largest share.
Which form dominates between the pKas
- Below pH 1.88: the cation, net charge +1.
- pH 1.88 to 3.65: the zwitterion, net charge 0.
- pH 3.65 to 9.60: the anion, net charge −1.
- Above pH 9.60: the dianion, net charge −2.
Open aspartic acid in the Amino Acid 3D Explorer and drag the pH to watch each group gain and lose its proton, with every pKa and its pI marked on the pH slider.
How the charge at pH 7.4 is calculated
Each group is treated on its own with the Henderson-Hasselbalch equation. An acidic group adds
−1/(1 + 10^(pKa − pH)), the share of molecules in which it has lost its proton, and a basic group
adds +1/(1 + 10^(pH − pKa)), the share in which it holds one.
Net charge of aspartic acid at pH 7.4
- α-carboxyl (pK₁ = 1.88): −1/(1 + 10^(1.88 − 7.4)) = −1.0000
- α-amino (pK₂ = 9.60): +1/(1 + 10^(7.4 − 9.60)) = +0.9937
- β-carboxyl (pKR = 3.65): −1/(1 + 10^(3.65 − 7.4)) = −0.9998
- q = −1.0000 + 0.9937 − 0.9998 = −1.0061
- q ≈ −1.01
How to calculate the pI of aspartic acid
Aspartic acid has three ionizable groups. Taken from the lowest pKa up, its neutral form exists between pK₁ (1.88, the α-carboxyl) and pKR (3.65, the β-carboxyl), so the pI is the mean of those two.
Isoelectric point of aspartic acid
- pI = (pK₁ + pKR)/2
- = (1.88 + 3.65)/2
- = 2.765 ≈ 2.77
The two printed pKas average to 2.765, exactly halfway between 2.76 and 2.77; Lehninger’s table gives 2.77, the value used on this page.
Averaging all three pKas, a common mistake, gives 5.04, 2.27 pH units above the pI.
Stereochemistry
L-aspartic acid is S at its α-carbon, as 18 of the 19 chiral amino acids are: its amino group outranks its carboxyl, which outranks its side chain. The R and S Configuration Explorer ranks the groups on a stereocenter step by step.
How aspartic acid compares with the other 19
- Aspartic acid is one of 2 amino acids in the class Lehninger calls negatively charged; the other is glutamic acid.
- Its molar mass, 133.10 g/mol, makes it the 10th heaviest of the 20.
- Its pI, 2.77, is the lowest of the 20.
- On Kyte and Doolittle’s hydropathy scale it scores −3.5, the joint 3rd most hydrophilic of the 20, with asparagine, glutamine and glutamic acid.
- It has 2 codons, as do 8 of the other 19.
Is aspartic acid an essential amino acid?
No. Aspartic acid is one of the 5 nonessential amino acids, which the body makes in the amounts it needs. The other 4 are alanine, asparagine, glutamic acid and serine.
Classification from the Institute of Medicine, Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (2005), chapter 10, which calls the three lists indispensable, conditionally indispensable and dispensable.
Codons for aspartic acid
Aspartic acid is encoded by 2 of the 61 sense codons in the standard genetic code: GAU and GAC in mRNA, written GAT and GAC on the DNA coding strand.
The DNA to Protein Translator reads codons in a sequence of your own, and the Protein Synthesis Simulator builds the chain on a ribosome, one amino acid per codon.
Work with aspartic acid
- Amino Acid 3D Explorer: aspartic acid in 3D, with its charge at any pH
- Molar mass of C₄H₇NO₄, worked out atom by atom in the Molar Mass Calculator
- Henderson-Hasselbalch Calculator: the acid to base ratio of one group at any pH
- Titration Curve Calculator: the curve for a weak acid titrated with a strong base
- All 20 amino acids: pKa values, pI, charge and codons in one table
- Aspartic acid on Wikipedia