In nature, the most common representatives are 2-amino acids (α-amino acids) with the general formula RCH(NH2)COOH. This chapter provides knowledge of amino acids. The main contents of this chapter include all of the following: structure and properties of amino acids, synthesis of amino acids: a combination of amine and carboxylic acidchemistry,.and other contents. | Chapter 26: Amino Acids In nature, the most common representatives are 2-amino acids (α-amino acids) with the general formula RCH(NH2)COOH. R = alkyl, acyl, amino, hydroxy, mercapto, sulfide, carboxy, guanidino, or imidazolyl groups Amino acids give rise to polyamides: polypeptides (proteins, enzymes) Amide linkage For proteins, n ≥ 8000 and MW > 1,000,000. Proteins are crucial for transport (O2, hemoglobin), energy storage, catalysis, control of reactions, template for RNA/DNA action, antibodies, etc. More than 500 natural amino acids known Among the 20 main amino acids, 8 cannot be synthesized by the body (essential amino acids) Names: We use common names. α-Stereocenter usually S (or, old nomenclature, L, from L-glyceraldehyde) Essential amino acids Used in reversible disulfide bridging Monosodium glutamate (MSG) R ! Amino acids are acidic and basic: Exist as zwitterions pKa = 9-10 pKa = 2-3 Zwitterion Glycine as a zwitterion H3NCH2COO– + Synthesis of Amino Acids . | Chapter 26: Amino Acids In nature, the most common representatives are 2-amino acids (α-amino acids) with the general formula RCH(NH2)COOH. R = alkyl, acyl, amino, hydroxy, mercapto, sulfide, carboxy, guanidino, or imidazolyl groups Amino acids give rise to polyamides: polypeptides (proteins, enzymes) Amide linkage For proteins, n ≥ 8000 and MW > 1,000,000. Proteins are crucial for transport (O2, hemoglobin), energy storage, catalysis, control of reactions, template for RNA/DNA action, antibodies, etc. More than 500 natural amino acids known Among the 20 main amino acids, 8 cannot be synthesized by the body (essential amino acids) Names: We use common names. α-Stereocenter usually S (or, old nomenclature, L, from L-glyceraldehyde) Essential amino acids Used in reversible disulfide bridging Monosodium glutamate (MSG) R ! Amino acids are acidic and basic: Exist as zwitterions pKa = 9-10 pKa = 2-3 Zwitterion Glycine as a zwitterion H3NCH2COO– + Synthesis of Amino Acids Hell-Volhard-Zelinsky, then Amination (racemic) Yields not great Better: Gabriel Synthesis ( RX RNH2) 2. Gabriel Synthesis Made from malonic ester by radical bromination (hν) General: (in combination with malonic ester synthesis) 3. Strecker Synthesis Recall: When HCN is used in the presence of ammonia, ., NH4CN or NH4Cl/NaCN, it adds to the corresponding imine to give an aminonitrile: Adolf Strecker (1822–1871) Strecker Remember reductive amination and Mannich reaction! Peptides Amino acids form peptide bonds Dimer = dipeptide, trimer = tripeptide, and so on. The chain is arranged in space by H bonding, electrostatic attractions, hydrophobic-hydrophilic interactions (with water), and rigidity of the amide bond. Rigidity and planarity of the peptide bond sp2 Trans to the left to the right Main chain R, R’, R”, etc. are called the side chains. All stereocenters are assumed to be S. Amino acid (linear) sequence = primary structure. Three dimensional arrangement: secondary, tertiary, and quaternary .
