Chapter 3 (part 1) - Chapter3...

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Chapter 3 Amino acids, peptides, and proteins
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Properties of Amino Acids capacity to polymerize novel acid-base properties varied structure and chemical functionality chirality
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Basic Amino Acid Structure α -carbon is chiral (except for glycine) at pH 7.0 uncharged amino acids are zwitterions amino acids have a tetrahedral structure side chain carboxyl group amino group α -carbon
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Amino Acid Classification Aliphatic Aromatic Sulfur containing Polar/uncharged basic/acidic Hydrophobic Hydrophillic
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Aliphatic (alkane) Amino Acids Proline (pro, P)- cyclic amino acid Glycine(gly, G)-only non-chiral amino acid, not hydrophobic Alanine (ala, A) – R-group = methyl-group Valine (Val, V) –  isopropyl side chain Leucine (Leu, L) – isobutyl side chain Isoleucine (Ile, I) - 2 chiral carbons - sec-butyl  side chain Hydrophobicity
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Aromatic Amino Acids All very hydrophobic All contain aromatic group Absorb UV ~ 280 nm Phenylalanine (Phe, F) Tyrosine (Tyr, Y) – -OH ionizable (pKa = 10.5) capable of H- bonding Tryptophan (Trp, W) – bicyclic indole ring capable of H-bonding
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Sulfur Containing Amino Acids Methionine (Met, M) – “start” amino acid, very  hydrophobic Cysteine (Cys, C) – sulfur in form of sulfhydryl,  important in disulfide linkages, weak acid, can  form hydrogen bonds. pKa of a thiol is ~ 10  compared to an alcohol which has a pKa ~ 16 Why is a thiol (SH) so much more acidic than an  alcohol (OH)?
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Disulfide bond or disulfide bridge
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Contain carboxyl group side chains (weaker acids than alpha-carboxyl-group) Negatively charged at physiological pH, present as conjugate bases (therefore – ate not –ic acids) Carboxyl groups function as nucleophiles in some enzymatic reactions Aspartate –  Glutamate –  Acidic Amino Acids H 2 N H C C CH 2 OH O C OH O H 2 N H C C CH 2 OH O CH 2 C OH O Aspartic acid Glutamic acid
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Chapter 3 (part 1) - Chapter3...

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