Charged amino acids are polar because they possess side chains that carry a net electrical charge, either positive or negative. This charge results in an uneven distribution of electrons, making them highly hydrophilic (water-loving) and thus polar.
Understanding Polarity and Charge
Polarity in molecules arises from differences in electronegativity between atoms, leading to unequal sharing of electrons. This creates partial positive (δ+) and partial negative (δ-) charges within the molecule. In the case of charged amino acids, the entire side chain carries a formal positive or negative charge, making them strongly polar.
Charged Amino Acids: Acidic and Basic
Amino acids with charged side chains are generally categorized as either acidic (negatively charged at physiological pH) or basic (positively charged at physiological pH).
Acidic Amino Acids:
- Aspartic Acid (Asp, D): Contains a carboxylic acid group (-COOH) in its side chain, which can lose a proton (H+) to become negatively charged (-COO-) at physiological pH.
- Glutamic Acid (Glu, E): Similar to aspartic acid, it also has a carboxylic acid group in its side chain that can become negatively charged at physiological pH.
These negatively charged side chains strongly interact with water molecules through electrostatic interactions and hydrogen bonding, making them highly polar.
Basic Amino Acids:
- Lysine (Lys, K): Possesses an amino group (-NH2) in its side chain that can accept a proton (H+) to become positively charged (-NH3+) at physiological pH.
- Arginine (Arg, R): Contains a guanidino group in its side chain, which is positively charged at physiological pH.
- Histidine (His, H): Has an imidazole ring in its side chain that can be protonated (positively charged) depending on the pH, making it sometimes considered polar uncharged and sometimes polar charged. It's often protonated at lower pH values.
The positive charge on these side chains attracts water molecules, making them very polar.
Impact of Charge on Polarity
The presence of a full positive or negative charge on the side chain of an amino acid has a much stronger effect on its polarity compared to amino acids with just partial charges (like those with hydroxyl or amide groups). This is because the electrostatic interaction between a fully charged ion and water molecules is significantly stronger than dipole-dipole interactions or hydrogen bonding between partially charged molecules and water.
Importance in Protein Structure and Function
The charged nature of these amino acids plays a crucial role in:
- Protein folding: Hydrophilic (polar) amino acids tend to be located on the protein's surface, interacting with the surrounding aqueous environment. Hydrophobic (nonpolar) amino acids tend to cluster inside the protein, away from water.
- Enzyme active sites: Charged amino acids often participate in enzyme catalysis, either by stabilizing charged transition states or by acting as proton donors or acceptors.
- Protein-protein interactions: Electrostatic interactions between charged amino acids on different proteins can mediate protein complex formation.
- Ion channels: Charged amino acids lining the pore of an ion channel determine ion selectivity.
In summary, charged amino acids are polar because their side chains carry a net electrical charge, leading to strong electrostatic interactions with water and other polar molecules. This polarity is essential for protein structure, function, and interactions within biological systems.
