2011_09_06_Amino Acid Structure and Properties

2011_09_06_Amino Acid Structure and Properties - Protein...

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1 Protein Structure and Function Lecture 2b: Amino acid structure and properties Amino acid structure N H 2 R H COOH R COO H 3 N H + Un-ionized form Dipolar form (zwitterion) Carboxylic acid Amine ( α -amino) Side chain
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2 Ionization state vs pH R COO H 3 N H + R COOH H 3 N H + R COO N H 2 H Predominant forms p H 1p H 7p H 1 1 • L- and D-amino acids Chirality at C-alpha R COO H NH 3 + R COO H 3 N H + Chiral center L-amino acid D-amino acid
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3 Aliphatic side chains H COO H 3 N H + CH 3 COO H 3 N H + CH COO H 3 N H + C H 3 CH 3 CH 2 CH C H 3 CH 3 COO H 3 N H + CH CH 2 C H 3 CH 3 COO H 3 N H + Glycine (Gly, G) Alanine (Ala, A) Valine (Val, V) Leucine (Leu, L) Isoleucine (Ile, I) View 3D Aromatic side chains COO H 3 N H + CH 2 COO H 3 N H + CH 2 OH COO H 3 N H + N H CH 2 Tryptophan (Trp, W) Tyrosine (Tyr, Y) Phenylalanine (Phe, F) View 3D Indole ring COO H 3 N H + N NH CH 2 H + Histidine (His, H) Imidazole ring
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4 Spectroscopic properties of aromatic amino acids UV Absorbance ¾ Phe, λ max 256 nm ¾ Tyr, λ max 275 nm ¾ Trp, λ max 280 nm Fluorescence ¾ Phe, λ max 282 nm ¾ Tyr, λ max 303 nm ¾ Trp, λ max 348 nm Environment affects the observed spectrum ¾ Can be used to probe conformational changes Basic side chains CH 2 CH 2 CH 2 CH 2 NH 3 COO H 3 N H + + CH 2 CH 2 CH 2 NH N H 2 NH 2 COO H 3 N H + + Arginine (Arg, R) Lysine (Lys, K) View 3D
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2011_09_06_Amino Acid Structure and Properties - Protein...

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