the relkylated,llo gan5yermarniyeversibleadoaltehvdestofonm es8cgmehsasaonteaatseoaat ker Srrhesls Nanine 26-3 Synthesis of Enantiomerically Pure Amino Acids 品 -Pepide o:Amn Acidiomrsand arRaeaeaoeet8mgg5nemesms:noH eptide bonds inhuntrmnav w.i..fo 3 3 The initially formed 2-propanedioate can be alkylated, allowing for the preparation of substituted amino acids. Amino acids are prepared from aldehydes by the Strecker synthesis. Nitriles ordinarily reversible add to aldehydes to form cyanohydrins: The Strecker synthesis utilizes a variation of this reaction in the presence of ammonia: 26-3 Synthesis of Enantiomerically Pure Amino Acids Resolution of the racemic mixtures of amino acids produced by the preceding methods can be accomplished by reaction with an optically active amine, such as brucine, followed by fractional crystallization: Unfortunately, in practice this method is tedious and can suffer from poor yields. An alternate approach is to form the stereocenter at C2 enantioselectively, as in enantioselective hydrogenations of α,β- unsaturated amino acids. Nature employs this strategy by using two enzymes: NADH, glutamate dehydrogenase and a transaminase: (S)-Glutamic acid is the biosynthetic precursor of glutamine, proline and arginine. (S)-Glutamic acid also functions to aminate other 2-oxo acids via a transaminase, thus making other amino acids available. Peptides and Proteins: Amino Acid Oliomers and Polymers 26-4 Amino acids form peptide bonds. 2-Amino acids are the monomer units in polypeptides. The amide bond formed between the carboxylic acid function of one amino acid and the amino group of a second amino acid is called a peptide bond. The individual amino acid units within the polypeptide are referred to as residues. In some proteins, two or more polypeptide chains are linked by disulfide bridges