Condensation Reactions: Linking Amino Acids and Releasing WaterAmino acids join end-to-end through condensation reactions.The carboxyl group (-COOH) of one amino acid reacts with the amino group (-NH₂) of the next.A hydrogen atom from the amino group and a hydroxyl group (-OH) from the carboxyl group are removed.These combine to form a molecule of water (H₂O), which is released as a byproduct.The bond that forms between the remaining carbon and nitrogen is called a peptide bond.It is a strong covalent bond.Two amino acids linked by a single peptide bond form a dipeptide.Amino acid + Amino acid → Dipeptide + WaterPeptide bond formation Common MistakeThe peptide bond forms between the carbon of the carboxyl group and the nitrogen of the amino group.Students often draw it between the wrong atoms.The bond is specifically C-N, written as -CO-NH-.From Dipeptides to Polypeptides: Building Long ChainsThe process does not stop at two amino acids.Each additional amino acid is added by another condensation reaction, extending the chain and releasing another water molecule.Short chains (fewer than about 20 amino acids) are called oligopeptides.Longer chains are called polypeptides, and these fold into functional proteins.Every polypeptide has two distinct ends.The N-terminus has a free amino group (-NH₂).The C-terminus has a free carboxyl group (-COOH).The chain always runs from N-terminus to C-terminus, giving it a defined direction.Backbone and R GroupsThe repeating sequence of atoms along the chain (N-C-C-N-C-C...) forms the polypeptide backbone.This backbone is identical in every polypeptide.What makes each polypeptide unique is the sequence of R groups that project outward from the backbone.The R groups determine the chemical properties of the polypeptide and drive how it folds into its final three-dimensional shape.ExampleGlycine (-NH₂-CH₂-COOH) reacts with alanine (-NH₂-CH(CH₃)-COOH).The -OH from glycine's carboxyl group and one -H from alanine's amino group are removed as water.The resulting dipeptide (glycylalanine) has a peptide bond (-CO-NH-) linking the two residues.How to Score the Drawing MarksExam techniqueWhen drawing a condensation reaction, always show the water molecule being released.Examiners look for this.When drawing a dipeptide, label the peptide bond, the backbone, and the two R groups.Hydrolysis: The Reverse Reaction That Powers DigestionThe reverse of condensation is hydrolysis.A water molecule is added across the peptide bond, breaking it and releasing the individual amino acids.Hydrolysis is how your body breaks down dietary protein during digestion.Active recallWhat small molecule is released when two amino acids undergo a condensation reaction?Between which two atoms does the peptide bond form?What is the difference between an oligopeptide and a polypeptide?What is the difference between the N-terminus and C-terminus of a polypeptide?What is hydrolysis and how does it relate to condensation?
