Proteins are constructed from amino acids, which serve as essential components for numerous biological mechanisms. The 20 standard amino acids each have a unique side chain (R-group) that provides distinct physicochemical properties that determine protein structure, dynamics and function. The detailed properties of these biomolecules serve as essential knowledge across disciplines from enzymology to drug design.
The following table presents essential properties of the 20 standard amino acids, which encompass molecular weight information as well as structural formulas along with ionization constants and their respective isoelectric points (pI). Understanding the behavior of amino acids under both physiological and experimental settings relies heavily on these key parameters.
| Amino Acid | 3-Letter Code | 1-Letter Code | Molecular Weight (Da) | Molecular Formula | Residue Formula | Residue Weight (-H2O) (Da) | pKa(α-COOH) | pKb (α-NH3+) | pKx (Side Chain) | pI (Isoelectric Point) |
| Alanine | Ala | A | 89.09 | C3H7NO2 | C3H5NO | 71.08 | 2.34 | 9.69 | – | 6 |
| Arginine | Arg | R | 174.2 | C6H14N4O2 | C6H12N4O | 156.19 | 2.17 | 9.04 | 12.48 | 10.76 |
| Asparagine | Asn | N | 132.12 | C4H8N2O3 | C4H6N2O2 | 114.11 | 2.02 | 8.8 | – | 5.41 |
| Aspartic Acid | Asp | D | 133.1 | C4H7NO4 | C4H5NO3 | 115.09 | 1.88 | 9.6 | 3.65 | 2.77 |
| Cysteine | Cys | C | 121.16 | C3H7NO2S | C3H5NOS | 103.15 | 1.96 | 10.28 | 8.18 | 5.07 |
| Glutamic Acid | Glu | E | 147.13 | C5H9NO4 | C5H7NO3 | 129.12 | 2.19 | 9.67 | 4.25 | 3.22 |
| Glutamine | Gln | Q | 146.15 | C5H10N2O3 | C5H8N2O2 | 128.13 | 2.17 | 9.13 | – | 5.65 |
| Glycine | Gly | G | 75.07 | C2H5NO2 | C2H3NO | 57.05 | 2.34 | 9.6 | – | 5.97 |
| Histidine | His | H | 155.16 | C6H9N3O2 | C6H7N3O | 137.14 | 1.82 | 9.17 | 6 | 7.59 |
| Isoleucine | Ile | I | 131.18 | C6H13NO2 | C6H11NO | 113.16 | 2.36 | 9.6 | – | 6.02 |
| Leucine | Leu | L | 131.18 | C6H13NO2 | C6H11NO | 113.16 | 2.36 | 9.6 | – | 5.98 |
| Lysine | Lys | K | 146.19 | C6H14N2O2 | C6H12N2O | 128.18 | 2.18 | 8.95 | 10.53 | 9.74 |
| Methionine | Met | M | 149.21 | C5H11NO2S | C5H9NOS | 131.2 | 2.28 | 9.21 | – | 5.74 |
| Phenylalanine | Phe | F | 165.19 | C9H11NO2 | C9H9NO | 147.18 | 1.83 | 9.13 | – | 5.48 |
| Proline | Pro | P | 115.13 | C5H9NO2 | C5H7NO | 97.12 | 1.99 | 10.6 | – | 6.3 |
| Serine | Ser | S | 105.09 | C3H7NO3 | C3H5NO2 | 87.08 | 2.21 | 9.15 | – | 5.68 |
| Threonine | Thr | T | 119.12 | C4H9NO3 | C4H7NO2 | 101.11 | 2.09 | 9.1 | – | 5.6 |
| Tryptophan | Trp | W | 204.23 | C11H12N2O2 | C11H10N2O | 186.22 | 2.83 | 9.39 | – | 5.89 |
| Tyrosine | Tyr | Y | 181.19 | C9H11NO3 | C9H9NO2 | 163.18 | 2.2 | 9.11 | 10.07 | 5.66 |
| Valine | Val | V | 117.15 | C5H11NO2 | C5H9NO | 99.13 | 2.32 | 9.62 | – | 5.96 |
Key Parameters Explained
- Molecular Weight: The mass of one mole of the amino acid in Daltons (Da).
- Molecular Formula: The full chemical formula of the amino acid, including all atoms.
- Residue Formula: The chemical formula of the amino acid as it appears in a polypeptide chain (minus a molecule of water).
- Residue Weight (-H2O): The effective molecular weight of the amino acid residue in a protein sequence.
- pKa (α-COOH): The dissociation constant of the carboxyl group; relevant for understanding protonation states at acidic pH.
- pKb (α-NH3+): The dissociation constant of the amino group; important for protonation under basic conditions.
- pKx (Side Chain): The pKa of any ionizable side chain (if present), impacting the amino acid's buffering capacity and reactivity.
- pI (Isoelectric Point): The pH at which the amino acid exists predominantly in its zwitterionic (neutral) form.
This table serves as a vital reference in disciplines such as biochemistry, molecular biology, and bioengineering, where understanding amino acid properties supports innovations in protein design, enzyme catalysis, peptide synthesis, and biomedical research. Scientists gain enhanced predictive capabilities and control over protein behavior for medical and industrial uses through the correlation of molecular properties and biological functions.
