## 12.8 Amino Acids ### 12.8.1 Introduction to Amino Acids Amino acids are organic compounds that contain two key functional groups: an amino group ($-NH_2$) and a carboxylic acid group ($-COOH$). These compounds serve as the fundamental building blocks for protein synthesis in living organisms. The twenty amino acids involved in protein synthesis are classified into two categories: - **Non-essential amino acids:** Ten of these amino acids can be synthesized by the human body and are therefore classified as non-essential. - **Essential amino acids:** The remaining ten amino acids cannot be synthesized by the human body and must be obtained through dietary sources. These are called essential amino acids. The general formula for an amino acid is represented as: $$R-CH(NH_2)-COOH$$ Where $R$ represents a variable side chain that distinguishes one amino acid from another. ### 12.8.2 Alpha-Amino Acids Most naturally occurring amino acids are alpha-amino acids (also called 2-amino acids). In these compounds, the amino group is attached to the carbon atom alpha to (adjacent to) the carboxyl group. The simplest amino acid is 2-aminoethanoic acid (glycine), where $R = H$: $$H_2N-CH_2-COOH$$ The amino acid 2-aminopropanoic acid (alanine) has $R = CH_3$: $$H_2N-CH(CH_3)-COOH$$ ### 12.8.3 Physical Properties of Amino Acids: Zwitterions Amino acids exhibit amphoteric behavior due to the presence of both a basic amino group ($-NH_2$) and an acidic carboxylic acid group ($-COOH$) in the same molecule. This leads to internal proton transfer through the following mechanism: - The acidic $-COOH$ group donates a proton ($H^+$). - The basic $-NH_2$ group accepts this proton. This results in the formation of a **zwitterion**, a dipolar ion carrying both a positive charge ($-NH_3^+$) and a negative charge ($-COO^-$) but with no overall net electrical charge: $$R-CH(NH_2)-COOH \rightleftharpoons R-CH(NH_3^+)-COO^-$$ The zwitterionic structure significantly affects the physical properties of amino acids: - **High melting points:** Amino acids exist predominantly as zwitterions, even in the solid state. The strong electrostatic (ionic) attractions between these charged species require substantial energy to overcome, resulting in unusually high melting points compared to similar organic compounds. ### 12.8.4 Formation of Peptide Bonds Amino acids can link together through a reaction involving their functional groups. The amino group ($-NH_2$) of one amino acid reacts with the carboxylic acid group ($-COOH$) of another amino acid molecule. This reaction is a **condensation reaction** in which a molecule of water is eliminated. The resulting linkage is called a **peptide bond**: $$R_1-CH(NH_2)-COOH + R_2-CH(NH_2)-COOH \rightarrow R_1-CH(NH_2)-CO-NH-CH(R_2)-COOH + H_2O$$ Key terminology: - **Peptide bond:** The $-CO-NH-$ linkage that joins two amino acid units. - **Dipeptide:** A molecule formed when two amino acid units are joined by a single peptide bond. - **Tripeptide:** A molecule formed when three amino acid units are joined. - **Polypeptide or protein:** When many amino acid units join together through multiple peptide bonds, they form a large macromolecule. ### 12.8.5 Behaviour of Amino Acids and Dipeptides in an Electric Field (Electrophoresis) Electrophoresis is an analytical technique used to separate charged molecules, including amino acids and peptides, based on their movement in an electric field. The migration of amino acids and dipeptides depends on the **pH of the medium**, which influences the degree of ionization of the amino and carboxyl groups: - **Low pH (acidic conditions):** Amino acids exist primarily in their protonated state, with $-NH_3^+$ and undissociated $-COOH$ groups. The molecule carries a net positive charge and migrates toward the cathode (negative electrode). - **High pH (basic conditions):** Amino acids exist primarily in their deprotonated state, with $-COO^-$ and neutral $-NH_2$ groups. The molecule carries a net negative charge and migrates toward the anode (positive electrode). - **Isoelectric point (pI):** At a specific pH known as the isoelectric point, the amino acid exists predominantly as a zwitterion with no net electrical charge and will not migrate in an electric field.