Chapter 3 - Amino Acids and Peptides

Chapter 3 - Amino Acids and Peptides - The pKa values of...

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Chapter 3 – Amino Acids and Peptides 1. Amino Acids Exist in a Three-Dimensional World a. Chiral – nonsuperimposable mirror images i. Amino acids have a chiral center at the α-C 1. (Except for Glycine) b. L- and D- configuration i. Based on the similarity of an AA to glyceraldehyde c. Amino acids are classified according to two major criteria: i. The polarity of the side chains ii. The presence of an acidic or basic group in the side chain 2. Amino Acids can Act as Both Acids and Bases a. What happens when we titrate an amino acid? i. Titration of Alanine
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ii. With rising pH, the overall net charge of an amino acid changes with each inflection point iii. The pKa values of the  -carboxyl α  groups are fairly low, around  2 iv.
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Unformatted text preview: The pKa values of the amino groups are much higher, 9 to 10.5 v. The pKa of the side-chain groups depend on the group’s chemical nature vi. The pH at which a molecule has no net charge is called the isoelectric pH (pI) b. The pI can be calculated by: i. pI = pK a1 + pK a2 2 c. Triprotic Amino acids i. Asp ii. Glu iii. His +2 at low pH iv. Cys v. Tyr vi. Lys +2 at low pH vii. Arg +2 at low pH 3. The Peptide Bond a. Which groups on amino acids react to form a peptide bond? i. Peptide formed between: α-Carboxyl and α-Amino 1. H 2 0 is eliminated in the process ii. Strengthened by resonance:...
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