The Chemistry of a Condensation Reaction
Note- A condensation reaction occurs when two molecules combine to form a larger molecule, releasing a smaller molecule as a byproduct.
- For amino acids, the byproduct is water (H₂O).
- Key Functional Groups:
- Amine Group (-NH₂): Contains nitrogen.
- Carboxyl Group (-COOH): Contains carbon and oxygen.
- The Reaction:
- The carboxyl group of one amino acid reacts with the amine group of another.
- A hydrogen atom (H) from the amine group and a hydroxyl group (OH) from the carboxyl group are removed.
- These removed parts combine to form a molecule of water (H₂O).
- The Peptide Bond: The remaining carbon from the carboxyl group forms a covalent bond with the nitrogen from the amine group.
- Drawing a peptide bond is a commonly examined question.
Word Equation for the Reaction
Amino acid 1 + Amino acid 2 → Dipeptide + Water
Example- Consider the reaction between glycine ($NH_2-CH_2-COOH$) and alanine ($NH_2-CH(CH_3)-COOH$):
- The carboxyl group ($-COOH$) of glycine reacts with the amine group ($-NH_2$) of alanine.
- A water molecule ($H_2O$) is released as a byproduct.
- The resulting dipeptide is glycylalanine, with a peptide bond ($-CO-NH-$) linking the two amino acids.
Visualizing a Generalized Dipeptide
- Imagine two amino acids connected by a peptide bond. The structure includes:
- The Backbone: A repeating sequence of atoms (N-C-C) forms the core of the dipeptide. This backbone is consistent across all peptides and polypeptides.
- The Side Chains (R-groups): Each amino acid has a unique R-group attached to the central carbon (alpha carbon). These R-groups project outward from the backbone and give the peptide its distinct properties.
- Steps to Draw a Dipeptide:
- First Amino Acid: Show its amine group (-NH₂), alpha carbon (Cα), carboxyl group (-COOH), and R-group.
- Peptide Bond Formation: Connect the carboxyl group of the first amino acid to the amine group of the second amino acid using a peptide bond (-CO-NH-).
- Second Amino Acid: Complete its structure with its alpha carbon, carboxyl group, and R-group.
- When drawing dipeptides or polypeptides, always remember that the peptide bond forms between the carbon of the carboxyl group and the nitrogen of the amine group.
Extending the Chain: Polypeptides and Proteins
- Condensation reactions don't stop at dipeptides.
- Additional amino acids can be added through further condensation reactions, forming polypeptides, the building blocks of proteins.
Key Features of Polypeptides:
- Directionality:
- Amino Terminal (N-terminus): Has a free amine group (-NH₂).
- Carboxyl Terminal (C-terminus): Has a free carboxyl group (-COOH).
- Backbone and R-groups:
- The backbone consists of a repeating N-C-C sequence.
- R-groups project outward, giving the polypeptide its unique chemical properties.
- Short chains of fewer than 20 amino acids are often referred to as oligopeptides, while longer chains are called polypeptides.
Why Peptide Bond Formation is Essential
- Protein Synthesis:
- Peptide bonds are the foundation of protein synthesis, which occurs in ribosomes within cells.
- Without peptide bonds, proteins, essential molecules for life, could not exist.
- Diversity of Life:
- With 20 different amino acids, the number of possible polypeptide sequences is nearly infinite.
- This allows organisms to produce a vast array of proteins, each with unique structures and functions.
- Biological Functions: Proteins perform countless roles, including:
- Catalyzing chemical reactions (enzymes).
- Transporting oxygen (hemoglobin).
- Providing structural support (collagen).
- How does the ability to form an infinite variety of polypeptides relate to the diversity of life on Earth?
- Reflect on how this concept connects to Theory of Knowledge discussions about the relationship between structure and function in biology.
- Confusing Condensation with Hydrolysis:
- Condensation: Formation of peptide bonds with the release of water.
- Hydrolysis: Breaking of peptide bonds by adding water.
- Incorrect Peptide Bond Representation:
- Ensure the bond forms between the carbon of the carboxyl group and the nitrogen of the amine group.
- Overlooking Directionality:
- Remember that polypeptides have a distinct N-terminus and C-terminus.
- Neglecting R-group Diversity:
- Each R-group affects the properties and functions of the resulting protein.
- Don't treat all amino acids as identical except for their backbone.
- It’s easy to overlook the fact that a water molecule is released during a condensation reaction.
- Always include this in word equations and diagrams.
- Write the word equation for the condensation reaction between two amino acids to form a dipeptide.
- Draw the structure of a generalized dipeptide, labeling the peptide bond, backbone, and R-groups.
- Explain why the peptide bond is identical regardless of the R-groups of the amino acids involved.
