Some amino acids are considered basic because their side chains contain nitrogen atoms that resemble ammonia, a well-known base. These nitrogen-containing side chains can accept protons, leading to a positive charge at physiological pH.
The Basis of Basicity in Amino Acids
The key to understanding why some amino acids are basic lies in their side chains. Specifically, three amino acids - arginine (Arg), lysine (Lys), and histidine (His) - possess side chains that contain nitrogen and behave like bases. This means they have the ability to accept protons (H+), which makes them basic.
Basic Amino Acids
| Amino Acid | Abbreviation | Side Chain Characteristics | pKa | Charge at Neutral pH |
|---|---|---|---|---|
| Arginine | Arg | Guanidino group containing multiple nitrogen atoms | ~12.5 | +1 |
| Lysine | Lys | Amino group at the end of a hydrocarbon chain | ~10.5 | +1 |
| Histidine | His | Imidazole ring containing two nitrogen atoms | ~6.0 | Approximately neutral or +1 |
Why these side chains act as bases
- Nitrogen's Role: Nitrogen atoms in the side chains have a lone pair of electrons which can bind to a proton (H+). This act of proton binding defines the chemical property of a base.
- High pKa Values: The pKa values of these basic amino acids' side chains are high enough that they tend to bind protons under physiological conditions. A high pKa means a greater tendency to be protonated at higher (more basic) pH.
- Arginine's guanidino group has a very high pKa, making it highly likely to be protonated and positively charged at neutral pH.
- Lysine's amino group also has a high pKa, making it positively charged at physiological pH.
- Histidine's imidazole ring has a pKa near physiological pH, so it exists in both protonated and deprotonated forms depending on the exact pH, making it crucial in enzyme active sites.
Practical Insight
These basic amino acids are crucial in biological systems, participating in protein structures and enzymatic activities where their ability to donate or accept protons is critical. The positive charge at neutral pH influences the protein's three-dimensional structure and its interaction with other molecules.
In summary, some amino acids are basic because their side chains contain nitrogen atoms that can readily accept protons, leading to a positive charge, a fundamental characteristic of a base.
