lecture 26 - Chapter 26:Biomolecules: Amino Acids,...

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Chapter 26:Biomolecules: Amino Acids, Peptides, and Proteins Based on McMurry’s Organic Chemistry , 7 th edition
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2 Proteins – Amides from Amino Acids Amino acids contain a basic amino group and an acidic carboxyl group Joined as amides between the NH 2 of one amino acid and the CO 2 H to the next amino acid Chains with fewer than 50 units are called peptides Protein: large chains that have structural or catalytic functions in biology
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3 Why this Chapter? Amino acids are the fundamental building blocks of proteins To see how amino acids are incorporated into proteins and the structures of proteins
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4 26.1 Structures of Amino Acids In neutral solution, the COOH is ionized and the NH 2 is protonated The resulting structures have “+” and “-” charges (a dipolar ion , or zwitterion ) They are like ionic salts in solution
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5 The Common Amino Acids 20 amino acids form amides in proteins All are α -amino acids - the amino and carboxyl are connected to the same C They differ by the other substituent attached to the α carbon, called the side chain, with H as the fourth substituent Proline is a five-membered secondary amine, with N and the α C part of a five-membered ring See table 26.1 to examine names, abbreviations, physical properties, and structures of 20 commonly occurring amino acids
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6 Abbreviations and Codes Alanine A, Ala Arginine R, Arg Asparagine N, Asn Aspartic acid D, Asp Cysteine C, Cys Glutamine Q, Gln Glutamic Acid E, Glu Glycine G, Gly Histidine H, His Isoleucine I, Ile Leucine L, Leu Lysine K, Lys Methionine M, Met Phenylalanine F, Phe Proline P, Pro Serine S, Ser Threonine T, Thr Tryptophan W, Trp Tyrosine Y, Tyr Valine V, Val
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7 Chirality of Amino Acids Glycine, 2-amino-acetic acid, is achiral In all the others, the α carbons of the amino acids are centers of chirality The stereochemical reference for amino acids is the Fischer projection of L-serine Proteins are derived exclusively from L-amino acids
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8 Types of side chains Neutral: Fifteen of the twenty have neutral side chains Asp and Glu have a second COOH and are acidic Lys, Arg, His have additional basic amino groups side chains (the N in tryptophan is a very weak base) Cys, Ser, Tyr (OH and SH) are weak acids that are good nucleophiles
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Histidine Contains an imidazole ring that is partially protonated in neutral solution Only the pyridine-like, doubly bonded nitrogen in histidine is basic. The pyrrole-like singly bonded
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lecture 26 - Chapter 26:Biomolecules: Amino Acids,...

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