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Lecture 2

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LECTURE - 2 Amino acids, Polypeptides and Proteins
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Polypeptides and Proteins Polypeptides are polymers of amino acids. A protein consists of one or more polypeptides.
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Amino group Carboxyl group α carbon
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Ionization of Amino Acids At acidic pH, the carboxyl group is protonated and the amino acid is in the cationic form At neutral pH, the carboxyl group is deprotonated but the amino group is protonated. The net charge is zero; such ions are called Zwitterions At alkaline pH, the amino group is neutral –NH 2 and the amino acid is in the anionic form.
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20 different amino acids with different R groups
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Amino Acids: Atom Naming • Organic nomenclature: start from one end • Biochemical designation: start from α -carbon and go down the R-group
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Most α -Amino Acids are Chiral • The α -carbon always has four substituents and is tetrahedral All (except proline) have an acidic carboxyl group, a basic amino group, and an alpha hydrogen connected to the α -carbon Each amino acid has an unique fourth substituent R In glycine, the fourth substituent is also hydrogen
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(dextro) (levo)
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Amino Acids: Classification Common amino acids can be placed in five basic groups depending on their R substituents: • Nonpolar, aliphatic (7) • Aromatic (3) • Polar, uncharged (5) • Positively charged (3) • Negatively charged (2)
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Nonpolar, Aliphatic R Groups These amino acid side chains are hydrophobic
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Aromatic R Groups These amino acid side chains absorb UV light at 270-280 nm
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Polar, Uncharged R Groups These amino acids side chains can form hydrogen bonds Cysteine can form disulfide bonds
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