Chapter20CarboxylicAcidsandNitriles.ppt

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Amino Group Protection An amide that is less stable than the protein amide is formed and then removed The tert-butoxycarbonyl amide (BOC) protecting group is introduced with di-tert-butyl dicarbonate Removed by brief treatment with trifluoroacetic acid * Peptide Coupling Amides are formed by treating a mixture of an acid and amine with dicyclohexylcarbodiimide (DCC) * Overall Steps in Peptide Synthesis * 26.8 Automated Peptide Synthesis: The Merrifield Solid-Phase Technique Peptides are connected to beads of polystyrene, reacted, cycled and cleaved at the end * 26.9 Protein Structure The primary structure of a protein is simply the amino acid sequence. The secondary structure of a protein describes how segments of the peptide backbone orient into a regular pattern. The tertiary structure describes how the entire protein molecule coils into an overall three-dimensional shape. The quaternary structure describes how different protein molecules come together to yield large aggregate structures * ?-Helix ?-helix stabilized by H-bonds between amide N–H groups and C=O groups four residues away ?-helical segments in their chains * ?-Pleated Sheet b-pleated sheet secondary structure is exhibited by polypeptide chains lined up in a parallel arrangement, and held together by hydrogen bonds between chains * Denaturation of Proteins The tertiary structure of a globular protein is the result of many intramolecular attractions that can be disrupted by a change of the environment, causing the protein to become denatured Solubility is drastically decreased as in heating egg white, where the albumins unfold and coagulate Enzymes also lose all catalytic activity when denatured * 26.10 Enzymes and Coenzymes An enzyme is a protein that acts as a catalyst for a biological reaction. Most enzymes are specific for substrates while enzymes involved in digestion, such as papin attack