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lecture_4 - The Peptide Bond What allows amino acids to...

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The Peptide Bond What allows amino acids to polymerize to form peptides and proteins is the unique covalent linkage called a peptide bond . The bond is the result of a head to tail condensation of the amino group of one amino acid and the carboxyl group of another. Formation of this bond results in the release of 1 mol of water per mol of peptide bond formed . A dipeptide contains two amino acids, a tripeptide contains three amino acids etc. In general, these structures are called oligopeptides . After 20 or so amino acids, oligopeptides begin to be called polypeptides Proteins are long polypeptides. Although the transition is vague, usually structures having molecular weights over 10,000 are called proteins. Reading Chap. 5 pp. 126-128, 137-150. Chap. 6 pp. 159-163 .
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Structure of the Peptide Bond X-ray diffraction studies of crystals of small peptides by Linus Pauling and R. B. Corey indicated that the peptide bond is rigid, and planer. Pauling pointed out that this is largely a consequence of the resonance interaction of the amide, or the ability of the amide nitrogen to delocalize its lone pair of electrons onto the carbonyl oxygen. Because of this resonance, the C=O bond is actually longer than normal carbonyl bonds , and the N–C bond of the peptide bond is shorter than the N–C a bond. Notice that the carbonyl oxygen and amide hydrogen are in a trans configuration, as opposed to a cis configuration. This configuration is energetically more favorable because of possible steric interactions in the other.
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The Polarity of the Peptide Bond
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Hydrolysis of the Peptide Bond
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Acid Hydrolysis of Peptide Bond Acid hydrolysis destroys several different amino acids. Notably, tryptophan is totally destroyed. Cysteine is destroyed, and asparagine and glutamine are hydrolyzed to the corresponding carboxylic acids with subsequent release of ammonia.
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Protein Sequences The primary structure of a protein represents the linear arrangement of amino acids via peptide bonds. By convention, the sequence
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