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lecture 3 - 3-1 MCB354 Lecture 3 Ionization of Amino Acids...

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3-1 MCB354 Lecture 3 Reading in Lehninger : Chapter 3, pp. 81-89 Ionization of Amino Acids Peptides
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3-2 Ionization of Amino Acids
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H+ H+ fully protonated: net charge: +1 0 -1 At pH 2.34 , equal mixture of + H3N-CH2-COOH ( +1 ) and + H3N-CH2-COO - ( 0 ), so net + ve charge = 0.5 Titration of glycine 3-3
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Chemical environment affects p K a 3-4
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net charge: +1 0 -1 -2 H+ H+ H+ Titration of glutamic acid 3-5 p K as governing formation of the +1 and -1 forms govern formation of uncharged glutamate, and pI = 2.19 + 4.25 2 = 3.22
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H+ net charge: +2 +1 0 -1 H+ H+ Titration of histidine 3-6
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3-7
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peptide bond Unfavorable, because OH - poor leaving group 3-8 Formation of a peptide bond Condensation Hydrolysis
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Ser-Gly-Tyr-Ala-Leu ( SGYAL ) a pentapeptide “N-terminus” “C-terminus” 3-9
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R -groups can ionize , and contribute to the acid-base behavior of the peptide. But, interactions with other R -groups and loss of the charges from the αα ’s α -amino and α - carboxyl groups can affect R-group p K a.
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