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Cissy Gao / Irene Meng / Zero Yao Today’s Object • Identify amino acids and understand their structure •Identify the peptide bonds in dipeptides and polypeptides •Explain why amino acids are soluble in water What is amino acid? • An amino acid is a carboxylic acid with an amine group at the 2-position. – A carboxylic acid contains a –COOH group. – An amine group contains a NH₂ group. • AAmino acids are the building blocks of proteins. general 2-amino acid CH₃—CH—COOH NH₂ 2-aminopropanoic acid [alanine] Structure Hydrogen atom Amino group Carboxyl group Central carbon Radical group Structure Summary: • An amino acid has a central carbon atom bonded to a hydrogen atom and three groups: – An amino group(-NH₂) – An carboxyl/acidic group(-COOH) – An R group [radical group] “R" can be quite a complicated group containing other active groups like -OH, -SH, other amine or carboxylic acid groups, and so on. • There are only 20 different amino acids. – Amino acids differ from one another by their R group Reaction: • Amino acids can react with both acids and bases. Reaction with acids CH₃—CH—COOH + H⁺ NH₂ [alanine] CH₃—CH—COOH +NH₃ Reaction with bases CH₃—CH—COOH + OH⁻ NH₂ CH₃—CH—COO⁻ + H₂O NH₂ After reacting with either an acid or base the amino acid is ionic and remains soluble in water. Two important properties: 1. Amino acids are highly soluble in water. Amino acids have both acid and base groups arranged such that the acid and base groups can “neutralize” each other. Acidic Ionic Basic zwitterion H atom is lost from COOH and gained by NH₂ Two important properties: 2. Amino acids link with each other to form “dipeptides” and “polypeptides”. • An amino acid can be referred to as a peptide. • When two amino acids bond together, they produce a dipeptide. Question #1: When three amino acids bond together, what is it called? Answer: Tripeptide Question #2: When more than three amino acids bond together, what is it called? Answer: Polypeptide Two important properties: 2. Amino acids link with each other to form “dipeptides” and “polypeptides”. O O H—N—CH₂—C—OH + H—N—CH₂—C—OH H H Dehydration synthesis -water is removed O O H—N—CH₂—C—N—CH₂—C—OH + H₂O H H Dipeptide Peptide bond [Amide linkage] Two important properties: 2. Amino acids link with each other to form “dipeptides” and “polypeptides”. • A series of amino acid molecules can be joined by a series of amide linkages to form a polypeptide. • A polypeptide is a single chain of amino acids. • The order or combination of these amino acids determines which protein is produced. Practice: 1. Identify which of the following is an amino acid. 2. What is this reaction called ? Dehydration synthesis Bonus: Besides peptide bond, what other bond is presented? Hydrogen bond Video