Lecture 2

6 75 aromatic amino acid structures glycine glyg has

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Unformatted text preview: cine Gly G Proline Pro P Cα is not chiral pKa = 10.5 benzene ring (260 nm) UV Absorbance phenol ring (278 nm) indole g roup (280 nm) Methionine and Cysteine Page 59 pKa = 8.4 Tyr BSA is a protein that has Trp, Try and Phe a mino acids. Note the large absorbance at 280 nm for T rp. Two C ys side chains can be cross-linked by forming a disulfide b ridge (-CH2-S-S-CH2 -, cystine); disulfide bridges may stabilize 3D protein structure 4 O O +H 3N +H C O- CH β-MercaptoEthanol (BME) S S Disulfide -O Cleaving Disulfides (BME, DTT, GSH) CH2 -S CH NH3+ C NH3+ O Serine (Ser, S) and Threonine (Thr, T) have uncharged polar side chains OH H2C S H2C CH C O- CH2 H2C -O Side Chains with Alcohol Groups C CH CH2 S 3N O BME O OH +H 3N C OH O- CH Cys CH2 H2C H2 C C H2 O CH2 S- S +H -S 3N S HO H2C OH CH2 H2C H2C -O S O- CH2 -S CH C CH2 S C CH Micelles and Soluble Proteins have polar/charged surfaces NH3+ O The Basics: Histidine, Lysine and Arginine Page 60 The Acids: aspartate and glutamate and their Amide Derivatives: asparagine and glutamine Page 60 pKa = 6.0 pKa = 10.5 imidazole Table 3.2 Know the pKa values of the 7 amino acid ionizable side-chains The ratio of unprotonated t o protonated R-group changes by an order of magnitude with each change in pH unit relative to pKa. Tyrosine pH Ratio ( unprot: prot) 9.5 1:10 10.5 1:1 11.5 10:1 alkylamino group pKa = 12.5 guanidino group pKa = 3.9 pKa = 4.1 dicarboxylic acids uncharged but highly polar Ionization of H istidine (a) Titration curve of histidine pK1 = 1.8 pK2 = 6.0 pK3 = 9.3 pI = pH at which net charge is zero pI = (6 + 9.3)/2 =7.65 To determine the p I: pick the pKa a bove & below point where charge is zero and average those 2 p Ka’ s 5...
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